Dicke model with disordered spin-boson couplings

We introduce and study the disordered Dicke model in which the spin-boson couplings are drawn from a random distribution with some finite width. Regarding the quantum phase transition, we show that, when the standard deviation sigma of the coupling strength gradually increases, the critical value of the mean coupling strength mu gradually decreases, and after a certain sigma there is no quantum phase transition at all; the system always lies in the superradiant phase. We derive an approximate expression for the quantum phase transition in the presence of disorder in terms of mu and sigma, which we numerically verify. Studying the thermal phase transition in the disordered Dicke model, we obtain an analytical expression for the critical temperature in terms of the mean and standard deviation of the coupling strength. We observe that, even when the mean of the coupling strength is zero, there is a finite temperature transition if the standard deviation of the coupling is sufficiently high. Disordered couplings in the Dicke model will exist in quantum dot superlattices, and we also sketch how they can be engineered and controlled with ultracold atoms or molecules in a cavity.