Nonlinear quantum heat transfer in hybrid structures: sufficient conditions for thermal rectification.

We present a unified description of heat flow in two-terminal hybrid quantum systems. Using simple models, we analytically study nonlinear aspects of heat transfer between various reservoirs--metals, solids, and spin baths--mediated by the excitation and the relaxation of a central (subsystem) mode. We demonstrate rich nonlinear current-temperature characteristics, originating from either the molecular anharmonicity or the reservoir (complex) energy spectra. In particular, we establish sufficient conditions for thermal rectification in two-terminal junctions. We identify two classes of rectifiers. In type- A rectifiers the energy-dependent density of states of the reservoirs are dissimilar. In type- B rectifiers the baths are identical, but include particles whose statistics differ from that of the subsystem, to which they asymmetrically couple. Nonlinear heat flow and specifically thermal rectification are thus ubiquitous effects that could be observed in a variety of systems--phononic, electronic, and photonic.