  We propose a mechanism to substantially rectify radiative heat flow by
matching thin films of metal-to-insulator transition materials and polar
dielectrics in the electromagnetic near field. By leveraging the distinct
scaling behaviors of the local density of states with film thickness for metals
and insulators, we theoretically achieve rectification ratios over 140-a
10-fold improvement over the state of the art-with nanofilms of vanadium
dioxide and cubic boron nitride in the parallel-plane geometry at
experimentally feasible gap sizes (~100 nm). Our rational design offers
relative ease of fabrication, flexible choice of materials, and robustness
against deviations from optimal film thicknesses. We expect this work to
facilitate the application of thermal diodes in solid-state thermal circuits
and energy conversion devices.
