Transport measurements of in-plane critical fields inYBa2Cu3O7−δto 300 T

Thin-film ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ has been studied with in-plane magnetic fields $B$ to 850 T, with good signal-to-noise to 300 T, using a GHz-transmission-line technique for electrical transport measurements in explosive-driven microsecond pulsed magnetic fields. At an ambient temperature of 1.6 K with $B\perp c$ axis we find an onset of dissipation at 150 T, the highest field reported, with a saturation of resistivity at 240 T. Comparison with the Pauli limit $B_p=155\mathrm{}$ T expected for this material suggests that the critical field is limited by spin paramagnetism.