Upper critical field of Ba1−χKχBiO3 single crystal

Abstract

We measured the upper critical field Hc2 on Ba1−χKχBiO3 (0.35<χ<0.40) single crystal down to 2 K in pulsed magnetic fields up to 35 T. Resistive transitions in the magnetic field systematically shift as the temperature decreases and the ϱ-H curves saturate for high enough magnetic field strength. The initial slope of the upper critical field (dHc2/dT)Tc is -0.5 T/K. As temperature decreases, the Hc2−T curve shows an upward curvature. At 2 K, we find Hc2=30 T, which gives a coherence length ξ=33 A. Such an Hc2 value is almost twice as large as what one can deduce by using the standard expression Hc2(0)=−0.693Tc(dHc2/ dT)Tc. From the comparison with several models, we conclude that this unusual temperature dependence of Hc2 can be due either to structural instability or to microscopic sample inhomogeneities.