PHASE-TRANSITIONS IN THE BA1-XKXBIO3 SYSTEM AND OTHER OXIDE SUPERCONDUCTORS

The mechanisms of dielectric-to-metal and metal-to-superconductor phas e transitions are proposed. The antiphase-ordered array of heavy holes localized on the apical oxygen-bismuth bonds creates a dielectric gap on the Fermi surface. With doping by potassium, light carriers are in troduced, which reduce the Coulomb potential of ordered heavy holes an d dielectric gap. Heavy holes tunneling between the neighboring apical O-1 and O-2 ions produce local charge fluctuations. Their interaction s with short-wavelength optical phonons yield the coupled tunneling-ph onon modes. Those local boson modes lead to the pairing of light-condu cting carriers and to superconductivity with a short coherence length. The local pairing may explain the observed upward curvature of H(c2)( T) in the range from T(c) = 30 K down to 4 K and up to 16 T. After obv ious rearrangements, the mechanism can be applied for all bismuth oxid e-based and copper oxide-based superconductors.