Upper Critical Magnetic-Field Measurements In Rba2cu3o7-Delta(R=Rare Earth) Compounds

Upper critical magnetic field initial slopes -d H c2 /d T , have been measured for RBa 2 Cu 3 O 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 c 2 ( T =0 K)=102–183 T, where the range is determined by the degree of paramagnetic limitation; (b) H c 2 ( 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.