Dresden 2020 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 23: Nonequilibrium Quantum Many-Body Systems 1 (joint session TT/DY)

TT 23.3: Talk

Tuesday, March 17, 2020, 10:00–10:15, HSZ 204

Collective behavior of an excitonic insulator in the quantum electromagnetic field — •Katharina Lenk and Martin Eckstein — Department of Physics, University of Erlangen-Nuremnberg, 91058 Erlangen, Germany

The strong coupling of light and matter in cavity quantum electrodynamics provides new avenues to engineer properties of complex mate- rials. In this talk, we investigate the behavior of a so-called excitonic insulator (EI) in a cavity. The EI is a phase driven by the Coulomb interaction, in which two bands of a semiconductor or semimetal spontaneously hybridize, leading to the opening of a gap. We consider the particular case in which the EI implies a breaking of the U(1) symmetry related to the conservation of charge in the individual bands. The coupling of a generic bosonic mode, such as the coordinate of a phonon, reduces the symmetry, adds a mass to the phase mode, and stabilizes the symmetry-broken phase. While this suggests that a similar mechanism may be at work for the coupling of the EI to a cavity mode, we show that the balancing of dipolar interactions and the dipolar light-matter coupling leaves the phase mode massless, in spite of the breaking of the U(1) symmetry.