Imaginary part of the optical conductivity of Ba1-xKxBiO3

The frequency dependence of the imaginary part of the infrared conductivity is calculated for a superconductor. Sharp structure, characteristic of superconductivity with an order parameter with s-wave symmetry, appears in the BCS limit as a minimum at a frequency equal to twice the gap value. This structure scales with temperature but gets progressively smeared and shifted as impurity scattering is increased. The relationship between low-frequency results and the zero-frequency limit is investigated. Experimental results on Ba1-xKxBiO3 are also presented. The low-frequency imaginary part of the conductivity displays a minimum at 2 Delta approximate to 12 meV, and provides unequivocal evidence of an s-wave superconducting order parameter. Strong-coupling (Eliashberg) results show similar trends. Using this formulation we find that the electron-phonon coupling in Ba1-xKxBiO3 must necessarily be small, with coupling constant lambda approximate to 0.2, in agreement with conclusions drawn from measurements of the real part of the conductivity. Thus Ba1-xKxBiO3 is an s-wave superconductor that is not driven by the conventional electron-phonon interaction.