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@article{faucris.107598084,
abstract = {We identify a fundamental challenge for any non-perturbative approach based on finite clusters resulting from the reduced symmetry on graphs, most importantly the breaking of translational symmetry, when targeting the properties of excited states. This can be traced back to the appearance of intruder states in the low-energy spectrum, which represent a major obstacle in quasi-degenerate perturbation theory. Here a generalized notion of cluster additivity is introduced, which is used to formulate an optimized scheme of graph-based continuous unitary transformations allowing to solve and to physically understand this major issue. Most remarkably, our improved scheme demands to go beyond the paradigm of using the exact eigenvectors on graphs. Copyright (C) EPLA, 2015},
author = {Coester, K. and Clever, S. and Herbst, F. and Capponi, Sylvain and Schmidt, Kai Phillip},
doi = {10.1209/0295-5075/110/20006},
faupublication = {no},
journal = {EPL - Europhysics Letters},
peerreviewed = {Yes},
title = {{A} generalized perspective on non-perturbative linked-cluster expansions},
volume = {110},
year = {2015}
}
@article{faucris.107600724,
abstract = {We investigate the low-energy properties as well as quantum and thermal phase transitions of the Baxter-Wu model in a transverse magnetic field. Our study relies on stochastic series expansion quantum Monte Carlo and on series expansions about the low- and high-field limits at zero temperature using the quantum finite-lattice method on the triangular lattice. The phase boundary consists of a second-order critical line in the four-state Potts model universality class starting from the pure Baxter-Wu limit meeting a first-order line connected to the zero-temperature transition point (h approximate to 2.4, T = 0). Both lines merge at a tricritical point approximately located at (h approximate to 2.3J, T approximate to J).},
author = {Capponi, Sylvain and Jahromi, Saeed S. and Alet, Fabien and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevE.89.062136},
faupublication = {no},
journal = {Physical Review E},
peerreviewed = {Yes},
title = {{Baxter}-{Wu} model in a transverse magnetic field},
volume = {89},
year = {2014}
}
@article{faucris.226824067,
abstract = {Kitaev's honeycomb-lattice spin-1/2 model has become a paradigmatic example for Z(2) quantum spin liquids, both gapped and gapless. Here we study the fate of these spin-liquid phases in differently stacked bilayer versions of the Kitaev model. Increasing the ratio between the interlayer Heisenberg coupling J(perpendicular to) and the intralayer Kitaev couplings K-x,K-y,K-z destroys the topological spin liquid in favor of a paramagnetic dimer phase. We study phase diagrams as a function of J(perpendicular to)/K and Kitaev coupling anisotropies using Majorana-fermion mean-field theory, and we employ different expansion techniques in the limits of small and large J(perpendicular to)/K. For strongly anisotropic Kitaev couplings, we derive effective models for the different layer stackings that we use to discuss the quantum phase transition out of the Kitaev phase. We find that the phase diagrams depend sensitively on the nature of the stacking and anisotropy strength. While in some stackings and at strong anisotropies we find a single transition between the Kitaev and dimer phases, other stackings are more involved. Most importantly, we prove the existence of two novel macrospin phases, which can be understood in terms of Ising chains that can be either coupled ferromagnetically or remain degenerate, thus realizing a classical spin liquid. In addition, our results suggest the existence of a flux phase with spontaneous interlayer coherence. We discuss prospects for experimental realizations.},
author = {Seifert, Urban F. P. and Gritsch, Julian and Wagner, Erik and Joshi, Darshan G. and Brenig, Wolfram and Vojta, Matthias and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.98.155101},
faupublication = {yes},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Bilayer} {Kitaev} models: {Phase} diagrams and novel phases},
volume = {98},
year = {2018}
}
@article{faucris.122455124,
abstract = {Motivated by the possibility of universal quantum computation under noise perturbations, we compute the phase diagram of the two-dimensional (2D) cluster state Hamiltonian in the presence of Ising terms and magnetic fields. Unlike in previous analysis of perturbed 2D cluster states, we find strong evidence of a very well-defined cluster phase, separated from a polarized phase by a line of first-and second-order transitions compatible with the 3D Ising universality class and a tricritical end point. The phase boundary sets an upper bound for the amount of perturbation in the system so that its ground state is still useful for measurement-based quantum computation purposes. Moreover, we also compute the local fidelity with the unperturbed 2D cluster state. Besides a classical approximation, we determine the phase diagram by combining series expansion and variational infinite projected entangled-pair states methods. Our work constitutes an analysis of the nontrivial effect of few-body perturbations in the 2D cluster state, which is of relevance for experimental proposals.},
author = {Orus, Roman and Kalis, Henning and Bornemann, Marcel and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevA.87.062312},
faupublication = {no},
journal = {Physical Review A},
peerreviewed = {Yes},
title = {{Bounds} on universal quantum computation with perturbed two-dimensional cluster states},
volume = {87},
year = {2013}
}
@article{faucris.109478424,
abstract = {The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin-ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modeling of the dynamical response demonstrates our complete quantitative understanding of these states.},
author = {Ward, S. and Mena, M. and Bouillot, P. and Kollath, C. and Giamarchi, T. and Schmidt, Kai Phillip and Normand, B. and Kramer, K. W. and Biner, D. and Bewley, R. and Guidi, T. and Boehm, M. and Mcmorrow, D. F. and Ruegg, Ch.},
doi = {10.1103/PhysRevLett.118.177202},
faupublication = {yes},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Bound} {States} and {Field}-{Polarized} {Haldane} {Modes} in a {Quantum} {Spin} {Ladder}},
volume = {118},
year = {2017}
}
@article{faucris.108546284,
abstract = {We analyze the properties of low-energy bound states in the transverse-field Ising model and in the XXZ model on the square lattice. To this end, we develop an optimized implementation of perturbative continuous unitary transformations. The Ising model is studied in the small-field limit which is found to be a special case of the toric code model in a magnetic field. To analyze the XXZ model, we perform a perturbative expansion about the Ising limit in order to discuss the fate of the elementary magnon excitations when approaching the Heisenberg point.},
author = {Dusuel, Sebastien and Kamfor, Michael and Schmidt, Kai Phillip and Thomale, Ronny and Vidal, Julien},
doi = {10.1103/PhysRevB.81.064412},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Bound} states in two-dimensional spin systems near the {Ising} limit: {A} quantum finite-lattice study},
volume = {81},
year = {2010}
}
@article{faucris.108534844,
abstract = {We studied the robustness of a generalized Kitaev's toric code with Z(N) degrees of freedom in the presence of local perturbations. For N = 2, this model reduces to the conventional toric code in a uniform magnetic field. A quantitative analysis was performed for the perturbed Z(3) toric code by applying a combination of high-order series expansions and variational techniques. We found strong evidence for first- and second-order phase transitions between topologically ordered and polarized phases. Most interestingly, our results also indicate the existence of topological multi-critical points in the phase diagram.},
author = {Schulz, Marc Daniel and Dusuel, Sebastien and Orus, Roman and Vidal, Julien and Schmidt, Kai Phillip},
doi = {10.1088/1367-2630/14/2/025005},
faupublication = {no},
journal = {New Journal of Physics},
peerreviewed = {Yes},
title = {{Breakdown} of a perturbed {Z}({N}) topological phase},
volume = {14},
year = {2012}
}
@article{faucris.121428384,
abstract = {In the two-leg S=1/2 ladders of Sr14Cu24O41, a modulation of the exchange coupling arises from the charge order within the other structural element, the CuO2 chains. In general, breaking translational invariance by modulation causes gaps within the dispersion of elementary excitations. We show that the gap induced by the charge order can drastically change the magnetic Raman spectrum, leading to the sharp peak observed in Sr14Cu24O41. This sharp Raman line gives insight into the charge-order periodicity and hence into the distribution of carriers. The much broader spectrum of La6Ca8Cu24O41 reflects the response of an undoped ladder in the absence of charge order.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Knetter C, Gruninger M, Uhrig GS},
doi = {10.1103/PhysRevLett.90.167201},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Charge}-order-induced sharp {Raman} peak in {Sr14Cu24O41}},
volume = {90},
year = {2003}
}
@article{faucris.310122176,
abstract = {We study the interplay between non-Hermitian dynamics and classical phase synchronization in a system of N bosonic modes commonly coupled to an auxiliary, driven mode. For any set of non-Hermitian bipartite interactions between the auxiliary and other modes, the system evolves towards a phase synchronized state. We provide analytical and numerical evidence of such classical phase synchronization for systems ranging from a few modes to the macroscopic limit of large N and analyze the effects of inhomogeneous frequency broadening and robustness under the action of external thermal noise.},
author = {Rohn, Jonas and Schmidt, Kai Phillip and Genes, Claudiu},
doi = {10.1103/PhysRevA.108.023721},
faupublication = {yes},
journal = {Physical Review A},
note = {CRIS-Team Scopus Importer:2023-09-08},
peerreviewed = {Yes},
title = {{Classical} phase synchronization in dissipative non-{Hermitian} coupled systems},
volume = {108},
year = {2023}
}
@article{faucris.271376319,
abstract = {We study the competition between two different topological orders in three dimensions by considering the X-cube model and the three-dimensional toric code. The corresponding Hamiltonian can be decomposed into two commuting parts, one of which displays a self-dual spectrum. To determine the phase diagram, we compute the high-order series expansions of the ground-state energy in all limiting cases. Apart from the topological order related to the toric code and the fractonic order related to the X-cube model, we found two new phases which are adiabatically connected to classical limits with nontrivial sub-extensive degeneracies. All phase transitions are found to be first order.},
author = {Mühlhauser, Matthias and Schmidt, Kai Phillip and Vidal, Julien and Walther, Matthias},
doi = {10.21468/SCIPOSTPHYS.12.2.069},
faupublication = {yes},
journal = {SciPost Physics},
note = {CRIS-Team Scopus Importer:2022-03-25},
peerreviewed = {Yes},
title = {{Competing} topological orders in three dimensions: {X}-cube versus toric code},
volume = {12},
year = {2022}
}
@article{faucris.229206398,
abstract = {Building on the growing evidence based on NMR, magnetization, neutron scattering, electron spin resonance, and specific heat that, under pressure, SrCu2(BO3)(2) has an intermediate phase between the dimer and the Neel phase, we study the competition between two candidate phases in the context of a minimal model that includes two types of intra- and interdimer interactions without enlarging the unit cell. We show that the empty plaquette phase of the Shastry-Sutherland model is quickly replaced by a quasi-one-dimensional full plaquette phase when intra- and/or interdimer couplings take different values, and that this full plaquette phase is in much better agreement with available experimental data than the empty plaquette one.},
author = {Boos, Carolin and Crone, S. P. G. and Niesen, I. A. and Corboz, P. and Schmidt, Kai Phillip and Mila, F.},
doi = {10.1103/PhysRevB.100.140413},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team WoS Importer:2019-11-15},
peerreviewed = {Yes},
title = {{Competition} between intermediate plaquette phases in {SrCu2}({BO3})(2) under pressure},
volume = {100},
year = {2019}
}
@article{faucris.123345024,
abstract = {The aim of this work is to study the physical properties of a one-way quantum computer in an effective low-energy cluster state. We calculate the optimal working conditions as a function of the temperature and of the system parameters. The central result of our work is that any effective cluster state implemented in a perturbative framework is fragile against special kinds of external perturbations. Qualitative aspects of our work are important for any implementation of effective low-energy models containing strong multisite interactions.},
author = {Klagges, Daniel and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevLett.108.230508},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Constraints} on {Measurement}-{Based} {Quantum} {Computation} in {Effective} {Cluster} {States}},
volume = {108},
year = {2012}
}
@article{faucris.312962394,
abstract = {We investigate the quantum-critical behavior between the rung-singlet phase with hidden string order and the Néel phase with broken SU(2)-symmetry in quantum spin ladders with algebraically decaying unfrustrated long-range Heisenberg interactions. To this end, we determine high-order series expansions of energies and observables in the thermodynamic limit about the isolated rung-dimer limit. This is achieved by extending the method of perturbative continuous unitary transformations (pCUT) to long-range Heisenberg interactions and to the calculation of generic observables. The quantumcritical breakdown of the rung-singlet phase then allows us to determine the critical phase transition line and the entire set of critical exponents as a function of the decay exponent of the long-range interaction. We demonstrate long-range mean-field behavior as well as a non-trivial regime of continuously varying critical exponents implying the absence of deconfined criticality contrary to a recent suggestion in the literature.},
author = {Adelhardt, Patrick and Schmidt, Kai Phillip},
doi = {10.21468/SciPostPhys.15.3.087},
faupublication = {yes},
journal = {SciPost Physics},
note = {CRIS-Team Scopus Importer:2023-10-20},
peerreviewed = {Yes},
title = {{Continuously} varying critical exponents in long-range quantum spin ladders},
volume = {15},
year = {2023}
}
@article{faucris.294860217,
abstract = {We apply continuous similarity transformations (CSTs) to the easy-axis antiferromagnetic XXZ model on the square lattice. The CST flow equations are truncated in momentum space by the scaling dimension d so that all contributions with d 2 are taken into account. The resulting quartic magnon-conserving effective Hamiltonian is analyzed in the zero-, one-, and two-magnon sector. In this way, a quantitative description of the ground-state energy, the one-magnon dispersion, and its gap as well as of two-magnon bound states is gained for anisotropies ranging from the gapped Ising model to the gapless Heisenberg model. We discuss the critical properties of the gap closing as well as the evolution of the one-magnon roton mininum. The excitation energies of two-magnon bound states are calculated, and their decay into the two-magnon continuum is determined via the inverse participation ratio.},
author = {Walther, Matthias and Hering, Dag-Bjoern and Uhrig, Goetz S. and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevResearch.5.013132},
faupublication = {yes},
journal = {Physical Review Research},
note = {CRIS-Team WoS Importer:2023-03-31},
peerreviewed = {Yes},
title = {{Continuous} similarity transformation for critical phenomena: {Easy}-axis antiferromagnetic {XXZ} model},
volume = {5},
year = {2023}
}
@article{faucris.120451144,
abstract = {We analyze the effect of local spin operators in the Kitaev model on the honeycomb lattice. We show, in perturbation around the isolated-dimer limit, that they create Abelian anyons together with fermionic excitations which are likely to play a role in experiments. We derive the explicit form of the operators creating and moving Abelian anyons without creating fermions and show that it involves multispin operations. Finally, the important experimental constraints stemming from our results are discussed.},
author = {Dusuel, Sebastien and Schmidt, Kai Phillip and Vidal, Julien},
doi = {10.1103/PhysRevLett.100.177204},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Creation} and manipulation of anyons in the {Kitaev} model},
volume = {100},
year = {2008}
}
@article{faucris.107595884,
abstract = {Quasiparticle properties of quantum magnets with long-range interactions are investigated by high-order linked-cluster expansions in the thermodynamic limit. It is established that perturbative continuous unitary transformations on white graphs are a promising and flexible approach to treat long-range interactions in quantum many-body systems. We exemplify this scheme for the one-dimensional transverse-field Ising chain with long-range interactions. For this model, the elementary quasiparticle gap is determined allowing to access the quantum-critical regime including critical exponents and multiplicative logarithmic corrections for the ferroand antiferromagnetic case.},
author = {Fey, Sebastian and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.94.075156},
faupublication = {yes},
journal = {Physical Review B},
peerreviewed = {unknown},
title = {{Critical} behavior of quantum magnets with long-range interactions in the thermodynamic limit},
volume = {94},
year = {2016}
}
@article{faucris.107843384,
abstract = {We study the transverse field Ising model on a kagome and a triangular lattice using high-order series expansions about the high-field limit. For the triangular lattice our results confirm a second-order quantum phase transition in the 3d XY universality class. Our findings for the kagome lattice indicate a notable instance of a disorder by disorder scenario in two dimensions. The latter follows from a combined analysis of the elementary gap in the high- and low-field limit which is shown to stay finite for all fields h. Furthermore, the lowest one-particle dispersion for the kagome lattice is extremely flat acquiring a dispersion only from order 8 in the 1/h limit. This behavior can be traced back to the existence of local modes and their breakdown which is understood intuitively via the linked cluster expansion. DOI: 10.1103/PhysRevB.87.054404},
author = {Powalski, M. and Coester, K. and Moessner, R. and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.87.054404},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {unknown},
title = {{Disorder} by disorder and flat bands in the kagome transverse field {Ising} model},
volume = {87},
year = {2013}
}
@article{faucris.226824334,
abstract = {We investigate the impact of quenched disorder on the zero-temperature dynamic structure factor of two leg quantum spin ladders. Using linked-cluster expansions and bond-operator mean-field theory, huge effects on individual quasiparticles but also on composite bound states arid two-quasiparticle continua are observed. This leads to intriguing quantum structures in dynamical conelation functions well observable in spectroscopic experiments.},
