THERMALLY ACTIVATED FLUX MOTION IN YBA2CU3O7 AND BI2SR2CA2CU3O10 SUPERCONDUCTORS

A thermally activated flux motion model is proposed to explain the microwave magneto absorption in YBau307 and Bi2Sr2CausOlo (axis c - 37 A, T, - 110 K) sintered samples. The physical quantity suitable for a good description of the phenomenon is the derivative of the microwave surface resistance AR(H)/A.H at low magnetic field. New measurements in Bi2Sr2Ca2CusOlo are presented. Under the hypothesis that the free fluxons suffer of a viscous force proportional to the flux on velocity, we are able to fit our data up to 0.9Tc using the Bardeen-Stephen viscous coefficient 0: Hc2 (t). To fit the experimental results close to T, a phenomenological crossover term proportional to (1 - T/T) is added to Bs (t) in the expression of the viscosity coefficient 7 for the vortex motion. © 1990 The Japan Society of Applied Physics.