Very low field flux penetration in halogenated cuprates

Abstract

International audienceThe low-frequency non linear magnetic susceptibility chi(H,T) of YBa2Cu3O7-deltaFx ceramics (delta = 0 and 0.3) is measured in the range 0 <= x <= 0.2, 15K <= T < Tc, with Tc the coherence critical temperature and ac field amplitude 10(-8) T < µ0H0 < 10(-2) T. We find that the real part chi'(H0,T) = f[H1(T,x)/H0] is a universal scaling function for all samples with H1(T,x) = H1(0,x)epsilon(phi) the experimentally determined first flux penetration field, epsilon = 1-T/Tc and phi = 2.7+/-0.2. We describe a model where the order parameter is localized in randomly coupled regions whose critical behaviour depends on couplings above a temperature-dependent threshold. The observed dependence H1(T) results from screening currents circulating in percolation fractal clusters