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

Substantial field-dependent microwave absorption is found in superconducting materials of the YBa2Cu3O7 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 G/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 μg specimens can be performed in a few seconds, this property may prove useful for material characterization