Spontaneous Symmetry Breaking of Cavity Vacuum and Emergent Gyrotropic Effects in Embedded moir\'{e} Superlattices

TL;DR

This paper demonstrates that ultrastrong coupling between a moiré superlattice and a cavity vacuum can spontaneously break spatial symmetry, resulting in degenerate ground states with opposite gyrotropic effects, tunable by cavity parameters.

Contribution

It introduces a new paradigm where cavity vacuum induces spontaneous symmetry breaking in electronic systems, specifically in moiré superlattices, leading to controllable gyrotropic phenomena.

Findings

Spontaneous parity symmetry breaking in electronic and cavity states.

Degenerate ground states with opposite gyrotropic Hall effects.

Tunable gyrotropic responses via cavity polarization and interlayer bias.

Abstract

In an electronic system, spontaneous symmetry breaking can arise from many-body interaction between electrons, leading to degenerate ground states distinguishable by emergent effects otherwise prohibited by the symmetry. Here we show that ultrastrong coupling of a mesoscopic electronic system to the vacuum of a cavity resonator can lead to another paradigm of spontaneous breaking of spatial symmetries in both systems. As a pertinent example, we consider the orbital gyrotropic effects in a moir\'{e} superlattice embedded in a THz split ring cavity resonator. Our mean-field and exact diagonalization calculations consistently demonstrate a spontaneous parity symmetry breaking in both the electronic ground state and the cavity vacuum, leading to two degenerate hybrid ground states distinguished by their opposite orbital gyrotropic Hall and magnetic effects. These sizable responses in the cavity-embedded moir\'{e} superlattice are highly tunable by both the cavity field polarization and interlayer bias on the moir\'{e} superlattice, providing an advanced platform for manipulating gyrotropic effects.