Andreev refrlection and proximity effect at the phase boundary of ^3He-^4He mixutures

点击翻译按钮获取中文摘要

A phase separated boundary in the ^3He^<-4> solution is an ideally flat surface. Especially below 1 mK, the upper phase is a P-wave superfluid and the Andreev scattering is theoretically predicted to exist there. To observe it, the liquid has to be cooled down to the ballistic temperature region. We made an excellent heat exchanger, in which a combination of fine silver 2 and platinum powder is sintered to have a thickness of 0.5 mm (effective surface area 350m^2) , resulting into the lowest temperature of about 170muK in pure liquid ^3@He. We then developed a blackbody radiator made of a silver hemisphere with 4 mm in inner diameter and 1 mm in thickness. On its surface there exists an orifice of 0.2 mm in diameter, whose axis is oriented for the ^3@He quasiparticle beam to hit the boundary surface at a small angle of 20 degree. Inside the radiator two semicircular vibrating wires, one to produce a quasiparticle beam and the other to measure the quasiparticle density there. To test the system, the free surface of superfluid ^3@He B was used. The emitted energy flux from the blackbody is proportional to an applied heater power in the heater vibrating wire for the cases with and without the surface. However there is a clear difference between the slopes for both cases. This fact indicates that the quantum Andreev reflection clearly exists at the free surface of superfluid ^3@He B.The obseved reflection rare is smaller than expected but can be explained with a quasiclassical theory if we take into account the loss due to the diffuse scattering at the orifice. The shape of the orifice is found very important to make a quatitative analysis. Further experiments are planned for various aspect ratios and for the different surface conditions of the orifice.