From Floquet to Dicke: Quantum Spin Hall Insulator Interacting with Quantum Light

Gulácsi, Balázs and Dóra, Balázs (2015) From Floquet to Dicke: Quantum Spin Hall Insulator Interacting with Quantum Light. PHYSICAL REVIEW LETTERS, 115 (16). ISSN 0031-9007

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Abstract

Time-periodic perturbations due to classical electromagnetic fields are useful to engineer the topo- logical properties of matter using the Floquet theory. Here we investigate the effect of quantized electromagnetic fields by focusing on the quantized light-matter interaction on the edge state of a quantum spin-Hall insulator. A Dicke-type superradiant phase transition occurs at arbitrary weak coupling, the electronic spectrum acquires a finite gap and the resulting ground state manifold is topological with Chern number ±1. When the total number of excitations is conserved, a photocur- rent is generated along the edge, being pseudo-quantized as ω ln(1/ω) in the low frequency limit, and decaying as 1/ω for high frequencies with ω the photon frequency. The photon spectral function exhibits a clean Goldstone mode, a Higgs like collective mode at the optical gap and the polariton continuum.

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