The oscillating magnetic microsphere: a tool for investigating vorticity in superconductors and superfluids

A spherical magnet (radius 0.1 mm) is levitating between the superconductin g electrodes of a parallel plate capacitor. Small vertical oscillations of the magnet about its equilibrium position can be excited. From the damping of the oscillations (in vacuum) dissipative processes due to flux motion in the superconductors have been measured. For YBCO thin-film electrodes at 4 K Q-values are 10(6) implying a surface impedance of the order of 10(-13) O mega. At larger amplitudes nonlinear flux dynamics due to current-dependent pinning energies is observed. For Nb electrodes the e-values exceed 10(6). When the capacitor is filled with superfluid helium laminar and turbulent drag of the oscillating sphere can be studied. Below 0.5 K we find a range of intermediate driving forces where intermittent switching between potenti al how and turbulence occurs. A statistical analysis of the data yields the probability distribution for either direction. (C) 2000 Elsevier Science B .V. Al rights reserved.