Long-range super-Planckian heat transfer between nanoemitters in a resonant cavity

We study radiative heat transfer between two nanoemitters placed inside different types of closed cavities by means of a fluctuational-electrodynamics approach. We highlight a very sharp dependence of this transfer on cavity width and connect this to the matching between the material-induced resonance and the resonant modes of the cavity. In resonant configurations, this allows for an energy-flux amplification of several orders of magnitude with respect to the one exchanged between two emitters in vacuum as well as between two blackbodies, even at separation distances much larger than the thermal wavelength. On the other hand, variations of the cavity width by a few percent allow a reduction of the flux by several orders of magnitude and even a transition to inhibition compared to the vacuum scenario. Our results pave the way to the design of thermal waveguides for the long-distance transport of super-Planckian heat flux and selective heat transfer in many-body systems.