Temperature dependence of magnetic penetration depth in B 1Àx K x BiO 3 superconductor

Abstract Temperature behavior of the magnetic penetration depth, kðT Þ, was measured in the Meissner state of B 1Àx K x BiO 3 (x ¼ 0:37) single crystal using tunnel diode oscillator technique. Measurements were carried out down to 0.4 K. At low temperatures, 0:013 6 T =T c 6 0:4, kðT Þ is exponentially flat, which provides a strong evidence for conventional s-wave BCS behavior. Magnetic penetration depth is an effective tool to study properties of superconductors. At low temperatures, its temperature dependence is directly related to the density of low energy quasiparticles, which in turn can be related to the anisotropy of the superconducting energy gap on the Fermi surface. For investigation of the low lying excitations and thus the anisotropy of the energy gap, the analysis is considerably less ambiguous if data are taken on high quality single crystal and temperatures well below T c =3. In this paper we report penetration depth measurements on B 0:63 K 0:37 BiO 3 (T c % 31 K) down to 0.4 K. The investigation of the superconductivity in B 1Àx K x BiO 3 (BKBO) system has been one of the major subjects in studies of high T c superconductivity in oxide materials. The importance of the BKBO system lies in the fact that some of its superconducting properties are consistent with the conventional s-wave isotropic superconductivity, but others resemble high-T c cuprates. In particular, substantial isotope effect [1,2], strong phonon spectrum from the neutron scattering measurement