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Evidence of a small exciton in the Ba 1 − x K x BiO 3 superconductor and its relationship with superconductivity

The effective mass of a free-carrier plasmon is measured by momentum-resolved electron-energy-loss spectroscopy. The effective mass of the plasmon is heavy along the [100] direction and light along the [110] direction in the optimal doped regime, where T c is highest. The anisotropy of the effective mass between [100] and [110] decreases with increasing doping and decreasing of T c . In the overdoped regime with T c almost zero (normal metal state), the effective mass of the free-carrier plasmon becomes isotropic. A linear correlation between T c and the effective mass difference along the two orientations is extracted from the experimental data. The dispersion of the plasmon for the optimal doped material cannot be explained by the band structure alone, but that for the overdoped material is consistent with the band-structure calculation. This indicates that the effective mass is strongly renormalized along [100] by an additional interaction in the optimally doped material of the highest T c . Of related interest is a group of small excitons in the parent compound of Ba 1 - x K x BiO 3 (BKBO), BaBiO 3 , with assigned symmetries that are s wave (optically forbidden) at 4.2 eV, p wave at 2.1 eV (optically allowed), and d wave at 0.5 eV (optically forbidden). We propose a mixing state of free carriers and small excitons to explain the anisotropic dispersion for the optimally doped regime and a free-carrier state free of small excitons for the overdoped material (normal metal state). The linear correlation between T c and mass renormalization provides insight that the superconductivity of BKBO may be related to electronic structure, possibly to the small exciton or WZK (Wang-Zhang-Klein) exciton.