Fluctuation conductivity and upper critical field of Y1-xRExNi2B2C (RE = Er, Ho, Dy, Gd) superconductors

We have investigated the upper critical fields of Y1-xRExNi2B2C (RE = Er, H o, Dy, Gd; 0 less than or equal to x less than or equal to 1) compounds by using the scaling analysis of the thermal fluctuation conductivity. The the rmal fluctuation conductivity can be scaled by a universal function both ab ove and below the mean-field transition temperature for all magnetic fields . A positive curvature is present in H-c2(T). The upward curvature of the H -c2(T) curve in the Y1-xRExNi2B2C system might suggest an intrinsic anomalo us pairing mechanism. Based on the value of upper critical field near T-c, the superconducting coherence length is estimated at 6.4 nm for an x = 0 sa mple. The Ginzburg-Landau parameter kappa estimated is similar to 11 in the pristine YNi2B2C system. (C) 2000 Elsevier Science Ltd. All rights reserve d.