Thulium barium copper oxide: A 90‐K superconductor with a potential 1‐MG upper critical field

Upper critical magnetic fields H_c2(T) have been measured resistively for the high‐temperature (T_c≳90 K) superconductors TmBa₂Cu₃O_9−δ and YBa₂Cu₃O_9−δ in applied magnetic fields H up to 9 T. If H at the resistive midpoint is defined as H_c2, then the upper critical field slopes (dH_c2/dT)_Tc at the superconducting transition temperatures T_c are respectively −2.8 and −1.1 T/K. The Tm‐specimen initial slope is apparently the highest yet reported for 90 K superconductors. Extrapolations from this slope, based on standard, three‐dimensional, dirty‐limit Werthamer, Helfand, Hohenberg, and Maki theory, yield (1) zero‐temperature upper critical fields H_c2(T=0)=99–175 T, where the range is determined by the degree of paramagnetic limitation; (2) H_c2(T=77 K)≊36 T, the upper critical field at the boiling point of liquid nitrogen.