ANOMALY IN MAGNETIC-FIELD DEPENDENCE OF MICROWAVE-ABSORPTION AT VORTEX PINNING TO DEPINNING TRANSITION IN BI2-XPBXSR2CA2CU3O10 SUPERCONDUCTOR

Previous measurements have shown that the pinning to depinning transit ion of vortices in the copper oxide superconductors can be observed at constant temperature by a change in the slope of the linear dependenc e of the microwave absorption on square-root B at a field B'. In this work measurements of the magnetic field dependence of the microwave ab sorption in N = 3 Bi-Pb-Sr-Ca-Cu-O at 9.2 GHz at constant temperature at small magnetic field intervals in the transition region reveal a ma rked departure from the square-root B dependence. On the other hand me asurements of the temperature dependence of the microwave absorption d ue to vortex dissipation at constant magnetic field at one degree temp erature intervals show an abrupt transition inconsistent with a second order character required for a transition from a vortex glass to a fl uid. The anomaly in the magnetic field dependence of the absorption at the depinning to pinning transition is shown to be consistent with a magnetic field dependent distribution of pinning barriers in the flux solid leading to an intermediate phase consisting of a mixture of vort ex fluid and solid.