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

Upper critical magnetic fields H/sub c/2(T) have been measured resistively for the high-temperature (T/sub c/approx. >90 K) superconductors TmBa2Cu3O/sub 9-//sub δ/ and YBa2Cu3O/sub 9-//sub δ/ in applied magnetic fields H up to 9 T. If H at the resistive midpoint is defined as H/sub c/2, then the upper critical field slopes (dH/sub c/2/dT)/sub T//sub >//sub c//sub =/ at the superconducting transition temperatures T/sub 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/sub c/2(T = 0) = 99--175 T, where the range is determined by the degree of paramagnetic limitation; (2) H/sub c/2(T = 77 K)≅36 T, the upper critical field at the boiling point of liquid nitrogen