R. Grosser et al., Vortex motion in superconducting YBa2Cu3O7-delta inferred from the dampingof the oscillations of a levitating magnetic microsphere, J L TEMP PH, 119(5-6), 2000, pp. 723-742
The damping of the oscillations of a small permanent magnet (spherical shap
e, radius 0.1 mm) levitating between two parallel YBCO surfaces is measured
as a function of oscillation amplitude and temperature. The losses in the
samples (epitaxial thin films, bulk granular and bulk melt-textured) are an
alyzed in terms of oscillating shielding currents flowing through trapped f
lux lines whose motion gives rise to electric fields. We find dissipation t
o originate from different mechanisms of flux dynamics. At small amplitudes
there is a linear regime described by a surface resistance varying from 10
(-9) Omega for the bulk samples down to 10(-13) Omega for the thin films at
low temperatures. With increasing amplitude various nonlinear regimes are
observed, firstly collective pinning with diverging energy barriers, second
ly in bulk samples above 85 K hysteric damping, and finally in thin films e
xponentially large losses which can be described by pinning energies vanish
ing linearly at large currents.