author = {Hörmann, Max and Wunderlich, Paul and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevLett.121.167201},
faupublication = {yes},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Dynamic} {Structure} {Factor} of {Disordered} {Quantum} {Spin} {Ladders}},
volume = {121},
year = {2018}
}
@article{faucris.243615108,
abstract = {We investigate the influence of quenched disorder on the dynamic structure factor of Heisenberg bilayers on the square, triangular, and kagome lattice in the quantum paramagnetic phase. Perturbative continuous unitary transformations and white graphs are employed to calculate the one-triplon contribution up to high orders in perturbation about the dimer limit for bimodal and continuous disorder. For the square lattice we find that the lifetime of the gap mode is increased by stronger quantum correlations, while stronger disorder effects are observed for the triangular lattice due to geometric frustration. For intradimer disorder, in-band energy gaps are observed for both lattices which can be understood in terms of a level repulsion on dimers with low and high intradimer exchange that are close in energy at the momentum where the in-band gap opens. For the highly frustrated kagome lattice disorder even decreases the gap energy. In addition, the localization length of the low-energy flat band is increased up to order 7 in perturbation theory. The interplay of quenched disorder, geometric frustration, and strong correlations leads therefore to rich structures in the dynamical structure factor of two-dimensional quantum magnets.},
author = {Hörmann, Max and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.102.094427},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team WoS Importer:2020-10-09},
peerreviewed = {Yes},
title = {{Dynamic} structure factor of {Heisenberg} bilayer dimer phases in the presence of quenched disorder and frustration},
volume = {102},
year = {2020}
}
@article{faucris.282414201,
abstract = {We investigate the dynamic structure factor of the antiferromagnetic Kitaev honeycomb model in a magnetic field by applying perturbative continuous unitary transformations about the high-field limit. One- and two-quasiparticle properties of the dressed elementary spin-flip excitations of the high-field polarized phase are calculated, which account for most of the spectral weight in the dynamic structure factor. We discuss the evolution of spectral features in these quasiparticle sectors in terms of one-quasiparticle dispersions, two-quasiparticle continua, the formation of antibound states, and quasiparticle decay. In particular, a comparably strong spectral feature above the upper edge of the upmost two-quasiparticle continuum represents three antibound states which form due to nearest-neighbor density-density interactions.},
author = {Schellenberger, Andreas and Hörmann, Max and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.106.104403},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team Scopus Importer:2022-09-30},
peerreviewed = {Yes},
title = {{Dynamic} structure factor of the antiferromagnetic {Kitaev} model in large magnetic fields},
volume = {106},
year = {2022}
}
@article{faucris.107641864,
abstract = {We present a robust scheme to derive effective models non-perturbatively for quantum lattice models when at least one degree of freedom is gapped. A combination of graph theory and the method of continuous unitary transformations (gCUTs) is shown to efficiently capture all zero-temperature fluctuations in a controlled spatial range. The gCUT can be used either for effective quasi-particle descriptions or for effective low-energy descriptions in case of infinitely degenerate subspaces. We illustrate the method for 1d and 2d lattice models yielding convincing results in the thermodynamic limit. We find that the recently discovered spin liquid in the Hubbard model on the honeycomb lattice lies outside the perturbative strong-coupling regime. Various extensions and perspectives of the gCUT are discussed. Copyright (C) EPLA, 2011},
author = {Yang, H. Y. and Schmidt, Kai Phillip},
doi = {10.1209/0295-5075/94/17004},
faupublication = {no},
journal = {EPL - Europhysics Letters},
peerreviewed = {Yes},
title = {{Effective} models for gapped phases of strongly correlated quantum lattice models},
volume = {94},
year = {2011}
}
@article{faucris.108535284,
abstract = {Motivated by the recent discovery of a spin-liquid phase for the Hubbard model on the honeycomb lattice at half-filling (Meng et al 2010 Nature 88 487), we apply both perturbative and non-perturbative techniques to derive effective spin Hamiltonians describing the low-energy physics of the Mottinsulating phase of the system. Exact diagonalizations of the so-derived models on small clusters are performed, in order to assess the quality of the effective low-energy theory in the spin-liquid regime. We show that six-spin interactions on the elementary loop of the honeycomb lattice are the dominant sub-leading effective couplings. A minimal spin model is shown to reproduce most of the energetic properties of the Hubbard model on the honeycomb lattice in its spin-liquid phase. Surprisingly, a more elaborate effective low-energy spin model obtained by a systematic graph expansion rather disagrees beyond a certain point with the numerical results for the Hubbard model at intermediate couplings.},
author = {Yang, Hong-Yu and Fabricio Albuquerque, A. and Capponi, Sylvain and Laeuchli, Andreas M. and Schmidt, Kai Phillip},
doi = {10.1088/1367-2630/14/11/115027},
faupublication = {no},
journal = {New Journal of Physics},
peerreviewed = {Yes},
title = {{Effective} spin couplings in the {Mott} insulator of the honeycomb lattice {Hubbard} model},
volume = {14},
year = {2012}
}
@article{faucris.108548044,
abstract = {We show that the spin-liquid phase of the half-filled Hubbard model on the triangular lattice can be described by a pure spin model. This is based on a high-order strong coupling expansion ( up to order 12) using perturbative continuous unitary transformations. The resulting spin model is consistent with a transition from three-sublattice long-range magnetic order to an insulating spin-liquid phase, and with a jump of the double occupancy at the transition. Exact diagonalizations of both models show that the effective spin model is quantitatively accurate well into the spin-liquid phase, and a comparison with the Gutzwiller projected Fermi sea suggests a gapless spectrum and a spinon Fermi surface.},
author = {Yang, Hong-Yu and Laeuchli, Andreas M. and Mila, Frederic and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevLett.105.267204},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Effective} {Spin} {Model} for the {Spin}-{Liquid} {Phase} of the {Hubbard} {Model} on the {Triangular} {Lattice}},
volume = {105},
year = {2010}
}
@article{faucris.258791811,
abstract = {We investigate bilayer graphene systems with layer switching domain walls separating the two energetically equivalent Bernal stackings in the presence of an external magnetic field. To this end, we calculate quantum transport and local densities of three microscopic models for a single domain wall: a hard wall, a defect due to shear, and a defect due to tension. The quantum transport calculations are performed with a recursive Green's function method. Technically, we discuss an explicit algorithm for the separation of a system into subsystems for the recursion and we present an optimization of the well known iteration scheme for lead self-energies for sparse chain couplings. We find strong physical differences for the three different types of domain walls in the integer quantum Hall regime. For a domain wall due to shearing of the upper graphene layer, there is a plateau formation in the magnetoconductance for sufficiently wide defect regions. For wide domain walls due to tension in the upper graphene layer, there is only an approximate plateau formation with fluctuations of the order of the elementary conductance quantum σ0. A direct transition between stacking regions like for the hard wall domain wall shows no plateau formation and is therefore not a good model for either of the previously mentioned extended domain walls.},
author = {Baßler, Nico and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.103.155422},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team Scopus Importer:2021-05-21},
peerreviewed = {Yes},
title = {{Effects} of domain walls in bilayer graphene in an external magnetic field},
volume = {103},
year = {2021}
}
@article{faucris.107597204,
abstract = {The classical Ising model on the frustrated three-dimensional (3D) swedenborgite lattice has disordered spin liquid ground states for all ratios of inter- and intraplanar couplings. Quantum fluctuations due to a transverse field give rise to several exotic phenomena. In the limit of weakly coupled kagome layers we find a 3D version of disorder by disorder degeneracy lifting. For large out-of-plane couplings one-dimensional macrospins are formed, which realize a disordered macrospin liquid phase on an emerging two-dimensional triangular lattice. We speculate about a possibly exotic version of quantum criticality that connects the polarized phase to the macrospin liquid.},
author = {Sikkenk, Tycho S. and Coester, Kris and Buhrandt, Stefan and Fritz, Lars and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.95.060401},
faupublication = {yes},
journal = {Physical Review B},
pages = {060401},
peerreviewed = {Yes},
title = {{Emergence} of a 2d macro-spin liquid in a highly frustrated 3d quantum magnet},
volume = {95},
year = {2017}
}
@article{faucris.108556624,
abstract = {We study the gapped phase of the Kitaev model on the honeycomb lattice using perturbative continuous unitary transformations. The effective low-energy Hamiltonian is found to be an extended toric code with interacting anyons. High-energy excitations are emerging free fermions which are composed of hard-core bosons with an attached string of spin operators. The excitation spectrum is mapped onto that of a single particle hopping on a square lattice in a magnetic field. We also illustrate how to compute correlation functions in this framework. The present approach yields analytical perturbative results in the thermodynamical limit without using the Majorana or the Jordan-Wigner fermionization initially proposed to solve this problem.},
author = {Schmidt, Kai Phillip and Dusuel, Sebastien and Vidal, Julien},
doi = {10.1103/PhysRevLett.100.057208},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Emergent} fermions and anyons in the {Kitaev} model},
volume = {100},
year = {2008}
}
@article{faucris.319718714,
abstract = {We investigate the potential to induce long-range spin interactions in a Mott insulator via the quantum electromagnetic field of a cavity. The coupling between light and spins is inherently nonlinear, and occurs via multiphoton processes like Raman scattering and two-photon absorption and emission with electronically excited intermediate states. Based on this, two pathways are elucidated: (i) In the absence of external driving, long-range interactions are mediated by the exchange of at least two virtual cavity photons. We show that these vacuum-mediated interactions can surpass local Heisenberg interactions in mesoscopic setups such as sufficiently small split-ring resonators. (ii) In a laser-driven cavity, interactions can be tailored through a hybrid scheme involving both external laser photons and cavity photons. This offers a versatile pathway for Floquet engineering of long-range interactions in macroscopic systems. In general, the derivation of these interactions requires careful consideration: Notably, we demonstrate that a simple phenomenological approach, based on a spin-photon Hamiltonian that captures Raman and two-photon processes with effective matrix elements, can be used only if the cavity is resonantly driven. Outside of these narrow resonant regimes as well as for the undriven case, a fourth-order series expansion within the underlying electronic model is necessary, which we perform to obtain long-range four-spin interactions in the half-filled Hubbard model. },
author = {Fadler, Paul and Schmidt, Kai Phillip and Li, Jiajun and Eckstein, Martin},
doi = {10.1103/PhysRevB.109.085149},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team Scopus Importer:2024-03-15},
peerreviewed = {Yes},
title = {{Engineering} photon-mediated long-range spin interactions in {Mott} insulators},
volume = {109},
year = {2024}
}
@article{faucris.122576124,
abstract = {Continuous unitary transformations are a powerful tool to extract valuable information out of quantum many-body Hamiltonians, in which the so-called flow equation transforms the Hamiltonian to a diagonal or block-diagonal form in second quantization. Yet, one of their main challenges is how to approximate the infinitely-many coupled differential equations that are produced throughout this flow. Here we show that tensor networks offer a natural and non-perturbative truncation scheme in terms of entanglement. The corresponding scheme is called "entanglement-CUT" or eCUT. It can be used to extract the low-energy physics of quantum many-body Hamiltonians, including quasiparticle energy gaps. We provide the general idea behind eCUT and explain its implementation for finite 1d systems using the formalism of matrix product operators. We also present proof-of-principle results for the spin-(1/2) 1d quantum Ising model and the 3-state quantum Potts model in a transverse field. Entanglement-CUTs can also be generalized to higher dimensions and to the thermodynamic limit. Copyright (C) EPLA, 2017},
author = {Sahin, Serkan and Schmidt, Kai Phillip and Orus, Roman},
doi = {10.1209/0295-5075/117/20002},
faupublication = {yes},
journal = {EPL - Europhysics Letters},
month = {Jan},
peerreviewed = {Yes},
title = {{Entanglement} continuous unitary transformations},
volume = {117},
year = {2017}
}
@article{faucris.276886569,
abstract = {There is a class of quantum phase transitions that do not fit into the traditional Landau paradigm, but are described in terms of fractionalized degrees of freedom and emergent gauge fields. Hong et al. find evidence of such a transition in a molecular spin-1/2 antiferromagnetic ladder compound under hydrostatic pressure.},
author = {Hong, Tao and Ying, Tao and Huang, Qing and Dissanayake, Sachith E. and Qiu, Yiming and Turnbull, Mark M. and Podlesnyak, Andrey A. and Wu, Yan and Cao, Huibo and Liu, Yaohua and Umehara, Izuru and Gouchi, Jun and Uwatoko, Yoshiya and Matsuda, Masaaki and Tennant, David A. and Chern, Gia-Wei and Schmidt, Kai Phillip and Wessel, Stefan},
doi = {10.1038/s41467-022-30769-8},
faupublication = {yes},
journal = {Nature Communications},
note = {CRIS-Team WoS Importer:2022-06-17},
peerreviewed = {Yes},
title = {{Evidence} for pressure induced unconventional quantum criticality in the coupled spin ladder antiferromagnet {C9H18N2CuBr4}},
volume = {13},
year = {2022}
}
@article{faucris.122202564,
abstract = {Exact ground states of a spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice with Heisenberg intradimer and Ising interdimer couplings are found by two independent rigorous procedures. The first method uses a unitary transformation to establish a mapping correspondence with an effective classical spin model, while the second method relies on the derivation of an effective hard-core boson model by continuous unitary transformations. Both methods lead to equivalent effective Hamiltonians providing a convincing proof that the spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice exhibits a zero-temperature magnetization curve with just two intermediate plateaus at one-third and one-half of the saturation magnetization, which correspond to stripe and checkerboard orderings of singlets and polarized triplets, respectively. The nature of the remarkable stripe order relevant to the one-third plateau is thoroughly investigated with the help of the corresponding exact eigenvector. The rigorous results for the spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice are compared with the analogous results for the purely classical Ising and fully quantum Heisenberg models. Finally, we discuss to what extent the critical fields of SrCu2(BO3)(2) and (CuCl)Ca2Nb3O10 can be described within the suggested Ising-Heisenberg model.},
author = {Verkholyak, Taras and Strecka, Jozef and Mila, Frederic and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.90.134413},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Exact} ground states of a spin-1/2 {Ising}-{Heisenberg} model on the {Shastry}-{Sutherland} lattice in a magnetic field},
volume = {90},
year = {2014}
}
@article{faucris.237944438,
abstract = {We introduce a quantum spin-1/2 model hosting two exact plaquette singlet ground states in extended parameter regimes. There is an exact phase transition between both phases, at which the system has an extensive ground-state degeneracy. Further, the model exhibits an extensive number of conserved quantities, which allow the prediction of additional phases. We exploit this feature and explore the phase diagram in detail for two specific parameter regimes. The model is based solely on Heisenberg interactions and seems sufficiently simple to be realized in quantum materials. A general scheme to determine exact singlet product states is briefly discussed.},
author = {Boos, Carolin and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.101.144428},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team WoS Importer:2020-05-02},
peerreviewed = {Yes},
title = {{Exact} plaquette singlet phases in an orthogonal-plaquette model},
volume = {101},
year = {2020}
}
@article{faucris.121085184,
abstract = {The transition from dimerized to uniform phases is studied in terms of spectral weights for spin chains using continuous unitary transformations. The spectral weights in the S=1 channel are computed perturbatively around the limit of strong dimerization. We find that the spectral weight is concentrated mainly in the subspaces with a small number of elementary triplets (triplons), even for vanishing dimerization. So, besides spinons, triplons may be used as elementary excitations in spin chains. We conclude that there is no necessity to use fractional excitations in low-dimensional, undoped, or doped quantum antiferromagnets.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Uhrig GS},
doi = {10.1103/PhysRevLett.90.227204},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Excitations} in one-dimensional {S} = 1/2 quantum antiferromagnets},
volume = {90},
year = {2003}
}
@article{faucris.109547284,
