Vibrating-wire measurements in vacuum, liquid³He and liquid³He-⁴He mixtures

Abstract

In order to overcome the “200µK - barrier” in the refrigeration of liquid³He-⁴He mixtures we have constructed an experimental cell using only pure materials to minimize possible origins for heat leaks into the liquid. With this arrangement we were able to cool a saturated6.8%- mixture to a temperature of ≤150µK. A vibrating wire which was immersed in pure³He floating on top of the phase-separated mixture was used as a thermometer. This wire was calibrated in a second experiment with pure³He only in the cell. In superfluid³He-B at T≤0.15 mK the damping of the wire due to the quasiparticles becomes very small, and we observe typical characteristics of the vacuum damping of the wire which was extensively examined before filling any liquid into the cell.