Critical Spectrum and Quantum Criticality in the Two-Photon Rabi-Stark Model

Abstract: We investigate the spectral properties and quantum criticality of the two-photon Rabi-Stark model. Using the exact solution of this model, we rigorously derive a condition for complete spectral collapse, where all bound states vanish. In this case, the energy gap closes at a critical coupling, signaling a continuous quantum phase transition. The corresponding gap exponent differs from those in both the one-photon Rabi-Stark model and the quantum Rabi model, suggesting a distinct universality class. Furthermore, mapping to an inverse square potential well reveals an infinite number of discrete bound states at high two-level atomic frequencies, with their energies approaching the threshold exponentially. Our results offer new insights into novel spectral phenomena in nonlinear quantum Rabi models, with potential implications for experimental realizations in circuit QED and trapped ion systems.