abstract = {We find unconventional Mott insulators in a quasi-2D version of the Shastry-Sutherland model in a magnetic field. In our realization on a four-leg tube geometry, these are stabilized by correlated hopping of localized magnetic excitations. Using perturbative continuous unitary transformations (pCUTs, plus classical approximation or exact diagonalization) and the density matrix renormalisation group method (DMRG), we identify prominent magnetization plateaus at magnetizations M = 1/8, 3/16, 1/4, and 1/2. While the plateau at M = 1/4 can be understood in a semiclassical fashion in terms of diagonal stripes, the plateau at M = 1/8 displays highly entangled wheels in the transverse direction of the tube. Finally, the M = 3/16 plateau is most likely to be viewed as a classical 1/8 structure on which additional triplets are fully delocalized around the tube. The classical approximation of the effective model fails to describe all these plateau structures which benefit from correlated hopping. We relate our findings to the full 2D system, which is the underlying model for the frustrated quantum magnet SrCu(BO3)(2).},
author = {Foltin, G. R. and Manmana, S. R. and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.90.104404},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Exotic} magnetization plateaus in a quasi-two-dimensional {Shastry}-{Sutherland} model},
volume = {90},
year = {2014}
}
@article{faucris.122195524,
abstract = {We describe a generic scheme to extract critical exponents of quantum lattice models from sequences of numerical data, which is, for example, relevant for nonperturbative linked-cluster expansions or nonperturbative variants of continuous unitary transformations. The fundamental idea behind our approach is a reformulation of the numerical data sequences as a series expansion in a pseudoparameter. This allows us to utilize standard series expansion extrapolation techniques to extract critical properties such as critical points and critical exponents. The approach is illustrated for the deconfinement transition of the antiferromagnetic spin-1/2 Heisenberg chain.},
author = {Coester, Kris and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevE.94.022101},
faupublication = {yes},
journal = {Physical Review E},
peerreviewed = {Yes},
title = {{Extracting} critical exponents for sequences of numerical data via series extrapolation techniques},
volume = {94},
year = {2016}
}
@article{faucris.122704164,
abstract = {We present a family of two-dimensional frustrated quantum magnets solely based on pure nearest-neighbor Heisenberg interactions which can be solved quasiexactly. All lattices are constructed in terms of frustrated quantum cages containing a chiral degree of freedom protected by frustration. The ground states of these models are dubbed ultimate quantum paramagnets and exhibit an extensive entropy at zero temperature. We discuss the unusual and extensively degenerate excitations in such phases. Implications for thermodynamic properties as well as for decoherence free quantum computation are discussed.},
author = {Schmidt, Kai Phillip and Laad, Mukul},
doi = {10.1103/PhysRevLett.104.237201},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Family} of {Exactly} {Solvable} {Models} with an {Ultimate} {Quantum} {Paramagnetic} {Ground} {State}},
volume = {104},
year = {2010}
}
@article{faucris.122524204,
abstract = {We consider noninteracting fermions on the honeycomb lattice in the presence of a magnetic vortex superlattice. It is shown that depending on the superlattice periodicity, a gap may open at zero energy. We derive an expression of the gap in the small-flux limit but the main qualitative features are found to be valid for arbitrary fluxes. This study provides an original example of a metal-insulator transition induced by a strongly modulated magnetic field in graphene. At the same time, our results directly apply to Kitaev's honeycomb model in a vortex superlattice.},
author = {Kamfor, Michael and Dusuel, Sebastien and Schmidt, Kai Phillip and Vidal, Julien},
doi = {10.1103/PhysRevB.84.153404},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Fate} of {Dirac} points in a vortex superlattice},
volume = {84},
year = {2011}
}
@article{faucris.108558604,
abstract = {The key feature of an orbital wave or orbiton is a significant dispersion, which arises from exchange interactions between orbitals on distinct sites. We study the effect of a coupling between orbitons and phonons in one dimension using continuous unitary transformations. Already for intermediate values of the coupling, the orbiton band width is strongly reduced and the spectral density is dominated by an orbiton-phonon continuum. However, we find sharp features within the continuum and an orbiton-phonon antibound state above. Both show a significant dispersion and should be observable experimentally.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Gruninger M, Uhrig GS},
doi = {10.1103/PhysRevB.76.075108},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Fate} of orbitons coupled to phonons},
volume = {76},
year = {2007}
}
@article{faucris.108535504,
abstract = {The cluster state represents a highly entangled state which is one central object for measurement-based quantum computing. Here we study the robustness of the cluster state on the two-dimensional square lattice at zero temperature in the presence of external magnetic fields by means of different types of high-order series expansions and variational techniques using infinite projected entangled pair states. The phase diagram displays a first-order phase transition line ending in two critical end points. Furthermore, it contains a characteristic self-dual line in parameter space allowing many precise statements. The self-duality is shown to exist on any lattice topology.},
author = {Kalis, Henning and Klagges, Daniel and Orus, Roman and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevA.86.022317},
faupublication = {no},
journal = {Physical Review A},
peerreviewed = {Yes},
title = {{Fate} of the cluster state on the square lattice in a magnetic field},
volume = {86},
year = {2012}
}
@article{faucris.121779284,
abstract = {The notion of a quasiparticle, such as a phonon, a roton or a magnon, is used in modern condensed matter physics to describe an elementary collective excitation. The intrinsic zero-temperature magnon damping in quantum spin systems can be driven by the interaction of the one-magnon states and multi-magnon continuum. However, detailed experimental studies on this quantum many-body effect induced by an applied magnetic field are rare. Here we present a high-resolution neutron scattering study in high fields on an S = 1/2 antiferromagnet C9H18N2CuBr4. Compared with the non-interacting linear spin-wave theory, our results demonstrate a variety of phenomena including field-induced renormalization of one-magnon dispersion, spontaneous magnon decay observed via intrinsic linewidth broadening, unusual non-Lorentzian two-peak structure in the excitation spectra and a dramatic shift of spectral weight from one-magnon state to the two-magnon continuum.},
author = {Hong, Tao and Qiu, Y. and Matsumoto, M. and Tennant, D. A. and Coester, K. and Schmidt, Kai Phillip and Awwadi, F. F. and Turnbull, M. M. and Agrawal, H. and Chernyshev, A. L.},
doi = {10.1038/ncomms15148},
faupublication = {yes},
journal = {Nature Communications},
peerreviewed = {Yes},
title = {{Field} induced spontaneous quasiparticle decay and renormalization of quasiparticle dispersion in a quantum antiferromagnet},
volume = {8},
year = {2017}
}
@article{faucris.108548484,
abstract = {We study the magnetic-field-induced quantum phase transition from a gapped quantum phase that has no magnetic long-range order into a gapless phase in the spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY). At temperatures below about 1 K, the specific heat in the gapless phase attains an asymptotic linear temperature dependence, characteristic of a Tomonaga-Luttinger liquid. Inelastic neutron scattering and the specific heat measurements in both phases are in good agreement with theoretical calculations, demonstrating that DIMPY is the first model material for an S = 1/2 two-leg spin ladder in the strong-leg regime.},
author = {Hong, Tao and Kim, Y. H. and Hotta, C. and Takano, Y. and Tremelling, G. and Turnbull, M. M. and Landee, C. P. and Kang, H. -J. and Christensen, N. B. and Lefmann, Kim and Schmidt, Kai Phillip and Uhrig, G. S. and Broholm, Collin},
doi = {10.1103/PhysRevLett.105.137207},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Field}-{Induced} {Tomonaga}-{Luttinger} {Liquid} {Phase} of a {Two}-{Leg} {Spin}-1/2 {Ladder} with {Strong} {Leg} {Interactions}},
volume = {105},
year = {2010}
}
@article{faucris.238901087,
abstract = {We develop a strategy for tensor network algorithms that allows to deal very efficiently with lattices of high connectivity. The basic idea is to fine grain the physical degrees of freedom, i.e., decompose them into more fundamental units which, after a suitable coarse graining, provide the original ones. Thanks to this procedure, the original lattice with high connectivity is transformed by an isometry into a simpler structure, which is easier to simulate via usual tensor network methods. In particular this enables the use of standard schemes to contract infinite 2D tensor networks-such as corner transfer matrix renormalization schemes-which are more involved on complex lattice structures. We prove the validity of our approach by numerically computing the ground-state properties of the ferromagnetic spin-1 transverse-field Ising model on the 2D triangular and 3D stacked triangular lattice, as well as of the hardcore and softcore Bose-Hubbard models on the triangular lattice. Our results are benchmarked against those obtained with other techniques, such as perturbative continuous unitary transformations and graph projected entangled pair states, showing excellent agreement and also improved performance in several regimes.},
author = {Schmoll, Philipp and Jahromi, Saeed S. and Hörmann, Max and Schmidt, Kai Phillip and Orus, Roman},
doi = {10.1103/PhysRevLett.124.200603},
faupublication = {yes},
journal = {Physical Review Letters},
note = {CRIS-Team WoS Importer:2020-06-02},
peerreviewed = {Yes},
title = {{Fine} {Grained} {Tensor} {Network} {Methods}},
volume = {124},
year = {2020}
}
@article{faucris.108751984,
abstract = {Spectral densities are computed in unprecedented detail for quantum anti ferromagnetic spin 1/2 two-leg ladders. These results were obtained due to a major methodical advance achieved by optimally chosen unitary transformations. The approach is based on dressed integer excitations. Considerable weight is found at high energies in the two-particle sector. Precursors of fractional spinon physics occur supporting the conclusion that there is no necessity to resort to fractional excitations in order to describe features at higher energies.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Knetter C, Schmidt KP, Gruninger M, Uhrig GS},
faupublication = {no},
journal = {Physical Review Letters},
pages = {art. no.-167204},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Fractional} and integer excitations in quantum antiferromagnetic spin 1/2 ladders},
volume = {87},
year = {2001}
}
@article{faucris.108558824,
abstract = {We propose a versatile approach to treat commonly arising constraints. It is illustrated for interacting magnons of the Heisenberg antiferromagnet on a square lattice. For systems of LxL sites a nonperturbative continuous unitary transformation (CUT) is used to derive an effective Hamiltonian conserving the number of magnons. They are bosonic particles with a hard-core constraint which is captured by a local, repulsive interaction U. The limits U ->infinity and L ->infinity are achieved by extrapolation. The residual spin gap Delta(1) is smaller than 0.01J reflecting the gapless nature of the magnons. The one-magnon dispersion displays all known characteristics.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Uhrig GS},
doi = {10.1103/PhysRevB.73.172407},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Hard}-core magnons in the {S}=1/2 {Heisenberg} model on the square lattice},
volume = {73},
year = {2006}
}
@article{faucris.226824864,
abstract = {Spontaneous symmetry-breaking quantum phase transitions play an essential role in condensed-matter physics(1-3). The collective excitations in the broken-symmetry phase near the quantum critical point can be characterized by fluctuations of phase and amplitude of the order parameter. The phase oscillations correspond to the massless Nambu-Goldstone modes whereas the massive amplitude mode, analogous to the Higgs boson in particle physics(4,5), is prone to decay into a pair of low-energy Nambu-Goldstone modes in low dimensions(2,6,7). Especially, observation of a Higgs amplitude mode in two dimensions is an outstanding experimental challenge. Here, using inelastic neutron scattering and applying the bondoperator theory, we directly and unambiguously identify the Higgs amplitude mode in a two-dimensional S = 1/2 quantum antiferromagnet C9H18N2CuBr4 near a quantum critical point in two dimensions. Owing to an anisotropic energy gap, it kinematically prevents such decay and the Higgs amplitude mode acquires an infinite lifetime.},
author = {Hong, Tao and Matsumoto, Masashige and Qiu, Yiming and Chen, Wangchun and Gentile, Thomas R. and Watson, Shannon and Awwadi, Firas F. and Turnbull, Mark M. and Dissanayake, Sachith E. and Agrawal, Harish and Toft-Petersen, Rasmus and Klemke, Bastian and Coester, Kris and Schmidt, Kai Phillip and Tennant, David A.},
doi = {10.1038/NPHYS4182},
faupublication = {yes},
journal = {Nature},
pages = {638-+},
peerreviewed = {Yes},
title = {{Higgs} amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point},
volume = {13},
year = {2017}
}
@article{faucris.216811912,
abstract = {Polarized inelastic neutron scattering experiments recently identified the amplitude (Higgs) mode in C9H18N2CuBr4, a two-dimensional near-quantum-critical spin-1/2 two-leg ladder compound, which exhibits a weak easy-axis exchange anisotropy. Here, we theoretically examine the dynamic spin structure factor of such planar coupled spin-ladder systems using large-scale quantum Monte Carlo simulations. This allows us to provide a quantitative account of the experimental neutron scattering data within a consistent quantum spin model. Moreover, we trace the details of the continuous evolution of the amplitude mode from a two-particle bound state of coupled ladders in the classical Ising limit all the way to the quantum spin-1/2 Heisenberg limit with fully restored SU(2) symmetry, where it gets overdamped by the two-magnon continuum in neutron scattering.},
author = {Ying, T. and Schmidt, Kai Phillip and Wessel, S.},
doi = {10.1103/PhysRevLett.122.127201},
faupublication = {yes},
journal = {Physical Review Letters},
note = {CRIS-Team Scopus Importer:2019-05-02},
peerreviewed = {Yes},
title = {{Higgs} {Mode} of {Planar} {Coupled} {Spin} {Ladders} and its {Observation} in {C9} {H18} {N2CuBr4}},
volume = {122},
year = {2019}
}
@article{faucris.215881565,
abstract = {Polarized inelastic neutron scattering experiments recently identified the amplitude (Higgs) mode in C9H18N2CuBr4, a two-dimensional near-quantum-critical spin-1/2 two-leg ladder compound, which exhibits a weak easy-axis exchange anisotropy. Here, we theoretically examine the dynamic spin structure factor of such planar coupled spin-ladder systems using large-scale quantum Monte Carlo simulations. This allows us to provide a quantitative account of the experimental neutron scattering data within a consistent quantum spin model. Moreover, we trace the details of the continuous evolution of the amplitude mode from a two-particle bound state of coupled ladders in the classical Ising limit all the way to the quantum spin-1/2 Heisenberg limit with fully restored SU(2) symmetry, where it gets overdamped by the two-magnon continuum in neutron scattering.},
author = {Ying, T. and Schmidt, Kai Phillip and Wessel, S.},
doi = {10.1103/PhysRevLett.122.127201},
faupublication = {yes},
journal = {Physical Review Letters},
note = {CRIS-Team WoS Importer:2019-04-12},
peerreviewed = {Yes},
title = {{Higgs} {Mode} of {Planar} {Coupled} {Spin} {Ladders} and its {Observation} in {C9H18N2CuBr4}},
volume = {122},
year = {2019}
}
@article{faucris.107999584,
abstract = {We present a high order perturbation approach to quantitatively calculate spectral densities in three distinct steps starting from the model Hamiltonian and the observables of interest. The approach is based on the perturbative continuous unitary transformation introduced previously. It is conceived to work particularly well in models allowing a clear identification of the elementary excitations above the ground state. These are then viewed as quasi-particles above the vacuum. The article focuses on the technical aspects and includes a discussion of series extrapolation schemes. The strength of the method is demonstrated for S = 1/2 two-leg Heisenberg ladders, for which results are presented.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Knetter C, Schmidt KP, Uhrig GS},
doi = {10.1140/epjb/e2004-00008-2},
faupublication = {no},
journal = {European Physical Journal B},
pages = {525-544},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{High} order perturbation theory for spectral densities of multi-particle excitations: {S}=1/2 two-leg {Heisenberg} ladder},
volume = {36},
year = {2003}
}
@article{faucris.267253883,
abstract = {We investigate the low-energy physics of non-Hermitian quantum spin models with PT symmetry. To this end, we consider the one-dimensional Ising chain and the two-dimensional toric code in a non-Hermitian staggered field. For both systems, dual descriptions in terms of non-Hermitian staggered Ising interactions in a conventional transverse field exist. We perform high-order series expansions about the high- and low-field limit for both systems to determine the ground-state energy per site and the one-particle gap. The one-dimensional non-Hermitian Ising chain is known to be exactly solvable. Its ground-state phase diagram consists of second-order quantum phase transitions, which can be characterized by logarithmic singularities of the second derivative of the ground-state energy and, in the symmetry-broken phase, the gap closing of the low-field gap. In contrast, the gap closing from the high-field phase is not accessible perturbatively due to the complex energy and the occurrence of exceptional lines in the high-field gap expression. For the two-dimensional toric code in a non-Hermitian staggered field, we study the quantum robustness of the topologically ordered phase by the gap closing of the low-field gap. We find that the well-known second-order quantum phase transition of the toric code in a uniform field extends into a large portion of the non-Hermitian parameter space. However, the series expansions become unreliable for a dominant anti-Hermitian field. Interestingly, the analysis of the high-field gap reveals the potential presence of an intermediate region.},
author = {Lenke, Lea and Mühlhauser, Matthias and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.104.195137},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team Scopus Importer:2021-12-17},
peerreviewed = {Yes},
title = {{High}-order series expansion of non-{Hermitian} quantum spin models},
volume = {104},
year = {2021}
}
@article{faucris.122203444,
