High Quality Tuning Forks in Superfluid 3He-B Below 200 μK

We have investigated the influence of the damping force acting on high quality tuning forks (Q∼106) of different sizes and geometries in superfluid 3He-B at temperatures below 200 μK and a pressure of 0.1 bar. The measurements show that at low velocities, the damping of the largest fork expressed in terms of its resonance characteristic width Δf2 rises up as its velocity increases. This is in contradiction to the damping of the fork due to Andreev reflection and it may be caused by the interaction of this fork with excitations trapped in the Andreev bond states. We present our preliminary experimental results.