FLUX-FLOW DISSIPATION IN GDBA2CU3O7 AT MICROWAVE-FREQUENCIES

We report measurements of the magnetic-field-dependent microwave dissipation in high-purity GdBa2Cu3O7-delta pellets with different grain sizes, by means of a cavity technique with no field modulation. The dissipation is due to the intragranular viscous vortex motion at fields greater than or similar to 0.02 T, and to weak-links at low fields (H < 0.02 T). The vortex-motion dissipation increases linearly with the magnetic field for temperatures T not too close to T(c), whereas near T(c) it bends and approaches a square-root behavior at higher fields. Moreover, a temperature-independent resonance is detected at H almost-equal-to 0.8 T, which is attributed to the Gd3+ ions, consistent with the data reported in literature. A phenomenological model for the flux-flow-induced surface resistance can take into account the linear behavior with the field and the bending near T(c), as well as the temperature dependence of the slope of the linear absorption not too close to T(c). The same results are obtained by a recent theory by Coffey and Clem for the surface resistance of a type-II superconductor in a magnetic field, in its low-field, zero-pinning limit. It is then confirmed that the field absorption (for H > 0.02 T) has to be ascribed to the free motion of flux lines.