EXCESS CONDUCTIVITY, CRITICAL REGION AND ANISOTROPY IN YBA2CU4O8

Conductivity fluctuation effects in two YBa2Cu4O8 samples from differe nt batches are studied in the light of system anisotropy. The Lawrence -Doniach model is not suitable, hence the Aslammov-Larkin model with A nderson-Zou's formulation rho(n)(T) = A + B/T has been used. In the me an field region both the samples show 3D order parameter fluctuations. For the mean-held critical temperature three alternatives and their c onsequences are discussed. The order parameter dimensionality remains the same in all the alternatives. To account for the anisotropy of the Y-124 samples a single-crystal-analog for ab-plane resistivity has be en invoked. The Ginzburg point and theoretical and experimental critic al region widths are calculated and discussed in the light of anisotro py. Small variations due to different batch synthesis do not affect th e analysis. A theoretical formulation without anisotropy factor does n ot hold good. The experimental critical region widths lie between the theoretical values of GL and those of Fisher ct al. (Phys. Rev., B42, 1991, 130). Compared to Y-123 system these widths and the fluctuations are larger. Binding energy considerations confirm this fact. Copyrigh t (C) 1996 Published by Elsevier Science