Upper critical fields of high-Tc superconducting La2-xSrxCuO4-y: Possibility of 140 tesla

The upper critical fields Bc2(T) of La2-xSrxCuO4(x=0.15 and x=0.20) were measured near Tc in dc fields to 23 T and at 4.2 K with pulsed fields to 45 T. Resistive midpoint transitions for the x=0.15 sample yield &=2.2 T/K, Tc=38.1 K, and Bc2(0)=58 T assuming a dirty type-II superconductor with no Pauli paramagnetic limiting. Data for the onset yield 4.50.5 K-1, 40.1 K, and 12515 T, respectively. Pulsed-field data show that fractions of both materials are superconducting above 45 T at 4.2 K. Calculations of limits of Bc2(T) are presented and experimental limitations of measurements in these high-field superconductors are indicated.