Conventional and unconventional Dicke models: Multistabilities and nonequilibrium dynamics

The Dicke model describes the collective behavior of a sub-wavelength--size ensemble of two-level atoms (i.e., spin-1/2) interacting identically with a single quantized radiation field of a cavity. Across a critical coupling strength it exhibits a zero-temperature phase transition from the normal state to the superradiant phase where the field is populated and the collective spin acquires a nonzero $x$-component, which can be imagined as ferromagnetic ordering of the atomic spins along $x$. Here we introduce a variant of this model where two sub-wavelength--size ensembles of spins interact with a single quantized radiation field with different strengths. Subsequently, we restrict ourselves to a special case where the coupling strengths are opposite (which is unitarily equivalent to equal-coupling strengths). Due to the conservation of the total spin in each ensemble individually, the system supports two distinct superradiant states with $x$-ferromagnetic and $x$-ferrimagnetic spin ordering, coexisting with each other in a large parameter regime. The stability and dynamics of the system in the thermodynamic limit are examined using a semiclassical approach, which predicts non-stationary behaviors due to the multistabilities. At the end, we also perform small-scale full quantum-mechanical calculations, with results consistent with the semiclassical ones.