We investigate the dynamical instabilities of a resonator coupled to a superconducting single-electron transistor (SSET) tuned to the Josephson quasiparticle (JQP) resonance. Starting from the quantum master equation of the system, we use a standard semiclassical approximation to derive a closed set of mean field equations which describe the average dynamics of the resonator and SSET charge. Using amplitude and phase coordinates for the resonator and assuming that the amplitude changes much more slowly than the phase, we explore the instabilities which arise in the resonator dynamics as a function of coupling to the SSET, detuning from the JQP resonance and the resonator frequency. We find that the locations (in parameter space) and sizes of the limit cycle states predicted by the mean field equations agree well with numerical solutions of the full master equation for sufficiently weak SSET–resonator coupling. The mean field equations also give a good qualitative description of the set of dynamical transitions in the resonator state that occur as the coupling is progressively increased.
We examine the paper basic equations of T. Domański and K.I. Wysokiński [Phys. Rev. B {\bf 59}, 173 (1999)], who calculated the critical superconducting temperature, T_c, in function of Coulomb correlations (U) for s- and d-wave order parameter symmetries. We give physical arguments, based on general theory of perturbation, that in their gap equation the Coulomb repulsion is counted twice. Because of that, we write down the right mean-field gap equation and solve it by using a normal state one-particle Green function which gives a Mott metal-insulator transition for
T>Tc and which is valid for any value of U. In consequence, we are not allowed to use mean-field theory for both U and pairing (V). Our numerical results for T_c vs U shows that U is detrimental to superconductivity which agrees with the numerical results of Domański and Wysokiński. Our results can be extended to any value of the pairing interaction if superconducting pairing fluctuations are taken into account.