Enhancement of efficiency in the Dicke model quantum heat engine

We analyze a quantum heat engine described by the full Dicke model. The system exhibit quantum phase transitions under certain conditions. We consider the system performing a Stirling thermodynamic cycle. We obtain an enhancement of efficiency when during the cycle the coupling parameter cross a critical value. We analyze the effect of unbalance between rotating and counter-rotating terms in the model. The maximum efficiency is obtained when the contributions of the counter-rotating and rotating terms are equal. The relation between ground state degeneracy, related to the quantum phase transition, and maximum efficiency is investigated.