Upper critical fields and high superconducting transition temperatures of La1.85Sr0.15CuO4 and La1.85Ba0.15CuO4

Abstract

The superconducting transition temperature T_c of La_1.85Sr_0.15CuO₄ is measured to be 34.7 K (midpoint) with a transition width (10–90%) of 2.6 K. The critical field slope at T_c is 2.13 T/K. Using the Werthamer, Helfand, and Hohenberg expression in the dirty limit we estimate the upper critical field H_c2(0) to be at least 50 T for this system. The measured transition width broadens initially with increasing field to 4 K, then saturates at this value for fields above 2 T. The critical field slope for the lower T_c material La_1.85Ba_0.15CuO₄ is measured to be 1.78 T/K, which yields an H_c2(0) of at least 36 T.