abstract = {We consider the string-net model on the honeycomb lattice for Ising anyons in the presence of a string tension. This competing term induces a nontrivial dynamics of the non-Abelian anyonic quasiparticles and may lead to a breakdown of the topological phase. Using high-order series expansions and exact diagonalizations, we determine the robustness of this doubled Ising phase, which is found to be separated from two gapped phases. An effective quantum dimer model emerges in the large tension limit, giving rise to two different translation symmetry-broken phases. Consequently, we obtain four transition points, two of which are associated with first-order transitions whereas the two others are found to be continuous and provide examples of recently proposed Bose condensation for anyons.},
author = {Schulz, Marc Daniel and Dusuel, Sebastien and Misguich, Gregoire and Schmidt, Kai Phillip and Vidal, Julien},
doi = {10.1103/PhysRevB.89.201103},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Ising} anyons with a string tension},
volume = {89},
year = {2014}
}
@article{faucris.248100243,
abstract = {We investigate the influence of light-matter interactions on correlated quantum matter by studying the paradigmatic Dicke-Ising model. This type of coupling to a confined, spatially delocalized bosonic light mode, such as provided by an optical resonator, resembles a quantized transverse magnetic field of tunable strength. As a consequence, the symmetry-broken magnetic state breaks down for strong enough light-matter interactions to a paramagnetic state. The nonlocal character of the bosonic mode can change the quantum phase transition in a drastic manner, which we analyze quantitatively for the simplest case of the Dicke-Ising chain geometry. The results show a direct transition between a magnetically ordered phase with zero photon density and a magnetically polarized phase with superradiant behavior of the light. Our predictions are equally valid for the dual quantized Ising chain in a conventional transverse magnetic field.},
author = {Rohn, Jonas and Hörmann, Max and Genes, Claudiu and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevResearch.2.023131},
faupublication = {yes},
journal = {Physical Review Research},
note = {CRIS-Team WoS Importer:2021-01-22},
peerreviewed = {Yes},
title = {{Ising} model in a light-induced quantized transverse field},
volume = {2},
year = {2020}
}
@article{faucris.108548704,
abstract = {We consider an extension of the Kitaev honeycomb model based on arbitrary dimer coverings satisfying matching rules. We focus on three different dimer coverings having the smallest unit cells for which we calculate the ground-state phase diagram. We also study one- and two-vortex properties for these coverings in the Abelian phases and show that vortex-vortex interactions can be attractive or repulsive. These qualitative differences are confirmed analytically by high-order perturbative expansions around the isolated-dimer limit. Similarities and differences with the original Kitaev honeycomb model are discussed.},
author = {Kamfor, Michael and Dusuel, Sebastien and Vidal, Julien and Schmidt, Kai Phillip},
doi = {10.1088/1742-5468/2010/08/P08010},
faupublication = {no},
journal = {Journal of Statistical Mechanics-Theory and Experiment},
keywords = {solvable lattice models;frustrated systems (theory);spin liquids (theory);quantum phase transitions (theory)},
peerreviewed = {Yes},
title = {{Kitaev} model and dimer coverings on the honeycomb lattice},
year = {2010}
}
@article{faucris.289678291,
abstract = {We describe applications of two-dimensional subwavelength quantum emitter arrays as efficient optical elements in the linear regime. For normally incident light, the cooperative optical response, stemming from emitter-emitter dipole exchanges, allows the control of the array's transmission, its resonance frequency, and bandwidth. Operations on fully polarized incident light, such as generic linear and circular polarizers as well as phase retarders can be engineered and described in terms of Jones matrices. Our analytical approach and accompanying numerical simulations identify optimal regimes for such operations and reveal the importance of adjusting the array geometry and of the careful tuning of the external magnetic fields amplitude and direction.},
author = {Baßler, Nico and Reitz, Michael and Schmidt, Kai Phillip and Genes, Claudiu},
doi = {10.1364/OE.476830},
faupublication = {yes},
journal = {Optics Express},
note = {CRIS-Team Scopus Importer:2023-02-24},
pages = {6003-6026},
peerreviewed = {Yes},
title = {{Linear} optical elements based on cooperative subwavelength emitter arrays},
volume = {31},
year = {2023}
}
@article{faucris.109545304,
abstract = {For arbitrary space dimension d, we investigate the quantum phase transitions of two paradigmatic spin models defined on a hypercubic lattice, the coupled-dimer Heisenberg model and the transverse-field Ising model. To this end, high-order linked-cluster expansions for the ground-state energy and the one-particle gap are performed up to order 9 about the decoupled-dimer and high-field limits, respectively. Extrapolations of the high-order series yield the location of the quantum phase transition and the correlation-length exponent. as a function of space dimension d. The results are complemented by 1/d expansions to next-to-leading order of observables across the phase diagrams. Remarkably, our analysis of the extrapolated linked-cluster expansion allows to extract the coefficients of the full 1/d expansion for the phase-boundary location in both models exactly in leading order and quantitatively for subleading corrections.},
author = {Coester, Kris and Joshi, Darshan G. and Vojta, Matthias and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.94.125109},
faupublication = {yes},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Linked}-cluster expansions for quantum magnets on the hypercubic lattice},
volume = {94},
year = {2016}
}
@article{faucris.277347811,
abstract = {We propose a hypergraph expansion which facilitates the direct treatment of quantum spin models with many-site interactions via perturbative linked cluster expansions. The main idea is to generate all relevant subclusters and sort them into equivalence classes essentially governed by hypergraph isomorphism. Concretely, a reduced K??nig representation of the hypergraphs is used to make the equivalence relation accessible by graph isomorphism. During this procedure we determine the embedding factor for each equivalence class, which is used in the final resummation in order to obtain the final result. As an instructive example we calculate the ground-state energy and a particular excitation gap of the plaquette Ising model in a transverse field on the three-dimensional cubic lattice.},
author = {Mühlhauser, Matthias and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevE.105.064110},
faupublication = {yes},
journal = {Physical Review E},
note = {CRIS-Team WoS Importer:2022-07-01},
peerreviewed = {Yes},
title = {{Linked} cluster expansions via hypergraph decompositions},
volume = {105},
year = {2022}
}
@article{faucris.108551124,
abstract = {We determine analytically the phase diagram of the toric code model in a parallel magnetic field which displays three distinct regions. Our study relies on two high-order perturbative expansions in the strong- and weak-field limits, as well as a large-spin analysis. Calculations in the topological phase establish a quasiparticle picture for the anyonic excitations. We obtain two second-order transition lines that merge with a first-order line, giving rise to a multicritical point as recently suggested by numerical simulations. We compute the values of the corresponding critical fields and exponents that drive the closure of the gap. We also give the one-particle dispersions of the anyonic quasiparticles inside the topological phase.},
author = {Vidal, Julien and Dusuel, Sebastien and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.79.033109},
faupublication = {no},
journal = {Physical Review B},
keywords = {magnetic fields;quantum theory},
month = {Jan},
peerreviewed = {Yes},
title = {{Low}-energy effective theory of the toric code model in a parallel magnetic field},
volume = {79},
year = {2009}
}
@article{faucris.226825122,
abstract = {We study topological properties of one-triplon bands in an extended Shastry-Sutherland model relevant for the frustrated quantum magnet SrCu2(BO3)(2). To this end perturbative continuous unitary transformations are applied about the isolated dimer limit allowing us to calculate the one-triplon dispersion up to high order in various couplings including intra-and interdimer Dzyaloshinskii-Moriya interactions and a general uniform magnetic field. We determine the Berry curvature and the Chern number of the different one-triplon bands. We demonstrate the occurrence of Chern numbers +/- 1 and +/- 2 for the case that two components of the magnetic field are finite. Finally, we also calculate the triplon Hall effect arising at finite temperatures.},
author = {Malki, Maik and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.95.195137},
faupublication = {yes},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Magnetic} {Chern} bands and triplon {Hall} effect in an extended {Shastry}-{Sutherland} model},
volume = {95},
year = {2017}
}
@article{faucris.108641544,
abstract = {The universal behaviour of the magnetic excitations in high-temperature superconductors is described in a model with static stripes retaining only the localized spin degrees of freedom. The stripes are represented by a model of coupled two-leg spin ladders. We start from the results obtained previously by continuous unitary transformations for an isolated spin ladder. A quantitative description of neutron scattering data is reached, using a model for a single cuprate layer with well established values of the exchange coupling constants. The neutron resonance peak is explained in terms of a saddle point in the dispersion of the magnetic excitations. Here we make predictions for bilayer systems with in-phase or out-of-phase stripe correlations. The results may serve as a guide for future experimental analyses.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Uhrig GS, Schmidt KP, Gruninger M},
faupublication = {no},
journal = {Journal of the Physical Society of Japan},
keywords = {magnetic excitations;spectral densities;stripe phase;inelastic neutron scattering;high-temperature superconductors},
month = {Jan},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Magnetic} {Excitations} in {Bilayer} {High}-{Temperature} {Superconductors} with {Stripe} {Correlations}},
volume = {74},
year = {2005}
}
@inproceedings{faucris.108642204,
abstract = {The magnetic excitations in the stripe phase of high-T-c superconductors are investigated in a model of spin ladders which are effectively coupled via charged stripes. Starting from the effective single-triplon model for the isolated spin ladder, the quasi-one-dimensional spin system can be described straightforwardly. Very good agreement is obtained with recent neutron scattering data on La15/8Ba1/8CuO4 (no spin gap) and YBa2Cu3O6.6 (gapped). The signature of quasi-one-dimensional spin physics in a single-domain stripe phase is predicted. (c) 2004 Elsevier B.V. All rights reserved.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Uhrig GS, Schmidt KP, Gruninger M},
doi = {10.1016/j.jmmm.2004.11.230},
faupublication = {no},
keywords = {superconductors;high T-c;neutron scattering;inelastic;antiferromagnetism;quantum;spin dynamics},
pages = {330-333},
peerreviewed = {Yes},
publisher = {ELSEVIER SCIENCE BV},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Magnetic} excitations in the stripe phase of high-{T}-c superconductors},
volume = {290},
year = {2005}
}
@inproceedings{faucris.108338604,
abstract = {Magnetic excitations in two-leg S = 1/2 ladders are studied both experimentally and theoretically. Experimentally, we report on the reflectivity, the transmittance and the optical conductivity sigma(omega) of undoped LaxCa14-xCu24O41 for x = 4, 5, and 5.2. Using two different theoretical approaches (Jordan-Wigner fermions and perturbation theory), we calculate the dispersion of the elementary triplets, the optical conductivity and the momentum-resolved spectral density of two-triplet excitations for 0.2 less than or equal to J(\\)/J(perpendicular to) less than or equal to 1.2. We discuss phonon-assisted two-triplet absorption, the existence of two-triplet bound states, the two-triplet continuum, and the size of the exchange parameters. (C) 2002 Elsevier Science Ltd. All rights reserved.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Gruninger M, Windt M, Nunner T, Knetter C, Schmidt KP, Uhrig GS, Kopp T, Freimuth A, Ammerahl U, Buchner B, Revcolevschi A},
faupublication = {no},
keywords = {infrared spectroscopy;magnetic properties},
pages = {2167-2173},
peerreviewed = {Yes},
publisher = {PERGAMON-ELSEVIER SCIENCE LTD},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Magnetic} excitations in two-leg spin 1/2 ladders: experiment and theory},
volume = {63},
year = {2002}
}
@article{faucris.121535744,
abstract = {We present specific-heat and neutron-scattering results for the S = 1/2 quantum antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)CuBr4. The material orders magnetically at T-N = 1.99(2) K, and magnetic excitations are accompanied by an energy gap of 0.30(2) meV due to spin anisotropy. The system is best described as coupled two-leg spin-1/2 ladders with the leg exchange J(leg) = 0.60(2) meV, rung exchange J(rung) = 0.64(9) meV, interladder exchange J(int) = 0.19(2) meV, and an interaction-anisotropy parameter lambda = 0.93(2), according to inelastic neutron-scattering measurements. In contrast to most spin ladders reported to date, the material is a rare example in which the interladder coupling is very near the critical value required to drive the system to a Neel-ordered phase without the assistance of a magnetic field.},
author = {Hong, Tao and Schmidt, Kai Phillip and Coester, K. and Awwadi, F. F. and Turnbull, M. M. and Qiu, Y. and Rodriguez-Rivera, J. A. and Zhu, M. and Ke, X. and Aoyama, C. P. and Takano, Y. and Cao, Huibo and Tian, Wei and Ma, Jie and Custelcean, Radu and Zhou, H. D. and Matsuda, Masaaki},
doi = {10.1103/PhysRevB.89.174432},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Magnetic} ordering induced by interladder coupling in the spin-1/2 {Heisenberg} two-leg ladder antiferromagnet {C9H18N2CuBr4}},
volume = {89},
year = {2014}
}
@article{faucris.107843824,
abstract = {The magnetization process of the orthogonal-dimer antiferromagnet SrCu2(BO3)(2) is investigated in high magnetic fields of up to 118 T. A 1/2 plateau is clearly observed in the field range 84 to 108 T in addition to 1/8, 1/4, and 1/3 plateaus at lower fields. Using a combination of state-of-the-art numerical simulations, the main features of the high-field magnetization, a 1/2 plateau of width 24 T, a 1/3 plateau of width 34 T, and no 2/5 plateau, are shown to agree quantitatively with the Shastry-Sutherland model if the ratio of inter-to intradimer exchange interactions J'/J = 0.63. It is further predicted that the intermediate phase between the 1/3 and 1/2 plateaus is not uniform but consists of a 1/3 supersolid followed by a 2/5 supersolid and possibly a domain-wall phase, with a reentrance into the 1/3 supersolid above the 1/2 plateau.},
author = {Matsuda, Y. H. and Abe, N. and Takeyama, S. and Kageyama, Hiroshi and Corboz, P. and Honecker, Andreas and Manmana, S. R. and Foltin, G. R. and Schmidt, Kai Phillip and Mila, F.},
doi = {10.1103/PhysRevLett.111.137204},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Magnetization} of {SrCu2}({BO3})(2) in {Ultrahigh} {Magnetic} {Fields} up to 118 {T}},
volume = {111},
year = {2013}
}
@inproceedings{faucris.122620564,
abstract = {We study an extended two-dimensional Shastry-Sutherland model in a magnetic field where besides the usual Heisenberg exchanges of the Shastry-Sutherland model two additional SU(2) invariant couplings are included. Perturbative continous unitary transformations are used to determine the leading order effects of the additional couplings on the pure hopping and on the long-range interactions between the triplons which are the most relevant terms for small magnetization. We then compare the energy of various magnetization plateaux in the classical limit and we discuss the implications for the two-dimensional quantum magnet SrCu(2)(BO(3))(2).},
author = {Schmidt, Kai Phillip and Dorier, Julien and Mila, Frederic},
doi = {10.1088/1742-6596/145/1/012047},
faupublication = {no},
month = {Jan},
peerreviewed = {Yes},
title = {{Magnetization} plateaux in an extended {Shastry}-{Sutherland} model},
volume = {145},
year = {2009}
}
@article{faucris.112950244,
abstract = {Using perturbative expansions and the contractor renormalization (CORE) algorithm, we obtain effective hard-core bosonic Hamiltonians describing the low-energy physics of S=1/2 spin-dimer antiferromagnets known to display supersolid phases under an applied magnetic field. The resulting effective models are investigated by means of mean-field analysis and quantum Monte Carlo simulations. A "leapfrog mechanism," through means of which extra singlets delocalize in a checkerboard-solid environment via correlated hoppings, is unveiled that accounts for the supersolid behavior.},
author = {Picon, J. -D. and Albuquerque, A. F. and Schmidt, Kai Phillip and Laflorencie, Nicolas and Troyer, Matthias and Mila, F.},
doi = {10.1103/PhysRevB.78.184418},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Mechanisms} for spin supersolidity in {S}=1/2 spin-dimer antiferromagnets},
volume = {78},
year = {2008}
}
@article{faucris.318014138,
abstract = {Quantum metasurfaces, i.e., two-dimensional subwavelength arrays of quantum emitters, can be employed as mirrors towards the design of hybrid cavities, where the optical response is given by the interplay of a cavity-confined field and the surface modes supported by the arrays. We show that stacked layers of quantum metasurfaces with orthogonal dipole orientation can serve as helicity-preserving cavities. These structures exhibit ultranarrow resonances and can enhance the intensity of the incoming field by orders of magnitude, while simultaneously preserving the handedness of the field circulating inside the resonator, as opposed to conventional cavities. The rapid phase shift in the cavity transmission around the resonance can be exploited for the sensitive detection of chiral scatterers passing through the cavity. We discuss possible applications of these resonators as sensors for the discrimination of chiral molecules. Our approach describes a new way of chiral sensing via the measurement of particle-induced phase shifts. },
author = {Baßler, Nico S. and Aiello, Andrea and Schmidt, Kai Phillip and Genes, Claudiu and Reitz, Michael},
doi = {10.1103/PhysRevLett.132.043602},
faupublication = {yes},
journal = {Physical Review Letters},
month = {Jan},
note = {CRIS-Team Scopus Importer:2024-02-09},
peerreviewed = {Yes},
title = {{Metasurface}-{Based} {Hybrid} {Optical} {Cavities} for {Chiral} {Sensing}},
volume = {132},
year = {2024}
}
@article{faucris.111591964,
