Superconductivity of BaPb 1−xBi xO 3 in the coexisting state of itinerant large polarons and localized small bipolarons

Abstract

The systematic analysis of strongly x-dependent continuum peaks in the Raman spectra of BaPb 1−xBi xO 3 manifested that the continuum is formed of five sets of peaks. These peaks are assigned to the breathing mode coupled with itinerant electrons and localized electrons. The mode coupled with the transition from the itinerant to the localized electronic state shows softening toward the opening point of the insulating pseudo-gap at x=0.15. The itinerant electronic state has a finite extent of wave function and the metal-insulator transition at x=0.35 is caused by the disappearance of the percolation conductance. The localized state is a self-trapped bipolaronic state. The strong interaction between the itinerant electrons and bipolarons is a possible origin of this superconductivity.