Can Warmer than Room Temperature Electrons Levitate Above a Liquid Helium Surface?

We address the problem of overheating of electrons trapped on the liquid helium surface by cyclotron resonance excitation. Previous experiments suggest that electrons can be heated to temperatures up to 1000 K, more than three orders of magnitude higher than the temperature of the helium bath in the sub-Kelvin range. In this work we attempt to discriminate between a redistribution of thermal origin and other out-of-equilibrium mechanisms that would not require so high temperatures like resonant photo-galvanic effects or negative mobilities. We argue that for a heating scenario the direction of the electron flow under cyclotron resonance can be controlled by the shape of the initial electron density profile, with a dependence that can be modeled accurately within the Poisson–Boltzmann theory framework. This provides an self-consistency check to probe whether the redistribution is indeed consistent with a thermal origin. We find that while our experimental results are consistent with the Poisson–Boltzmann theoretical dependence, some deviations suggest that other physical mechanisms can also provide a measurable contribution. Analyzing our results with the heating model we find that the electron temperatures increase with electron density under the same microwave irradiation conditions. This unexpected density dependence calls for a microscopic treatment of the energy relaxation of overheated electrons.