abstract = {The frustrated quantum magnet SrCu2(BO3)(2) shows a remarkably rich phase diagram in an external magnetic field including a sequence of magnetization plateaux. The by far experimentally most studied and most prominent magnetization plateau is the 1/8 plateau. Theoretically, one expects that this material is well described by the Shastry-Sutherland model. But, recent microscopic calculations indicate that the 1/8 plateau is energetically not favored. Here, we report on a very simple microscopic mechanism which naturally leads to a 1/8 plateau for realistic values of the magnetic exchange constants. We show that the 1/8 plateau with a square unit cell benefits most compared to other plateau structures from quantum fluctuations which, to a large part, are induced by Dzyaloshinskii-Moriya interactions. Physically, such couplings result in kinetic terms in an effective hard-core-boson description leading to a renormalization of the energy of the different plateau structures which we treat in this work on the mean-field level. The stability of the resulting plateaux are discussed. Furthermore, our results indicate a series of stripe structures above 1/8 and a stable magnetization plateau at 1/6. Most qualitative aspects of our microscopic theory agree well with a recently formulated phenomenological theory for the experimental data of SrCu2(BO3)(2). Interestingly, our calculations point to a rather large ratio of the magnetic couplings in the Shastry-Sutherland model such that nonperturbative effects become essential for the understanding of the frustrated quantum magnet SrCu2(BO3)(2).},
author = {Nemec, M. and Foltin, G. R. and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.86.174425},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Microscopic} mechanism for the 1/8 magnetization plateau in {SrCu2}({BO3})(2)},
volume = {86},
year = {2012}
}
@article{faucris.306930501,
abstract = {Studying multi-particle elementary excitations has provided unique access to understand collective many-body phenomena in correlated electronic materials, paving the way towards constructing microscopic models. In this work, we perform O K-edge resonant inelastic X-ray scattering (RIXS) on the quasi-one-dimensional cuprate Sr 14Cu 24O 41 with weakly-doped spin ladders. The RIXS signal is dominated by a dispersing sharp mode ~ 270 meV on top of a damped incoherent component ~ 400-500 meV. Comparing with model calculations using the perturbative continuous unitary transformations method, the two components resemble the spin-conserving ΔS = 0 two-triplon bound state and continuum excitations in the spin ladders. Such multi-spin response with long-lived ΔS = 0 excitons is central to several exotic magnetic properties featuring Majorana fermions, yet remains unexplored given the generally weak cross-section with other experimental techniques. By investigating a simple spin-ladder model system, our study provides valuable insight into low-dimensional quantum magnetism.},
author = {Tseng, Yi and Paris, Eugenio and Schmidt, Kai Phillip and Zhang, Wenliang and Asmara, Teguh Citra and Bag, Rabindranath and Strocov, Vladimir N. and Singh, Surjeet and Schlappa, Justine and Rønnow, Henrik M. and Schmitt, Thorsten},
doi = {10.1038/s42005-023-01250-9},
faupublication = {yes},
journal = {Communications Physics},
note = {CRIS-Team Scopus Importer:2023-06-30},
peerreviewed = {Yes},
title = {{Momentum}-resolved spin-conserving two-triplon bound state and continuum in a cuprate ladder},
volume = {6},
year = {2023}
}
@article{faucris.123307624,
abstract = {We study the half-filled Hubbard model on a one-parameter family of vortex-full square lattices ranging from the isotropic case to weakly coupled Hubbard dimers. The ground-state phase diagram consists of four phases: A semimetal and a band insulator which are connected to the weak-coupling limit, and a magnetically ordered Neel phase and a valence bond solid (VBS) which are linked to the strong-coupling Mott limit. The phase diagram is obtained by quantum Monte Carlo (QMC) and continuous unitary transformations (CUTs). The CUT is performed in a two-step process: Nonperturbative graph-based CUTs are used in the Mott insulating phase to integrate out charge fluctuations. The resulting effective spin model is tackled by perturbative CUTs about the isolated dimer limit yielding the breakdown of the VBS by triplon condensation. We find three scenarios when varying the interaction for a fixed anisotropy of hopping amplitudes: (i) one direct phase transition from Neel to semimetal, (ii) two phase transitions VBS to Neel and Neel to semimetal, or (iii) a smooth crossover from VBS to the band insulator. Our results are consistent with the absence of spin-liquid phases in the whole phase diagram.},
author = {Ixert, D. and Assaad, F. F. and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.90.195133},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Mott} physics in the half-filled {Hubbard} model on a family of vortex-full square lattices},
volume = {90},
year = {2014}
}
@article{faucris.118492044,
abstract = {The excitation spectrum of the 2-leg S = 1/2 Heisenberg ladder is examined perturbatively. Using an optimally chosen continuous unitary transformation, we expand the Hamiltonian and the Raman operator about the limit of isolated rungs leading to high order series expansions allowing to calculate spectral densities quantitatively. The 2-particle sector is examined for total momentum k = 0. We show that triplet-triplet interaction gives rise to a band splitting. (C) 2002 Elsevier Science B.V. All rights reserved.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Knetter C, Schmidt KP, Uhrig GS},
faupublication = {no},
journal = {Physica B-Condensed Matter},
keywords = {quantum spin-systems;multi-particle excitations;spectral densities},
pages = {527-528},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Multi}-particle excitations and spectral densities in quantum spin-systems},
volume = {312},
year = {2002}
}
@article{faucris.294152886,
abstract = {Spin waves (magnons) in two dimensions are the potential glue in high-temperature superconductors so that their quantitative understanding is mandatory. Yet even for the fundamental case of the undoped Heisenberg model on the square lattice a consistent picture is still lacking. Significant spectral continua are taken as evidence of the existence of fractional excitations (spinons), but descriptions in terms of spinons fail to show the established absence of an energy gap. Here a fully consistent picture of the dynamics in the square-lattice quantum antiferromagnet is provided which agrees with the experimental findings. The key step is to capture (i) the strong attractive interaction between the spin waves and (ii) the vertex corrections of the observables.© },
author = {Powalski, Michael and Schmidt, Kai Phillip and Uhrig, Goetz S.},
doi = {10.21468/SciPostPhys.4.1.001},
faupublication = {yes},
journal = {SciPost Physics},
month = {Jan},
note = {CRIS-Team Scopus Importer:2023-03-27},
peerreviewed = {Yes},
title = {{Mutually} attracting spin waves in the square-lattice quantum {Antiferromagnet}},
volume = {4},
year = {2018}
}
@article{faucris.108551344,
abstract = {Inelastic neutron scattering was used to determine the spin Hamiltonian for the singlet ground-state system of fully deuterated BPCB, (C5D12N)(2)CuBr4. A two-leg spin-1/2 ladder model, with J(perpendicular to) = (1.09 +/- 0.01) meV and J(parallel to) = (0.296 +/- 0.005) meV, accurately describes the data. The experimental limit on the effective interladder exchange constant is vertical bar J(int)(eff)vertical bar less than or similar to 0.006 meV, and the limit on total diagonal, intraladder exchange is vertical bar J(F)+J(F')vertical bar <= 0.1 meV. Including the effects of copper to bromide covalent spin transfer on the magnetic form factor, the experimental ratios of intraladder bond energies are consistent with the predictions of continuous unitary transformation.},
author = {Savici, A. T. and Granroth, G. E. and Broholm, C. L. and Pajerowski, D. M. and Brown, C. M. and Talham, D. R. and Meisel, M. W. and Schmidt, Kai Phillip and Uhrig, G. S. and Nagler, S. E.},
doi = {10.1103/PhysRevB.80.094411},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Neutron} scattering evidence for isolated spin-1/2 ladders in ({C5D12N})(2){CuBr4}},
volume = {80},
year = {2009}
}
@article{faucris.122529264,
abstract = {Inelastic neutron scattering measurements are reported for a powder sample of the spin-1/2 quantum paramagnet Cu(quinoxaline)Br-2. Magnetic neutron scattering is identified above an energy gap of 1.9 meV. Analysis of the sharp spectral maximum at the onset indicates that the material is magnetically quasi-one-dimensional. Consideration of the wave vector dependence of the scattering and polymeric structure further identifies the material as a two-legged spin-1/2 ladder. Detailed comparisons of the data to various models of magnetism in this material based on the single-mode approximation and the continuous unitary transformation are presented. The latter theory provides an excellent account of the data with leg exchange J(parallel to)=2.0 meV and rung exchange J(perpendicular to)=3.3 meV.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Hong T, Kenzelmann M, Turnbull MM, Landee CP, Lewis BD, Schmidt KP, Uhrig GS, Qiu Y, Broholm C, Reich D},
doi = {10.1103/PhysRevB.74.094434},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Neutron} scattering from a coordination polymer quantum paramagnet},
volume = {74},
year = {2006}
}
@article{faucris.109549264,
abstract = {For coupled-dimer Heisenberg magnets, a paradigm of magnetic quantum phase transitions, we develop a systematic expansion in 1/d, the inverse number of space dimensions. The expansion employs a formulation of the bond-operator technique and is based on the observation that a suitably chosen product-state wave function yields exact zero-temperature expectation values of local observables in the d -> infinity limit, with corrections vanishing as 1/d. We demonstrate the approach for a model of dimers on a hypercubic lattice, which generalizes the square-lattice bilayer Heisenberg model to arbitrary d. In this paper, we use the 1/d expansion to calculate static and dynamic observables at zero temperature in the paramagnetic singlet phase, up to the quantum phase transition, and compare the results with numerical data available for d = 2. Contact is also made with previously proposed refinements of bond-operator theory as well as with a perturbative expansion in the interdimer coupling. In a companion paper, the present 1/d expansion will be extended to the ordered phase, where it is shown to consistently describe the entire phase diagram including the quantum critical point.},
author = {Joshi, Darshan G. and Coester, Kris and Schmidt, Kai Phillip and Vojta, Matthias},
doi = {10.1103/PhysRevB.91.094404},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Nonlinear} bond-operator theory and 1/d expansion for coupled-dimer magnets. {I}. {Paramagnetic} phase},
volume = {91},
year = {2015}
}
@article{faucris.107599184,
abstract = {We use nonperturbative linked-cluster expansions to determine the ground-state energy per site of the spin-one Heisenberg model on the kagome lattice. To this end, a parameter is introduced allowing us to interpolate between a fully trimerized state and the isotropic model. The ground-state energy per site of the full graph decomposition up to graphs of six triangles (18 spins) displays a complex behavior as a function of this parameter close to the isotropic model which we attribute to divergencies of partial series in the graph expansion of quasi-1D unfrustrated chain graphs. More concretely, these divergencies can be traced back to a quantum critical point of the one-dimensional unfrustrated chain of coupled triangles. Interestingly, the reorganization of the nonperturbative linked-cluster expansion in terms of clusters with enhanced symmetry yields a ground-state energy per site of the isotropic two-dimensional model that is in quantitative agreement with other numerical approaches in favor of a spontaneous trimerization of the system. Our findings are of general importance for any nonperturbative linked-cluster expansion on geometrically frustrated systems.},
author = {Ixert, Dominik and Tischler, Tobias and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.92.174422},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Nonperturbative} linked-cluster expansions for the trimerized ground state of the spin-one kagome {Heisenberg} model},
volume = {92},
year = {2015}
}
@article{faucris.109546624,
abstract = {We introduce a generic scheme to perform nonperturbative linked cluster expansions in long-range ordered quantum phases. Clusters are considered to be surrounded by an ordered reference state leading to effective edge fields in the exact diagonalization on clusters, which break the associated symmetry of the ordered phase. Two approaches, based either on a self-consistent solution of the order parameter or on minimal sensitivity with respect to the ground-state energy per site, are formulated to find the optimal edge field in each NLCE order. Furthermore, we investigate the scaling behavior of the NLCE data sequences towards the infinite-order limit. We apply our scheme to gapped and gapless ordered phases of XXZ Heisenberg models on various lattices and for spins 1/2 and 1 using several types of cluster expansions ranging from a full-graph decomposition, rectangular clusters, up to more symmetric square clusters. It is found that the inclusion of edge fields allows to regularize nonperturbative linked-cluster expansions in ordered phases yielding convergent data sequences. This includes the long-range spin-ordered ground state of the spin-1/2 and spin-1 Heisenberg model on the square and triangular lattice as well as the trimerized valence bond crystal of the spin-1 Heisenberg model on the kagome lattice.},
author = {Ixert, Dominik and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.94.195133},
faupublication = {yes},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Nonperturbative} linked-cluster expansions in long-range ordered quantum systems},
volume = {94},
year = {2016}
}
@inproceedings{faucris.108008824,
abstract = {We present a novel extrapolation scheme for high order series expansions. The idea is to express the series, obtained in orders of an external variable, in terms of an internal parameter of the system. Here we apply this method to the 1-triplet dispersion in an antiferromagnetic S = 1/2 Heisenberg ladder. By the use of the internal parameter the accuracy of the truncated series is enhanced tremendously.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Knetter C, Uhrig GS},
faupublication = {no},
pages = {1481-1484},
peerreviewed = {No},
publisher = {ACTA PHYSICA POLONICA B, JAGELLONIAN UNIV, INST PHYSICS},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Novel} extrapolation for strong coupling expansions},
volume = {34},
year = {2003}
}
@article{faucris.118492484,
abstract = {Phonon-assisted two-magnon absorption is studied in the spin(7)1/2 two-leg ladders of (Ca, La) 14Cu24O41 for E parallel to c (legs) and E parallel to a (rungs). We verify the theoretically predicted existence of two-magnon singlet bound states, which give rise to peaks at approximate to 2140 and 2800 cm(-1). The two-magnon continuum is observed at approximate to 4000 cm(-1). Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J(parallel to) approximate to 1020-1100 cm(-1), J(parallel to)/J(perpendicular to) approximate to 1-1.2. At high energies, the magnetic contribution to sigma(omega) is strikingly similar in the ladders and in the undoped high-T-c cuprates, which emphasizes the importance of strong quantum fluctuations in the latter.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Windt M, Gruninger M, Nunner T, Knetter C, Schmidt KP, Uhrig GS, Kopp T, Freimuth A, Ammerahl U, Buchner B, Revcolevschi A},
doi = {10.1103/PhysRevLett.87.127002},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Observation} of two-magnon bound states in the two-leg ladders of ({Ca}, {La})(14){Cu24O41}},
volume = {87},
year = {2001}
}
@article{faucris.107679264,
abstract = {We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41 as a starting point for investigating doped ladders and their tendency toward superconductivity. A key feature was the separation of one-triplon and two-triplon scattering. Two-triplon scattering is observed quantitatively for the first time and so access is realized to the important strong magnetic quantum fluctuations. The spin gap is found to be 26.4 +/- 0.3 meV. The data are successfully modeled using the continuous unitary transformation method, and the exchange constants are determined by fitting to be J(leg) = 186 meV and J(rung) = 124 meV along the leg and rung, respectively; a substantial cyclic exchange of J(cyc) = 31 meV is confirmed.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Notbohm S, Ribeiro P, Lake B, Tennant DA, Schmidt KP, Uhrig GS, Hess C, Klingeler R, Behr G, Buchner B, Reehuis M, Bewley RI, Frost CD, Manuel P, Eccleston RS},
doi = {10.1103/PhysRevLett.98.027403},
faupublication = {no},
journal = {Physical Review Letters},
month = {Jan},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{One}- and two-triplon spectra of a cuprate ladder},
volume = {98},
year = {2007}
}
@inproceedings{faucris.108752864,
abstract = {Transmission and reflectivity of LaxCa14-xCu24O41 two-leg spin-(1)/(2) ladders were measured in the mid-infrared regime between 500 and 12 000 cm(-1). This allows us to determine the optical conductivity a, directly and with high sensitivity. Here we show data for x = 4 and 5 with the electrical field polarized parallel to the rungs (E\\a) and to the legs (E\\c). Three characteristic peaks are identified as magnetic excitations by comparison with two different theoretical calculations. (C) 2002 Elsevier Science B.V. All rights reserved.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Gruninger M, Windt M, Nunner T, Knetter C, Schmidt KP, Uhrig GS, Kopp T, Freimuth A, Ammerahl U, Buchner B, Revcolevschi A},
faupublication = {no},
keywords = {spin ladder;bound state;phonon-assisted 2-magnon absorption},
pages = {617-618},
peerreviewed = {No},
publisher = {ELSEVIER SCIENCE BV},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Optical} spectroscopy of ({La}, {Ca})(14){Cu24O41} spin ladders: comparison of experiment and theory},
volume = {312},
year = {2002}
}
@inproceedings{faucris.108645944,
abstract = {Low-dimensional quantum spin systems display fascinating excitation spectra. In recent years, optical spectroscopy was shown to be a powerful tool for the study of these spectra by means of phonon-assisted infrared absorption. We discuss the results of antiferromagnetic S=1/2 cuprates with various topologies: the spinon continuum observed in the weakly coupled chains of CaCu2O3, two-triplet bound states and the continuum of the two-leg ladders in (La,Ca)(14)Cu24O41, and the bimagnon-plus-phonon spectrum of the bilayer YBa2Cu3O6, an undoped parent compound of the 2D high-T-c cuprates. Various theoretical approaches (dynamical DMRG, continuous unitary transformations (CUT), and spin-wave theory) are used for a quantitative analysis. Particular attention is paid to the role of the cyclic four-spin exchange.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Gruninger M, Windt M, Benckiser E, Nunner TS, Schmidt KP, Uhrig GS, Kopp T},
faupublication = {no},
month = {Jan},
pages = {95-111},
peerreviewed = {No},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Optical} spectroscopy of low-dimensional quantum spin systems},
volume = {43},
year = {2003}
}
@article{faucris.106698724,
