Lenk K, Eckstein M (2020)
Publication Type: Journal article
Publication year: 2020
Book Volume: 102
Article Number: 205129
Journal Issue: 20
DOI: 10.1103/PhysRevB.102.205129
We investigate the equilibrium state and the collective modes of an excitonic insulator (EI) in a Fabry-Pérot cavity. In an EI, two bands of a semiconductor or semimetal spontaneously hybridize due to the Coulomb interaction between electrons and holes, leading to the opening of a gap. The coupling to the electromagnetic field reduces the symmetry of the system with respect to phase rotations of the excitonic order parameter from U(1) to Z2. While the reduction to a discrete symmetry would, in general, lead to a gapped phase mode and enhance the stability of the ordered phase, the coupling to the cavity leaves the mean-field ground state unaffected. Its energy remains invariant under U(1) phase rotations, in spite of the lower Z2 symmetry imposed by the cavity. In a dipolar gauge, this can be traced back to the balancing of the linear light-matter coupling and the dipolar self-interaction at zero frequency. At nonzero frequency, however, the collective excitations do reflect the lower Z2 symmetry. While our model is studied using a mean-field decoupling of the light-matter approximation (which is exact in the single-mode limit), the results show that fluctuations beyond mean field could play a crucial role in finding the true phase at finite temperature.
APA:
Lenk, K., & Eckstein, M. (2020). Collective excitations of the U (1)-symmetric exciton insulator in a cavity. Physical Review B, 102(20). https://doi.org/10.1103/PhysRevB.102.205129
MLA:
Lenk, Katharina, and Martin Eckstein. "Collective excitations of the U (1)-symmetric exciton insulator in a cavity." Physical Review B 102.20 (2020).
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