Evaporative cooling of He-3-He-4 mixture films

Heat transport by saturated He-3-He-4 films has been studied at temperature s 50..350 mK and the bulk concentration of He-3 ranging from 0.1 ppm to 5%. The cooling of the film, when locally heated above 160 mK, is mainly via 2 D flow of surface He-3 from colder area followed by evaporation of He-3. At certain heating power the 2D flow becomes a bottleneck, the heated spot ru ns out of He-3 and its temperature abruptly increases. The critical power i s neatly proportional do the surface density of He-3. For higher He-3 conce ntrations another distinct step in temperature has been observed at a lower heating power. It is attributed to the existence of an excited He-3 surfac e state whose population starts at He-3 surface density of 3.5 x 10(14) cm( -2). The second state is located about 1.2 K higher in energy than the grou nd state and provides an additional channel for the 2D flow of He-3.