The open driven two-level system at conical intersections of quasienergies

We study the stationary state of an ac-driven two-level system under particle exchange with a fermionic environment. A particular question addressed is whether there exist limits in which the populations of the Floquet states are determined by their quasienergies or their mean energies, respectively. The focus lies on parameters in the vicinity of conical intersections of quasienergies at which the two kinds of energies behave rather differently, such that the characteristics of the two intuitive limits are most pronounced. A main finding is a crossover from a Floquet-Gibbs-like state at low temperatures to a mean-energy dominated state at intermediate temperatures. Analytical estimates are confirmed by numerical calculations.