Phase transitions in the Ba1?xKxBiO3 system and other oxide superconductors

The mechanisms of dielectric-to-metal and metal-to-superconductor phase transitions are proposed. The antiphase-ordered array of heavy holes localized on the apical oxygen-bismuth bonds creates a dielectric gap on the Fermi surfce. With doping by potassium, light carriers are introduced, which reduce the Coulomb potential of ordered heavy holes and dielectric gap. Heavy holes tunneling between the neighboring apical O^–1 and O^–2 ions produce local charge fluctuations. Their interactions with short-wavelength optical phonons yield the coupled tunneling-phonon modes. Those local boson modes lead to the pairing of light-conducting carriers and to superconductivity with a short coherence length. The local pairing may explain the observed upward curvature ofH_c2(T) in the range fromT_c=30 K down to 4 K and up to 16 T. After obvious rearrangements, the mechanism can be applied for all bismuth oxide-based and copper oxide-based superconductors.