Upper critical fields of high-${\mathrm{T}}_{\mathrm{c}}$ superconducting ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$: Possibility of 140 tesla

The upper critical fields $B_{c2}$(T) of ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$(x=0.15 and x=0.20) were measured near $T_c$ 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, $T_c$=38.1 K, and $B_{c2}$(0)=58 T assuming a dirty type-II superconductor with no Pauli paramagnetic limiting. Data for the onset yield 4.5±0.5 ${\mathrm{K}}^{\mathrm{-}1}$, 40.1 K, and 125±15 T, respectively. Pulsed-field data show that fractions of both materials are superconducting above 45 T at 4.2 K. Calculations of limits of $B_{c2}$(T) are presented and experimental limitations of measurements in these high-field superconductors are indicated.