Evaporative Cooling of 3He-4He Mixture Films

Heat transport by saturated ³He-⁴He films has been studied at temperatures 50..350 mK and the bulk concentration of ³He ranging from 0.1 ppm to 5%. The cooling of the film, when locally heated above 160 mK, is mainly via 2D flow of surface ³He from colder area followed by evaporation of ³He. At certain heating power the 2D flow becomes a bottleneck, the heated spot runs out of ³He and its temperature abruptly increases. The critical power is nearly proportional to the surface density of ³He. For higher ³He concentrations another distinct step in temperature has been observed at a lower heating power. It is attributed to the existence of an excited ³He surface state whose population starts at ³He surface density of 3.5×10¹⁴ cm^–2. The second state is located about 1.2 K higher in energy than the ground state and provides an additional channel for the 2D flow of ³He.