Field-dependent microwave absorption in high T sub c superconductors.

Substantial field-dependent microwave absorption is found in superconducting materials of the YBa sub 2 Cu sub 3 O sub 7 class. Abrupt changes occur over a 20 G range at zero field which are qualitatively different for polycrystalline and single- crystal materials. A strong hysteresis in polycrystalline samples is quenched by modulation fields of 0.2 T sub (pp). Formally the dissipative processes observed may be described in terms of flux relaxation, but the underlying physical mechanisms remain unresolved. As measurements on 100 mu g specimens can be performed in seconds, this property may prove useful for material characterization.