Abstract
We show that strong electron–electron interactions in quantum materials can give rise to electronic transitions that couple strongly to cavity fields, and collective enhancement of these interactions can result in ultrastrong effective coupling strengths. As a paradigmatic example we consider a Fermi–Hubbard model coupled to a single-mode cavity and find that resonant electron-cavity interactions result in the formation of a quasi-continuum of polariton branches. The vacuum Rabi splitting of the two outermost branches is collectively enhanced and scales with , where L is the number of electronic sites, and the maximal achievable value for geff is determined by the volume of the unit cell of the crystal. We find that geff for existing quantum materials can by far exceed the width of the first excited Hubbard band. This effect can be experimentally observed via measurements of the optical conductivity and does not require ultrastrong coupling on the single-electron level. Quantum correlations in the electronic ground state as well as the microscopic nature of the light–matter interaction enhance the collective light–matter interaction compared to an ensemble of independent two-level atoms interacting with a cavity mode.
Highlights
Collective phenomena in light–matter interactions are of tremendous interest in quantum physics
Original content from this We show that strong electron–electron interactions in quantum materials can give rise to electronic work may be used under transitions that couple strongly to cavity fields, and collective enhancement of these interactions can the terms of the Creative Commons Attribution 3.0 result in ultrastrong effective coupling strengths
We find that the optical conductivity of this system features two peaks that are separated in energy by the collectively enhanced vacuum Rabi frequency geff μ 2L, where L is the number of electronic sites
Summary
Collective phenomena in light–matter interactions are of tremendous interest in quantum physics. The strong coupling of magnetic excitations to microwave cavities was investigated in [9,10,11,12,13,14], and two-dimensional electron gases coupled to THz cavities were studied in [15,16,17,18,19,20] In all these systems [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20], Coulomb interactions between electrons play a minor role and are not directly involved in the formation of polaritons
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