M. Niemetz et al., The oscillating magnetic microsphere: a tool for investigating vorticity in superconductors and superfluids, PHYSICA B, 280(1-4), 2000, pp. 559-560
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.