Upper critical magnetic field measurements in RBa 2Cu 3O 7−δ (R= rareearth) compounds

Abstract

Upper critical magnetic field initial slopes -d H c2/d T, have been measured for RBa 2Cu 3O 7−δ compounds where R=Y , Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu. If H at the resistive midpoint is defined as H c2, then -d H c2/d T falls within the range of 0.81 (R=Lu) to 2.86 (R=Eu) T/K. Extrapolations to T=0 K using the standard, three-dimensional, dirty-limit theory of theory of Werthamer, Helfand, Hohenberg and Maki yield (for R=Eu) (a) zero-temperature upper critical fields H c2 ( T=0 K)=102–183 T, where the range is determined by the degree of paramagnetic limitation; (b) H c2 ( T=77 K) ≃ 43 T, the upper critical field at the boiling point of liquid nitrogen. Excessive broadening of the resistive transition to the superconducting state which lowers | -d H c2/ dT| is discussed in terms of intergranular conduction and crystal anisotropy with comparison drawn to single crystal data.