DEPENDENCE OF THE FLUX-CREEP TIME-SCALE ON SAMPLE-SIZE FOR MELT-TEXTURED YBA2CU3O7 BY AC-SUSCEPTIBILITY MEASUREMENTS

The complex ac susceptibility chi was measured as a function of temper ature T, ac field frequency f, and amplitude H-ac for three textured Y Ba2Cu3O7 samples with different sizes d. The effects of f and H-ac on the peak temperature T-p in chi('') were analyzed based on nonlinear f lux diffusion theory. The current-density- (j-) dependent effective ac tivation barriers U(j) proportional to ln(j(0)/j) have been extracted in a time window of milliseconds and an the same as those obtained in other time windows. Flux-creep time scales t(0) of 10(-5) and 10(-7) s have been extracted for the large (d approximate to 1 mm) and small ( d approximate to 10 mu m) samples, respectively. The fact that to incr eases with sample size shows that it is a macroscopic quantity, as pre dicted by the nonlinear flux diffusion theory.