Evidence of a small exciton in the Ba_{1x}K_{x}BiO_{3} superconductor and its relationship with superconductivity
Abstract
The effective mass of a freecarrier plasmon is measured by momentumresolved electronenergyloss 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 freecarrier 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 bandstructure 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_{1x}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 freecarrier 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 (WangZhangKlein) exciton.
 Publication:

Physical Review B
 Pub Date:
 January 2003
 DOI:
 10.1103/PhysRevB.67.024505
 Bibcode:
 2003PhRvB..67b4505W
 Keywords:

 74.70.b;
 71.18.+y;
 78.20.e;
 Superconducting materials;
 Fermi surface: calculations and measurements;
 effective mass g factor;
 Optical properties of bulk materials and thin films