A Novel Floquet Superradiant Phase Transition in Landau Polaritons

Superradiant quantum phase transitions (SQPTs) have been predicted in light–matter coupled systems operating in the ultrastrong coupling regime, most notably within the Dicke model. In realistic systems, however, the presence of the diamagnetic term in the light–matter Hamiltonian generally imposes a no-go theorem that prevents such transitions. Here, we demonstrate that Floquet driving can circumvent this constraint in a Landau polariton system, leading to a novel SQPT. We consider a two-dimensional electron gas embedded in a terahertz cavity and subjected to a perpendicular static magnetic field. An additional time-periodic magnetic field modulates both the cyclotron resonance frequency and the light–matter coupling strength, while leaving the diamagnetic term unchanged. Using the Floquet–Magnus expansion, we show that an effective static component of the light–matter coupling drives the system into an SQPT once a critical threshold is exceeded, characterized by a macroscopic occupation of the cavity mode and collective excitations across Landau levels. This scenario, unique to Landau polariton systems, provides a route to realizing an SQPT that is otherwise forbidden in equilibrium by no-go theorems.