UPPER CRITICAL-FIELD OF BA1-XKXBIO3 SINGLE-CRYSTAL

We measured the upper critical field H(c2) on Ba1-xKxBiO3 (0.35<x<0.40 ) single crystal down to 2 K in pulsed magnetic fields upto 35 T. Resi stive transitions in the magnetic field systematically shift as the te mperature decreases and the rho-H curves saturate for high enough magn etic field strength. The initial slope of the upper critical field (dH (c2)/dT)Tc is -0.5 T/K. As temperature decreases, the H(c2)-T curve sh ows an upward curvature. At 2 K, we find H(c2)=30 T, which gives a coh erence length xi = 33 angstrom. Such an H(c2) value is almost twice as large as what one can deduce by using the standard expression H(c2)(0 ) = -0.693T(c)(dH(c2)/dT)Tc. From the comparison with several models, we conclude that this unusual temperature dependence of H(c2) can be d ue either to structural instability or to microscopic sample inhomogen eities.