abstract = {We introduce a white-graph expansion for the method of perturbative continuous unitary transformations when implemented as a linked-cluster expansion. The essential idea behind an expansion in white graphs is to perform an optimized bookkeeping during the calculation by exploiting the model-independent effective Hamiltonian in second quantization and the associated inherent cluster additivity. This approach is shown to be especially well suited for microscopic models with many coupling constants, since the total number of relevant graphs is drastically reduced. The white-graph expansion is exemplified for a two-dimensional quantum spin model of coupled two-leg XXZ ladders.},
author = {Coester, K. and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevE.92.022118},
faupublication = {no},
journal = {Physical Review E},
peerreviewed = {Yes},
title = {{Optimizing} linked-cluster expansions by white graphs},
volume = {92},
year = {2015}
}
@article{faucris.122455564,
abstract = {We introduce a toric code model on the dice lattice which is exactly solvable and displays topological order at zero temperature. In the presence of a magnetic field, the flux dynamics is mapped to the highly frustrated transverse field Ising model on the kagome lattice. This correspondence suggests an intriguing disorder by disorder phenomenon in a topologically ordered system implying that the topological order is extremely robust due to the geometric frustration. Furthermore, a connection between fully frustrated transverse field Ising models and topologically ordered systems is demonstrated which opens an exciting physical playground due to the interplay of topological quantum order and geometric frustration.},
author = {Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.88.035118},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Persisting} topological order via geometric frustration},
volume = {88},
year = {2013}
}
@article{faucris.108557064,
abstract = {We analyze the gapped phase of the Kitaev honeycomb model perturbatively in the isolated-dimer limit. Our analysis is based on the continuous unitary transformations method, which allows one to compute the spectrum as well as matrix elements of operators between eigenstates at high order. The starting point of our study consists of an exact mapping of the original honeycomb spin system onto a square-lattice model involving an effective spin and a hard-core boson. We then derive the low-energy effective Hamiltonian up to order 10 which is found to describe an interacting-anyon system, contrary to the order 4 result which predicts a free theory. These results give the ground-state energy in any vortex sector and thus also the vortex gap, which is relevant for experiments. Furthermore, we show that the elementary excitations are emerging free fermions composed of a hard-core boson with an attached spin- and phase-operator string. We also focus on observables and compute, in particular, the spin-spin correlation functions. We show that they admit a multiplaquette expansion that we derive up to order 6. Finally, we study the creation and manipulation of anyons with local operators, show that they also create fermions, and discuss the relevance of our findings for experiments in optical lattices.},
author = {Vidal, Julien and Schmidt, Kai Phillip and Dusuel, Sebastien},
doi = {10.1103/PhysRevB.78.245121},
faupublication = {no},
journal = {Physical Review B},
keywords = {anyons;boson systems;fermion systems;optical lattices;perturbation techniques;spin systems},
peerreviewed = {Yes},
title = {{Perturbative} approach to an exactly solved problem: {Kitaev} honeycomb model},
volume = {78},
year = {2008}
}
@article{faucris.108557504,
abstract = {We analyze the Kitaev model on the triangle-honeycomb lattice whose ground state has recently been shown to be a chiral spin liquid. We consider two perturbative expansions: the isolated-dimer limit containing Abelian anyons and the isolated-triangle limit. In the former case, we derive the low-energy effective theory and discuss the role played by multiplaquette interactions. In this phase, we also compute the spin-spin correlation functions for any vortex configuration. In the isolated-triangle limit, we show that the effective theory is, at lowest nontrivial order, the Kitaev honeycomb model at the isotropic point. We also compute the next-order correction which opens a gap and yields non-Abelian anyons.},
author = {Dusuel, Sebastien and Schmidt, Kai Phillip and Vidal, Julien and Zaffino, Rosa Letizia},
doi = {10.1103/PhysRevB.78.125102},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Perturbative} study of the {Kitaev} model with spontaneous time-reversal symmetry breaking},
volume = {78},
year = {2008}
}
@article{faucris.122201024,
abstract = {We show that the critical fields of the magnetization plateaus of the Shastry-Sutherland model decrease significantly upon increasing the ratio of inter-to intradimer coupling and accordingly that the magnetization plateaus of SrCu2(BO3)(2) shift to lower field under pressure, making the first two plateaus at 1/8 and 2/15 potentially accessible to neutron scattering experiments. These conclusions are based on the derivation of an effective classical model of interacting pinwheel-shaped spin-2 bound states using a combination of perturbative and graph-based continuous unitary transformations, showing that pinwheel crystals are indeed the lowest-energy plateau structures at low magnetization and that a simple model of intermediate-range two-body repulsion between pinwheels is able to account quantitatively for the plateau sequence.},
author = {Schneider, David A. and Coester, Kris and Mila, Frederic and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.93.241107},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {unknown},
title = {{Pressure} dependence of the magnetization plateaus of {SrCu2}({BO3})(2)},
volume = {93},
year = {2016}
}
@article{faucris.317985258,
abstract = {We construct a projection-based cluster-additive transformation that block-diagonalizes wide classes of lattice Hamiltonians H = H0 + V. Its cluster additivity is an essential ingredient to set up perturbative or non-perturbative linked-cluster expansions for degenerate excitation subspaces of H0. Our transformation generalizes the minimal transformation known amongst others under the names Takahashi’s transformation, Schrieffer-Wolff transformation, des Cloiseaux effective Hamiltonian, canonical van Vleck effective Hamiltonian or two-block orthogonalization method. The effective cluster-additive Hamiltonian and the transformation for a given subspace of H, that is adiabatically connected to the eigenspace of H0 with eigenvalue e0^{n}, solely depends on the eigenspaces of H connected to e0^{m} with e0^{m} ≤ e0^{n}. In contrast, other cluster-additive transformations like the multi-block orthogonalization method or perturbative continuous unitary transformations need a larger basis. This can be exploited to implement the transformation efficiently both perturbatively and non-perturbatively. As a benchmark, we perform perturbative and non-perturbative linked-cluster expansions in the low-field ordered phase of the transverse-field Ising model on the square lattice for single spin-flips and two spin-flip bound-states.},
author = {Hörmann, Max and Schmidt, Kai Phillip},
doi = {10.21468/SciPostPhys.15.3.097},
faupublication = {yes},
journal = {SciPost Physics},
note = {Created from Fastlane, Scopus look-up},
peerreviewed = {Yes},
title = {{Projective} cluster-additive transformation for quantum lattice models},
volume = {15},
year = {2023}
}
@article{faucris.245899133,
abstract = {The critical breakdown of a one-dimensional quantum magnet with long-range interactions is studied by investigating the high-field polarized phase of the anisotropic XY model in a transverse field for the ferromagnetic and antiferromagnetic cases. While for the limiting case of the isotropic long-range XY model we can extract the elementary one-quasiparticle dispersion analytically and calculate two-quasiparticle excitation energies quantitatively in a numerical fashion, for the long-range Ising limit as well as in the intermediate regime we use perturbative continuous unitary transformations on white graphs in combination with classical Monte Carlo simulations for the graph embedding to extract high-order series expansions in the thermodynamic limit. This enables us to determine the quantum-critical breakdown of the high-field polarized phase by analyzing the gap closing including associated critical exponents and multiplicative logarithmic corrections. In addition, for the ferromagnetic isotropic XY model we determined the critical exponents z and v analytically by a bosonic quantum-field theory.},
author = {Adelhardt, Patrick and Koziol, Jan and Schellenberger, Andreas and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.102.174424},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team WoS Importer:2020-11-27},
peerreviewed = {Yes},
title = {{Quantum} criticality and excitations of a long-range anisotropic {XY} chain in a transverse field},
volume = {102},
year = {2020}
}
@article{faucris.228169160,
abstract = {To gain a better understanding of the interplay between frustrated long-range interactions and zero-temperature quantum fluctuations, we investigate the ground-state phase diagram of the transverse-field Ising model with algebraically decaying long-range Ising interactions on quasi-one-dimensional infinite-cylinder triangular lattices. Technically, we apply various approaches including low- and high-field series expansions. For the classical long-range Ising model, we investigate cylinders with an arbitrary even circumference. We show the occurrence of gapped stripe-ordered phases emerging out of the infinitely degenerate nearest-neighbor Ising ground-state space on the two-dimensional triangular lattice. Further, while cylinders with circumferences 6, 10, 14, etc., are always in the same stripe phase for any decay exponent of the long-range Ising interaction, the family of cylinders with circumferences 4, 8, 12, etc., displays a phase transition between two different types of stripe structures. For the full long-range transverse-field Ising model, we concentrate on cylinders with circumference four and six. The ground-state phase diagram consists of several quantum phases in both cases including an x-polarized phase, stripe-ordered phases, and clock-ordered phases which emerge from an order-by-disorder scenario already present in the nearest-neighbor model. In addition, the generic presence of a potential intermediate gapless phase with algebraic correlations and associated Kosterlitz-Thouless transitions is discussed for both cylinders.},
author = {Koziol, Jan and Fey, Sebastian and Kapfer, Sebastian and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.100.144411},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team WoS Importer:2019-10-22},
peerreviewed = {Yes},
title = {{Quantum} criticality of the transverse-field {Ising} model with long-range interactions on triangular-lattice cylinders},
volume = {100},
year = {2019}
}
@article{faucris.217464944,
abstract = {We study the critical breakdown of two-dimensional quantum magnets in the presence of algebraically decaying long-range interactions by investigating the transverse-field Ising model on the square and triangular lattice. This is achieved technically by combining perturbative continuous unitary transformations with classical Monte Carlo simulations to extract high-order series for the one-particle excitations in the high-field quantum paramagnet. We find that the unfrustrated systems change from mean-field to nearest-neighbor universality with continuously varying critical exponents. In the frustrated case on the square lattice the system remains in the universality class of the nearest-neighbor model independent of the long-range nature of the interaction, while we argue that the quantum criticality for the triangular lattice is terminated by a first-order phase transition line.},
author = {Fey, Sebastian and Kapfer, Sebastian and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevLett.122.017203},
faupublication = {yes},
journal = {Physical Review Letters},
month = {Jan},
note = {CRIS-Team Scopus Importer:2019-05-14},
peerreviewed = {Yes},
title = {{Quantum} {Criticality} of {Two}-{Dimensional} {Quantum} {Magnets} with {Long}-{Range} {Interactions}},
volume = {122},
year = {2019}
}
@article{faucris.247569192,
abstract = {We demonstrate that two toric code layers on the square lattice coupled by an Ising interaction display two distinct phases with intrinsic topological order. The second-order quantum phase transition between the weakly coupled Z(2) x Z(2) and the strongly coupled Z(2) topological order can be described by the condensation of bosonic quasiparticles from both sides and belongs to the three-dimensional Ising* universality class. This can be shown by an exact duality transformation to the transverse-field Ising model on the square lattice, which builds on the existence of an extensive number of local Z(2) conserved parities. These conserved quantities correspond to the product of two adjacent star operators on different layers. Notably, we show that the low-energy effective model derived about the limit of large Ising coupling is given by an effective single-layer toric code in terms of the conserved quantities of the Ising toric code bilayer. The two topological phases are further characterized by the topological entanglement entropy which serves as a nonlocal order parameter.},
author = {Wiedmann, Raymond and Lenke, Lea and Walther, Matthias and Mühlhauser, Matthias and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.102.214422},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team WoS Importer:2021-01-08},
peerreviewed = {Yes},
title = {{Quantum} critical phase transition between two topologically ordered phases in the {Ising} tonic code bilayer},
volume = {102},
year = {2020}
}
@article{faucris.261338373,
abstract = {The quantum-critical properties of the transverse-field Ising model with algebraically decaying interactions are investigated by means of stochastic series expansion quantum Monte Carlo, on both the one-dimensional linear chain and the two-dimensional square lattice. We extract the critical exponents nu and beta as a function of the decay exponent of the long-range interactions. For ferromagnetic Ising interactions, we resolve the limiting regimes known from field theory, ranging from the nearest-neighbor Ising to the long-range Gaussian universality classes, as well as the intermediate regime with continuously varying critical exponents. In the long-range Gaussian regime, we treat the effect of dangerous irrelevant variables on finite-size scaling forms. For antiferromagnetic and therefore competing Ising interactions, the stochastic series expansion algorithm displays growing autocorrelation times leading to a reduced performance. Nevertheless, our results are consistent with the nearest-neighbor Ising universality for all investigated interaction ranges both on the linear chain and the square lattice.},
author = {Koziol, Jan and Langheld, Anja and Kapfer, Sebastian and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.103.245135},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team WoS Importer:2021-07-09},
peerreviewed = {Yes},
title = {{Quantum}-critical properties of the long-range transverse-field {Ising} model from quantum {Monte} {Carlo} simulations},
volume = {103},
year = {2021}
}
@article{faucris.107602044,
abstract = {We investigate the transverse field Ising model on a diamond chain using series expansions about the high-field limit and exact diagonalizations. For the unfrustrated case we accurately determine the quantum critical point and its expected 2d Ising universality separating the polarized and the Z(2) symmetry broken phase. In contrast, we find strong evidence for a disorder by disorder scenario for the fully-frustrated transverse field Ising model, i.e., except for the pure Ising model, having an extensive number of ground states, the system is always in a quantum disordered polarized phase. The low-energy excitations in this polarized phase are understood in terms of exact local modes of the model. Furthermore, an effective low-energy description for an infinitesimal transverse field allows us to pinpoint the quantum disordered nature of the ground state via mapping to an effective transverse field Ising chain and to determine the induced gap to the elementary effective domain wall excitation very accurately.},
author = {Coester, K. and Malitz, W. and Fey, S. and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.88.184402},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Quantum} disorder and local modes of the fully-frustrated transverse field {Ising} model on a diamond chain},
volume = {88},
year = {2013}
}
@article{faucris.277348061,
abstract = {We investigate the quantum phase diagram of the K-layer Ising toric code corresponding to K layers of twodimensional toric codes coupled by Ising interactions. While for small Ising interactions the system displays 1K2 topological order originating from the toric codes in each layer, the system shows 12 topological order in the high-Ising limit. The latter is demonstrated for general K by deriving an effective low-energy model in Kth-order degenerate perturbation theory, which is given as an effective anisotropic single-layer toric code in terms of collective pseudospins 1/2 referring to the two ground states of isolated Ising chain segments. For the specific cases K = 3 and K = 4 we apply high-order series expansions to determine the gap series in the low- and high-Ising limit. Extrapolation of the elementary energy gaps gives convincing evidence that the ground-state phase diagram consists of a single quantum critical point and our findings suggest a quantum phase transition in the 3D Ising* universality class for both K separating both types of topological order, which is consistent with former findings for the bilayer Ising toric code.},
author = {Schamriss, Lukas and Lenke, Lea and Mühlhauser, Matthias and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.105.184425},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team WoS Importer:2022-07-01},
peerreviewed = {Yes},
title = {{Quantum} phase transitions in the {K}-layer {Ising} toric code},
volume = {105},
year = {2022}
}
@article{faucris.122206744,
abstract = {We investigate the low-energy spectral properties and robustness of the topological phase of color code, which is a quantum spin model for the aim of fault-tolerant quantum computation, in the presence of a uniform magnetic field or Ising interactions, using high-order series expansion and exact diagonalization. In a uniform magnetic field, we find first-order phase transitions in all field directions. In contrast, our results for the Ising interactions unveil that for strong enough Ising couplings, the Z(2) x Z(2) topological phase of color code breaks down to symmetry broken phases by first-or second-order phase transitions.},
author = {Jahromi, Saeed S. and Masoudi, S. Farhad and Kargarian, Mehdi and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.88.214411},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Quantum} phase transitions out of a {Z}(2) x {Z}(2) topological phase},
