Interactions, superconducting Tc, and fluctuation magnetization for two coupled dots in the crossover between the Gaussian orthogonal and unitary ensembles

We study a system of two quantum dots connected by a hopping bridge. Both the dots and connecting region are assumed to be in universal crossover regimes between Gaussian orthogonal and unitary ensembles. Using a diagrammatic approach appropriate for energy separations much larger than the level spacing, we obtain the ensemble-averaged one- and two-particle Green's functions. It turns out that the diffusion and Cooperon parts of the two-particle Green's function can be described by separate scaling functions. We then use this information to investigate a model interacting system in which one dot has an attractive s -wave reduced Bardeen-Cooper-Schrieffer interaction, while the other is noninteracting but subject to an orbital magnetic field. We find that the critical temperature is nonmonotonic in the flux through the second dot in a certain regime of interdot coupling. Likewise, the fluctuation magnetization above the critical temperature is also nonmonotonic in this regime, can be either diamagnetic or paramagnetic, and can be deduced from the Cooperon scaling function.