ANOMALOUS PROPERTIES AND NATURE OF BA1-XKXBIO3 SUPERCONDUCTIVITY

On the base of temperature dependencies of the specific heat in magnetic fields, thermal expansion and magnetic properties of high quality high-T-c Ba1-xKXBiO3 (x=0,38 . 0,40) it was found that H-c2(T) had anomalous upward curvature up to fields 20T and coefficient of thermal expansion was negative at low temperatures. The results contradict the picture of simple electron-phonon coupling. We have presented a model of the electronic ordering in Ba1-xKXBiO3 which drives the system to the dielectric-metal phase transition with doping. It was shown that electronic spectrum peculiarities near the Brillouin zone boundary determine the anomalous properties and upward curvature of H-c2(T) in BaKBiO. The mechanism of BaKBiO superconductivity is discussed.