Probing Bogoliubov Quasiparticles in Superfluid ^3 He with a ‘Vibrating-Wire Like’ MEMS Device

We have measured the interaction between superfluid ^3 He–B and a micro-machined goalpost-shaped device at temperatures below 0.2 T_c . The measured damping follows well the theory developed for vibrating wires, in which the Andreev reflection of quasiparticles in the flow field around the moving structure leads to a nonlinear frictional force. At low velocities, the damping force is proportional to velocity, while it tends to saturate for larger excitations. Above a velocity of 2.6 mm s ^-1 , the damping abruptly increases, which is interpreted in terms of Cooper-pair breaking. Interestingly, this critical velocity is significantly lower than that reported with other mechanical probes immersed in superfluid ^3 He. Furthermore, we report on a nonlinear resonance shape for large motion amplitudes that we interpret as an inertial effect due to quasiparticle friction, but other mechanisms could possibly be invoked as well.