Free-charge-carrier plasmons in Ba1-xKxBiO3: A close relation to cuprate superconductors

Broad-range reflectance and ellipsometric spectra of high-quality epitaxial superconducting Ba1-xKxBiO3 thin films are reported. From these data, accurate spectral functions of Ba1-xKxBiO3 have been obtained. They are found to be quite similar to the in-plane spectra of the superconducting cuprates; in particular, the frequency dependence of the reflectance is nearly linear, while the dielectric loss function is nearly quadratic below the plasma edge in all of these compounds. It is therefore unlikely that acoustic plasmons or magnetic fluctuations are the cause of these anomalous spectral features.