Harnessing thermal waves for heat pumping

Based on the nonlinear propagation of thermal diffusion waves, we demonstrate the existence of a net heat current even in the absence of a mean temperature gradient. Unlike the steady-state heat current, this current of thermal waves is not driven by the material thermal conductivity, but rather by its temperature derivative. The heat current propagates outward or inward of its excitation source when the thermal conductivity increases or decreases with temperature, respectively. The modulation of the heat flux direction can thus be achieved by varying the material temperature around the thermal conductivity peak exhibited by various dielectrics. For a silicon plate with a thermal conductivity peak at 40 K that supports the propagation of thermal waves with an excitation amplitude of 10 K, we observe a net heat flux exceeding 600.1em0exW0.1em0excm^-2 at 20 K and falling below -20 W cm^-2 at 65 K. Larger excitation amplitudes allow higher or lower heat fluxes to be achieved, which makes them easy to observe and potentially apply for harvesting or evacuating thermal energy in systems undergoing natural temperature fluctuations on earth and in outer space.