volume = {88},
year = {2013}
}
@article{faucris.226825387,
abstract = {We use linked-cluster expansions to analyze the quantum phase transitions between symmetry-unbroken trivial and topological Haldane phases in two different spin-one chains. The first model is the spin-one Heisenberg chain in the presence of a single-ion anisotropy, while the second one is the dimerized spin-one Heisenberg chain. For both models, we determine the ground-state energy and the one-particle gap inside the nontopological phase as a high-order series using perturbative continuous unitary transformations. Extrapolations of the gap series are applied to locate the quantum critical point and to extract the associated critical exponent. We find that this approach works unsatisfactorily for the anisotropic chain, since the quality of the extrapolation appears insufficient due to the large correlation length exponent. In contrast, extrapolation schemes display very good convergence for the gap closing in the case of the dimerized spin-one Heisenberg chain.},
author = {Adelhardt, Patrick and Gritsch, Julian and Hille, M. and Reiß, David and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.96.235123},
faupublication = {yes},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Quantum} phase transitions to topological {Haldane} phases in spin-one chains studied by linked-cluster expansions},
volume = {96},
year = {2017}
}
@article{faucris.222697869,
abstract = {We study the robustness of 3D intrinsic topogical order under external perturbations by investigating the paradigmatic microscopic model, the 3D toric code in an external magnetic field. Exact dualities as well as variational calculations reveal a ground-state phase diagram with first and second-order quantum phase transitions. The variational approach can be applied without further approximations only for certain field directions. In the general field case, an approximative scheme based on an expansion of the variational energy in orders of the variational parameters is developed. For the breakdown of the 3D intrinsic topological order, it is found that the (im-) mobility of the quasiparticle excitations is crucial in contrast to their fractional statistics.},
author = {Reiss, David A. and Schmidt, Kai Phillip},
doi = {10.21468/SciPostPhys.6.6.078},
faupublication = {yes},
journal = {SciPost Physics},
note = {CRIS-Team WoS Importer:2019-07-19},
peerreviewed = {Yes},
title = {{Quantum} robustness and phase transitions of the {3D} toric code in a field},
volume = {6},
year = {2019}
}
@article{faucris.235421492,
abstract = {The quantum robustness of fracton phases is investigated by studying the influence of quantum fluctuations on the X-Cube model and Haah's code, which realize a type-I and type-II fracton phase, respectively. To this end, a finite uniform magnetic field is applied to induce quantum fluctuations in the fracton phase, resulting in zero-temperature phase transitions between fracton phases and polarized phases. Using high-order series expansions and a variational approach, all phase transitions are classified as strongly first order, which turns out to be a consequence of the (partial) immobility of fracton excitations. Indeed, single fractons as well as few-fracton composites can hardly lower their excitation energy by delocalization due to the intriguing properties of fracton phases, as demonstrated in this work explicitly in terms of fracton quasiparticles.},
author = {Mühlhauser, Matthias and Walther, Matthias and Reiss, D. A. and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.101.054426},
faupublication = {yes},
journal = {Physical Review B},
note = {CRIS-Team WoS Importer:2020-03-06},
peerreviewed = {Yes},
title = {{Quantum} robustness of fracton phases},
volume = {101},
year = {2020}
}
@article{faucris.108753084,
abstract = {The Raman response in the antiferromagnetic 2-leg S = 1/2 Heisenberg ladder is calculated for various couplings by continuous unitary transformations. For leg couplings above 80% of the rung coupling a characteristic 2-peak structure occurs with a point of zero intensity within the continuum. Experimental data for CaV2O5 and LayCa14-yCu24O41 are analyzed and the coupling constants are determined. Evidence is found that the Heisenberg model is not sufficient to describe cuprate ladders. We argue that a cyclic exchange term is the appropriate extension.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Knetter C, Uhrig GS},
faupublication = {no},
journal = {EPL - Europhysics Letters},
pages = {877-883},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Raman} response in antiferromagnetic two-leg {S}=1/2 {Heisenberg} ladders},
volume = {56},
year = {2001}
}
@article{faucris.117876044,
abstract = {A unified picture for the Raman response of magnetic excitations in cuprate spin-ladder compounds is obtained by comparing calculated two-triplon Raman line shapes to those of the prototypical compounds SrCu2O3 (Sr123), Sr14Cu24O41 (Sr14), and La6Ca8Cu24O41 (La6Ca8). The theoretical model for the two-leg ladder contains Heisenberg exchange couplings J(parallel to) and J(perpendicular to) plus an additional four-spin interaction J(cyc). Within this model Sr123 and Sr14 can be described by x:=J(parallel to)/J(perpendicular to)=1.5, x(cyc):=J(cyc)/J(perpendicular to)=0.2, J(perpendicular to)(Sr123)=1130 cm(-1) and J(perpendicular to)(Sr14)=1080 cm(-1). The couplings found for La6Ca8 are x=1.2, x(cyc)=0.2, and J(perpendicular to)(La6Ca8)=1130 cm(-1). The unexpected sharp two-triplon peak in the ladder materials compared to the undoped two-dimensional cuprates can be traced back to the anisotropy of the magnetic exchange in rung and leg direction. With the results obtained for the isotropic ladder, we calculate the Raman line shape of a two-dimensional square lattice using a toy model consisting of a vertical and a horizontal ladder. A direct comparison of these results with Raman experiments for the two-dimensional cuprates R5CuO4(R=La,Nd), Sr2CuO2Cl2, and YBa2Cu3O6+delta yields a good agreement for the dominating two-triplon peak. We conclude that short-range quantum fluctuations are dominating the magnetic Raman response in both ladders and planes. We discuss possible scenarios responsible for the high-energy spectral weight of the Raman line shape, i.e., phonons, the triple-resonance, and multiparticle contributions.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Gossling A, Kuhlmann U, Thomsen C, Loffert A, Gross C, Assmus W},
doi = {10.1103/PhysRevB.72.094419},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Raman} response of magnetic excitations in cuprate ladders and planes},
volume = {72},
year = {2005}
}
@article{faucris.123351844,
abstract = {We investigate the stability of the topological phase of the toric code model in the presence of a uniform magnetic field by means of variational and high-order series expansion approaches. We find that when this perturbation is strong enough, the system undergoes a topological phase transition whose first-or second-order nature depends on the field orientation. When this transition is of second order, it is in the Ising universality class except for a special line on which the critical exponent driving the closure of the gap varies continuously, unveiling a new topological universality class.},
author = {Dusuel, Sebastien and Kamfor, Michael and Orus, Roman and Schmidt, Kai Phillip and Vidal, Julien},
doi = {10.1103/PhysRevLett.106.107203},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Robustness} of a {Perturbed} {Topological} {Phase}},
volume = {106},
year = {2011}
}
@article{faucris.107844044,
abstract = {The robustness of the topological color code, which is a class of error-correcting quantum codes, is investigated under the influence of a uniform magnetic field on the honeycomb lattice. Our study relies on two high-order series expansions using perturbative continuous unitary transformations in the limit of low and high fields, exact diagonalization, and a classical approximation. We show that the topological color code in a single parallel field is isospectral to the Baxter-Wu model in a transverse field on the triangular lattice. It is found that the topological phase is stable up to a critical field beyond which it breaks down to the polarized phase by a first-order phase transition. The results also suggest that the topological color code is more robust than the toric code in the parallel magnetic field. DOI: 10.1103/PhysRevB.87.094413},
author = {Jahromi, Saeed S. and Kargarian, Mehdi and Masoudi, S. Farhad and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.87.094413},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {unknown},
title = {{Robustness} of a topological phase: {Topological} color code in a parallel magnetic field},
volume = {87},
year = {2013}
}
@article{faucris.122201244,
abstract = {A quantitative description of magnons in long-range ordered quantum antiferromagnets is presented which is consistent from low to high energies. It is illustrated for the generic S = 1/2 Heisenberg model on the square lattice. The approach is based on a continuous similarity transformation in momentum space using the scaling dimension as the truncation criterion. Evidence is found for significant magnon-magnon attraction inducing a Higgs resonance. The high-energy roton minimum in the magnon dispersion appears to be induced by strong magnon-Higgs scattering.},
author = {Powalski, M. and Uhrig, G. S. and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevLett.115.207202},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Roton} {Minimum} as a {Fingerprint} of {Magnon}-{Higgs} {Scattering} in {Ordered} {Quantum} {Antiferromagnets}},
volume = {115},
year = {2015}
}
@article{faucris.124148024,
abstract = {Using continuous unitary transformations (CUTs) we calculate the one-triplet gap for the antiferromagnetic S = 1/2 two-leg spin ladder with additional four-spin exchange interactions in a high order series expansion about the limit of isolated rungs. By applying an efficient extrapolation technique we calculate the transition line between the rung-singlet phase and a spontaneously dimerized phase with dimers on the legs. Using this extrapolation technique we are able to analyze the crossover from strong rung coupling to weakly coupled chains.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Monien H, Uhrig GS},
doi = {10.1103/PhysRevB.67.184413},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Rung}-singlet phase of the {S}=(1)/(2) two-leg spin-ladder with four-spin cyclic exchange},
volume = {67},
year = {2003}
}
@article{faucris.286184530,
abstract = {The hyperscaling relation and standard finite-size scaling (FSS) are known to break down above the upper critical dimension due to dangerous irrelevant variables. We establish a coherent formalism for FSS at quantum phase transitions above the upper critical dimension following the recently introduced Q-FSS formalism for thermal phase transitions. Contrary to long-standing belief, the correlation sector is affected by dangerous irrelevant variables. The presented formalism recovers a generalized hyperscaling relation and FSS form. Using this new FSS formalism, we determine the full set of critical exponents for the long-range transverse-field Ising chain in all criticality regimes ranging from the nearest-neighbor to the long-range mean-field regime. For the same model, we also explicitly confirm the effect of dangerous irrelevant variables on the characteristic length scale.},
author = {Langheld, Anja and Koziol, Jan and Adelhardt, Patrick and Kapfer, Sebastian and Schmidt, Kai Phillip},
doi = {10.21468/SciPostPhys.13.4.088},
faupublication = {yes},
journal = {SciPost Physics},
note = {CRIS-Team Scopus Importer:2022-12-02},
peerreviewed = {Yes},
title = {{Scaling} at quantum phase transitions above the upper critical dimension},
volume = {13},
year = {2022}
}
@article{faucris.107665624,
abstract = {We investigate the effect of a transverse magnetic field on the toric code model. We show that this problem can be mapped onto the Xu-Moore model and thus onto the quantum compass model, which are known to be self-dual. We analyze the low-energy spectrum by means of perturbative continuous unitary transformations and determine accurately the energy gaps of various symmetry sectors. Our results are in very good agreement with exact diagonalization data for all values of the parameters except at the self-dual point where level crossings are responsible for a first-order phase transition between a topological phase and a polarized phase. Interestingly, bound states of two and four quasiparticles with fermionic and bosonic statistics emerge, and display dispersion relations of reduced dimensionality.},
author = {Vidal, Julien and Thomale, Ronny and Schmidt, Kai Phillip and Dusuel, Sebastien},
doi = {10.1103/PhysRevB.80.081104},
faupublication = {no},
journal = {Physical Review B},
keywords = {bound states;energy gap;fermion systems;magnetic transitions;perturbation theory;quasiparticles},
peerreviewed = {Yes},
title = {{Self}-duality and bound states of the toric code model in a transverse field},
volume = {80},
year = {2009}
}
@article{faucris.309433949,
abstract = {The established approach of perturbative continuous unitary transformations (pCUTs) constructs effective quantum many-body Hamiltonians as perturbative series that conserve the number of one quasiparticle type. We extend the pCUT method to similarity transformations - dubbed pcst++ - allowing for multiple quasiparticle types with complex-valued energies. This enlarges the field of application to closed and open quantum many-body systems with unperturbed operators corresponding to arbitrary superimposed ladder spectra. To this end, a generalized counting operator is combined with the quasiparticle generator for open quantum systems recently introduced by Schmiedinghoff and Uhrig [G. Schmiedinghoff and G. S. Uhrig, SciPost Phys. 13, 122 (2022)2542-465310.21468/SciPostPhys.13.6.122]. The pcst++ then yields model-independent block-diagonal effective Hamiltonians and Lindbladians, allowing a linked-cluster expansion in the thermodynamic limit similar to the conventional pCUT method. We illustrate the application of the pcst++ method by discussing representative closed, open, and non-Hermitian quantum systems.},
author = {Lenke, Lea and Schellenberger, Andreas and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevA.108.013323},
faupublication = {yes},
journal = {Physical Review A},
note = {CRIS-Team Scopus Importer:2023-08-18},
peerreviewed = {Yes},
title = {{Series} expansions in closed and open quantum many-body systems with multiple quasiparticle types},
volume = {108},
year = {2023}
}
@article{faucris.108559484,
abstract = {We investigate the consequences of correlated hopping on the ground state properties of hard-core bosons on a square lattice as revealed by extensive exact diagonalizations and quantum Monte Carlo simulations. While for noninteracting hard-core bosons the effective attraction induced by the correlated hopping leads to phase separation at low density, we show that a modest nearest-neighbor repulsion suppresses phase separation, leading to a remarkable low-density pairing phase with no single particle Bose-Einstein condensation but long-range two-particle correlations, signaling a condensation of pairs. We also explain why the unusual properties of the pairing phase are a real challenge for standard one-worm quantum Monte Carlo simulations.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Dorier J, Lauchli A, Mila F},
doi = {10.1103/PhysRevB.74.174508},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Single}-particle versus pair condensation of hard-core bosons with correlated hopping},
volume = {74},
year = {2006}
}
@article{faucris.123355584,
abstract = {We study the physics of cold polar molecules loaded into an optical lattice in the regime of strong three-body interactions, as put forward recently by Buchler et al. [Nature Phys. 3, 726 (2007)]. To this end, quantum Monte Carlo simulations, exact diagonalization, and a semiclassical approach are used to explore hard-core bosons on the 2D square lattice which interact solely by long-ranged three-body terms. The resulting phase diagram shows a sequence of solid and supersolid phases. Our findings are directly relevant for future experimental implementations and open a new route towards the discovery of a lattice supersolid phase in experiment.},
author = {Schmidt, Kai Phillip and Dorier, Julien and Laeuchli, Andreas M.},
doi = {10.1103/PhysRevLett.101.150405},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Solids} and {Supersolids} of {Three}-{Body} {Interacting} {Polar} {Molecules} on an {Optical} {Lattice}},
volume = {101},
year = {2008}
}
@misc{faucris.123372964,
abstract = {This article summarizes and extends the recent developments in the microscopic modeling of the magnetic excitations in cuprate two-leg ladder systems. The microscopic Hamiltonian comprises dominant Heisenberg exchange terms plus an additional four-spin interaction which is about five times smaller. We give an over-view over the relevant energies like the one-triplon dispersion, the energies of two-triplon bound states and the positions of multi-triplon continua and over relevant spectral properties like spectral weights and spectral densities in the parameter regime appropriate for cuprate systems. It is concluded that an almost complete understanding of the magnetic excitations in undoped cuprate ladders has been obtained as measured by inelastic neutron scattering, inelastic light (Raman) scattering and infrared absorption.},
author = {Schmidt, Kai Phillip and Uhrig, G. S.},
faupublication = {no},
keywords = {magnetic excitations;spin ladders;spin liquids;four-spin ring exchange;high-T-c superconductivity;spectral densities;continuous unitary transformations},
peerreviewed = {automatic},
title = {{Spectral} properties of magnetic excitations in cuprate two-leg ladder systems},
year = {2005}
}
@article{faucris.108013884,
abstract = {Spectral densities are calculated for the dimerized and frustrated S=1/2 chain using the method of continuous unitary transformations. The transformation to an effective triplon model is realized in a perturbative fashion up to high orders about the limit of isolated dimers. An efficient description in terms of triplons (elementary triplets) is possible: a detailed analysis of the spectral densities is provided for strong and intermediate dimerization including the influence of frustration. Precise predictions are made for inelastic neutron scattering experiments probing the S=1 sector and for optical experiments (Raman scattering, infrared absorption) probing the S=0 sector. Bound states and resonances influence the important continua strongly. The comparison with the field theoretic results reveals that the sine-Gordon model describes the low-energy features for strong to intermediate dimerization only at critical frustration.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Knetter C, Uhrig GS},
doi = {10.1103/PhysRevB.69.104417},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Spectral} properties of the dimerized and frustrated {S}=1/2 chain},
volume = {69},
year = {2004}
}
@article{faucris.107603364,
abstract = {Dynamical correlation functions of the toric code in a uniform magnetic field are studied inside the topological phase, in the small-field limit. Such an experimentally measurable quantity displays rich field-dependent features that can be understood via the interplay of the kinetics and the interaction of the anyonic excitations. In particular, it is sensitive to the two-quasiparticle bound states that are present in the spectrum for a wide range of magnetic fields. Interestingly, such collective modes can even constitute the lowest-energy excitations of the system.},
author = {Kamfor, Michael and Dusuel, Sebastien and Vidal, Julien and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.89.045411},
faupublication = {no},
journal = {Physical Review B},
month = {Jan},
peerreviewed = {Yes},
title = {{Spectroscopy} of a topological phase},
volume = {89},
year = {2014}
}
@article{faucris.107596984,
abstract = {We show that the highly frustrated transverse-field Ising model on the three-dimensional pyrochlore lattice realizes a first-order phase transition without symmetry breaking between the low-field Coulomb quantum spin liquid and the high-field polarized phase. The quantum phase transition is located quantitatively by comparing low-and high-field series expansions. Furthermore, the intriguing properties of the elementary excitations in the polarized phase are investigated. We argue that this model can be achieved experimentally by applying mechanical strain to a classical spin-ice material composed of non-Kramers spins such as Ho2Ti2O7. Taken together with our results, this provides an experimental platform to study quantum spin liquid physics.},
author = {Roechner, Julia and Balents, Leon and Schmidt, Kai Phillip},
doi = {10.1103/PhysRevB.94.201111},
faupublication = {yes},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Spin} liquid and quantum phase transition without symmetry breaking in a frustrated three-dimensional {Ising} model},
volume = {94},
year = {2016}
}
@article{faucris.108557724,
abstract = {We show that correlated hopping of triplets, which is often the dominant source of kinetic energy in dimer-based frustrated quantum magnets, produces a remarkably strong tendency to form supersolid phases in a magnetic field. These phases are characterized by simultaneous modulation and ordering of the longitudinal and transverse magnetization, respectively. Using quantum Monte Carlo and a semiclassical approach for an effective hard-core boson model with nearest-neighbor repulsion on a square lattice, we prove, in particular, that a supersolid phase can exist even if the repulsion is not strong enough to stabilize an insulating phase at half-filling. Experimental implications for frustrated quantum antiferromagnets in a magnetic field at zero and finite temperature are discussed.},
author = {Schmidt, Kai Phillip and Dorier, J. and Laeuchli, A. M. and Mila, F.},
doi = {10.1103/PhysRevLett.100.090401},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Supersolid} phase induced by correlated hopping in spin-1/2 frustrated quantum magnets},
volume = {100},
year = {2008}
}
@article{faucris.107674204,
abstract = {We discuss the appearance of supersolid phases for interacting hardcore bosons on the square lattice when, in addition to the standard nearest neighbor hopping and repulsion, correlated or next-nearest neighbor hopping is present. Having in mind dimer-based quantum magnets in a field described by effective bosonic models of this kind, we put special emphasis on a comparison between the different cases of relative signs of the kinetic processes, which correspond to unfrustrated or frustrated magnetic models. In the unfrustrated case, we compare Quantum Monte Carlo simulations with a mean-field (classical) approach, which is shown to give qualitatively correct results. Using this classical approach for the frustrated case, we find that, the phase diagram is generically richer than in the unfrustrated case. We also investigate in detail the differences between standard next-nearest neighbour and correlated hopping over the diagonal, with the conclusion that both cases are similar if checkerboard order is present at half-filling, while a supersolid phase can be stabilized without any adjacent solid phase only in the case of correlated hopping.},
author = {Mila, Frederic and Dorier, Julien and Schmidt, Kai Phillip},
faupublication = {no},
journal = {Progress of Theoretical Physics Supplement},
month = {Jan},
pages = {355-374},
peerreviewed = {Yes},
title = {{Supersolid} {Phases} of {Hardcore} {Bosons} on the {Square} {Lattice}: {Correlated} {Hopping}, {Next}-{Nearest} {Neighbor} {Hopping} and {Frustration}},
year = {2008}
}
@article{faucris.306943085,
abstract = {We propose a general approach to analyse diagonal ordering patterns in bosonic lattice models with algebraically decaying density-density interactions on arbitrary lattices. The key idea is a systematic search for the energetically best order on all unit cells of the lattice up to a given extent. Using resummed couplings we evaluate the energy of the ordering patterns in the thermodynamic limit using finite unit cells. We apply the proposed approach to the atomic limit of the extended Bose-Hubbard model on the triangular lattice at fillings f = 1/2 and f = 1. We investigate the ground-state properties of the antiferromagnetic long-range Ising model on the triangular lattice and determine a six-fold degenerate plain-stripe phase to be the ground state for finite decay exponents. We also probe the classical limit of the Fendley-Sengupta-Sachdev model describing Rydberg atom arrays. We focus on arrangements where the atoms are placed on the sites or links of the Kagome lattice. Our method provides a general framework to treat cristalline structures resulting from long-range interactions.},
author = {Koziol, Jan and Duft, Antonia and Morigi, Giovanna and Schmidt, Kai Phillip},
doi = {10.21468/SciPostPhys.14.5.136},
faupublication = {yes},
journal = {SciPost Physics},
note = {CRIS-Team Scopus Importer:2023-06-30},
peerreviewed = {Yes},
title = {{Systematic} analysis of crystalline phases in bosonic lattice models with algebraically decaying density-density interactions},
volume = {14},
year = {2023}
}
@article{faucris.108643304,
abstract = {The Mott insulating phase of a one-dimensional bosonic gas trapped in an optical lattice is described by a Bose-Hubbard model. A continuous unitary transformation is used to map this model onto an effective model conserving the number of elementary excitations. We obtain quantitative results for the kinetics and for the spectral weights of the low-energy excitations for a broad range of parameters in the insulating phase. By these results, recent Bragg spectroscopy experiments are explained. Evidence for a significant temperature of the order of the microscopic energy scales is found.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Reischl A, Schmidt KP, Uhrig GS},
doi = {10.1103/PhysRevA.72.063609},
faupublication = {no},
journal = {Physical Review A},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Temperature} in one-dimensional bosonic {Mott} insulators},
volume = {72},
year = {2005}
}
@article{faucris.108557944,
abstract = {Motivated by the remarkable properties of SrCu(2)(BO(3))(2) in a magnetic field, we use perturbative continuous unitary transformations to determine the magnetization plateaux of the Shastry-Sutherland model, unveiling an unexpected sequence of plateaux progressively appearing at 2/9, 1/6, 1/9, and 2/15 upon increasing the interdimer coupling. We predict that a 1/6 plateau should be present in SrCu(2)(BO(3))(2), even if residual interactions beyond the Shastry-Sutherland are strong enough to modify the other plateaux below 1/3. The method is extended to calculate the magnetization profile within the plateaux, leading to a local structure around triplons that agrees with NMR results on SrCu(2)(BO(3))(2).},
author = {Dorier, J. and Schmidt, Kai Phillip and Mila, F.},
doi = {10.1103/PhysRevLett.101.250402},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Theory} of {Magnetization} {Plateaux} in the {Shastry}-{Sutherland} {Model}},
volume = {101},
year = {2008}
}
@article{faucris.108985624,
abstract = {By high-temperature series expansion and exact complete diagonalization the magnetic susceptibility chi(T) and the specific heat C(T) of a two-leg S=1/2 ladder with cyclic (four-spin) exchange are computed. Both methods yield convincing results for not too small temperatures. We find that a small amount of cyclic exchange influences the thermodynamic properties significantly. Our results can serve as reliable basis for an efficient analysis of experimental data.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Buhler A, Low U, Schmidt KP, Uhrig GS},
doi = {10.1103/PhysRevB.67.134428},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Thermodynamic} properties of spin ladders with cyclic exchange},
volume = {67},
year = {2003}
}
@article{faucris.107685204,
abstract = {In this work we give a consistent picture of the thermodynamic properties of bosons in the Mott insulating phase when loaded adiabatically into one-dimensional optical lattices. We find a crucial dependence of the temperature in the optical lattice on the doping level of the Mott insulator. In the undoped case, the temperature is of the order of the large onsite Hubbard interaction. In contrast, at a finite doping level the temperature jumps almost immediately to the order of the small hopping parameter. These two situations are investigated on the one hand by considering limiting cases like the atomic limit and the case of free fermions. On the other hand, they are examined using a quasi-particle conserving continuous unitary transformation extended by an approximate thermodynamics for hardcore particles.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Reischl A, Uhrig GS},
doi = {10.1140/epjd/e2006-00003-9},
faupublication = {no},
journal = {European Physical Journal D},
pages = {343-352},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Thermodynamics} of adiabatically loaded cold bosons in the {Mott} insulating phase of one-dimensional optical lattices},
volume = {38},
year = {2006}
}
@article{faucris.108646384,
abstract = {For many-particle systems defined on lattices we investigate the global structure of effective Hamiltonians and observables obtained by means of a suitable basis transformation. We study transformations which lead to effective Hamiltonians conserving the number of excitations. The same transformation must be used to obtain effective observables. The analysis of the structure shows that effective operators give rise to a simple and intuitive perspective on the initial problem. The systematic calculation of n-particle irreducible quantities becomes possible constituting a significant progress. Details on how to implement the approach perturbatively for a large class of systems are presented.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Knetter C, Schmidt KP, Uhrig GS},
faupublication = {no},
journal = {Journal of Physics A: Mathematical and General},
pages = {7889-7907},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{The} structure of operators in effective particle-conserving models},
volume = {36},
year = {2003}
}
@article{faucris.248107194,
abstract = {We provide numerical evidence in favor of spontaneous chiral symmetry breaking and the concomitant appearance of an Abelian chiral spin liquid for three-component fermions on the triangular lattice described by an SU(3) symmetric Hubbard model with hopping amplitude -t (t > 0) and on-site interaction U. This chiral phase is stabilized in the Mott phase with one particle per site in the presence of a uniform pi flux per plaquette, and in the Mott phase with two particles per site without any flux. Our approach relies on effective spin models derived in the strong-coupling limit in powers of t/U for general SU(N) and arbitrary uniform charge flux per plaquette, which are subsequently studied using exact diagonalizations and variational Monte Carlo simulations for N = 3, as well as exact diagonalizations of the SU(3) Hubbard model on small clusters. Up to third order in t/U, and for the time-reversal symmetric cases (flux 0 or pi), the low-energy description is given by the J-K model with Heisenberg coupling J and real ring exchange K. The phase diagram in the full J-K parameter range contains, apart from three already known, magnetically long-range ordered phases, two previously unreported phases: (i) a lattice nematic phase breaking the lattice rotation symmetry and (ii) a spontaneous time-reversal and parity symmetry breaking Abelian chiral spin liquid. For the Hubbard model, an investigation that includes higher-order itinerancy effects supports the presence of a phase transition inside the insulating region, occurring at (t/U)(c) approximate to 0.07 [(U/t)(c) approximate to 13] between the three-sublattice magnetically ordered phase at small t/U and this Abelian chiral spin liquid.},
author = {Boos, Carolin and Ganahl, C. J. and Lajko, M. and Nataf, P. and Lauchli, A. M. and Penc, Karlo and Schmidt, Kai Phillip and Mila, F.},
doi = {10.1103/PhysRevResearch.2.023098},
faupublication = {yes},
journal = {Physical Review Research},
note = {CRIS-Team WoS Importer:2021-01-22},
peerreviewed = {Yes},
title = {{Time}-reversal symmetry breaking {Abelian} chiral spin liquid in {Mott} phases of three-component fermions on the triangular lattice},
volume = {2},
year = {2020}
}
@article{faucris.122456004,
abstract = {We examine the zero-temperature phase diagram of the two-dimensional Levin-Wen string-net model with Fibonacci anyons in the presence of competing interactions. Combining high-order series expansions around three exactly solvable points and exact diagonalizations, we find that the non-Abelian doubled Fibonacci topological phase is separated from two nontopological phases by different second-order quantum critical points, the positions of which are computed accurately. These trivial phases are separated by a first-order transition occurring at a fourth exactly solvable point where the ground-state manifold is infinitely many degenerate. The evaluation of critical exponents suggests unusual universality classes. DOI: 10.1103/PhysRevLett.110.147203},
author = {Schulz, Marc Daniel and Dusuel, Sebastien and Schmidt, Kai Phillip and Vidal, Julien},
doi = {10.1103/PhysRevLett.110.147203},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
title = {{Topological} {Phase} {Transitions} in the {Golden} {String}-{Net} {Model}},
volume = {110},
year = {2013}
}
@article{faucris.108561684,
abstract = {We explore the properties of magnetic excitations on the trellis lattice which is relevant for the so-called telephone-number compounds A(14)Cu(24)O(41) with A=(La,Sr,Ca) and the system SrCu2O3. The trellis lattice consists of two-leg ladders which are coupled in a strongly frustrated fashion. We use the effective model obtained for a single two-leg spin ladder to calculate the two-dimensional one-triplon dispersion and the corresponding one-triplon contribution to the dynamical structure factor. Special attention is laid on signatures of the frustrating interladder magnetic exchange. A detailed suggestion is made for an experimental detection of this exchange in inelastic neutron-scattering experiments.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Schmidt KP, Uhrig GS},
doi = {10.1103/PhysRevB.75.224414},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Two} dimensionality of magnetic excitations on the trellis lattice: ({La},{Sr},{Ca})(14){Cu24O41} and {SrCu2O3}},
volume = {75},
year = {2007}
}
@article{faucris.107643844,
abstract = {Using density matrix renormalization group (DMRG) and perturbative continuous unitary transformations (PCUTs), we study the magnetization process in a magnetic field for all coupling strengths of a quasi-1D version of the 2D Shastry-Sutherland lattice, a frustrated spin tube made of two orthogonal dimer chains. At small inter-dimer coupling, plateaus in the magnetization appear at 1/6, 1/4, 1/3, 3/8, and 1/2. As in 2D, they correspond to a Wigner crystal of triplons. However, close to the boundary of the product singlet phase, plateaus of a new type appear at 1/5 and 3/4. They are stabilized by the localization of bound states of triplons. Their magnetization profile differs significantly from that of single-triplon plateaus and leads to specific NMR signatures. We address the possibility to stabilize such plateaus in further geometries by analyzing small finite clusters using exact diagonalizations and the PCUTs. Copyright (C) EPLA, 2011},
author = {Manmana, S. R. and Picon, J. -D. and Schmidt, Kai Phillip and Mila, F.},
doi = {10.1209/0295-5075/94/67004},
faupublication = {no},
journal = {EPL - Europhysics Letters},
peerreviewed = {Yes},
title = {{Unconventional} magnetization plateaus in a {Shastry}-{Sutherland} spin tube},
volume = {94},
year = {2011}
}
@article{faucris.108646604,
abstract = {Based on a two-dimensional model of coupled two-leg spin ladders, we derive a unified picture of recent neutron scattering data of stripe-ordered La15/8Ba1/8CuO4, namely, of the low-energy magnons around the superstructure satellites and of the triplon excitations at higher energies. The resonance peak at the antiferromagnetic wave vector Q(AF) in the stripe-ordered phase corresponds to a saddle point in the dispersion of the magnetic excitations. Quantitative agreement with the neutron data is obtained for J=130-160 meV and J(cyc)/J=0.2-0.25.},
author = {Schmidt, Kai Phillip and et al.},
author_hint = {Uhrig GS, Schmidt KP, Gruninger M},
doi = {10.1103/PhysRevLett.93.267003},
faupublication = {no},
journal = {Physical Review Letters},
peerreviewed = {Yes},
support_note = {Author relations incomplete. You may find additional data in field 'author{\_}hint'},
title = {{Unifying} magnons and triplons in stripe-ordered cuprate superconductors},
volume = {93},
year = {2004}
}
@article{faucris.123352504,
abstract = {We combine the results of perturbative continuous unitary transformations with a mean-field calculation to determine the evolution of the single-mode, i.e., one-triplon, contribution to the dynamic structure factor of a two-leg S=1/2 ladder on increasing temperature from zero to a finite value. The temperature dependence is induced by two effects: (i) no triplon can be excited on a rung where a thermally activated triplon is present and (ii) conditional excitation processes take place if a thermally activated triplon is present. Both effects diminish the one-triplon spectral weight upon heating. It is shown that the second effect is the dominant vertex correction in the calculation of the dynamic structure factor. The matrix elements describing the conditional triplon excitation in the two-leg Heisenberg ladder with additional four-spin ring exchange are calculated perturbatively up to order 9. The calculated results are compared to those of an inelastic neutron scattering experiment on the cuprate-ladder compound La(4)Sr(10)Cu(24)O(41) showing convincing agreement for established values of the exchange constants.},
author = {Exius, I. and Schmidt, Kai Phillip and Lake, Bella and Tennant, D. A. and Uhrig, G. S.},
doi = {10.1103/PhysRevB.82.214410},
faupublication = {no},
journal = {Physical Review B},
peerreviewed = {Yes},
title = {{Vertex} corrections in the dynamic structure factor in spin ladders},
volume = {82},
year = {2010}
}