Resonant Raman scattering in superconducting Ba$_{1-x}$K$_x$BiO$_3$

The effect of the photon energy of the exciting laser radiation on the Raman spectra of Ba$_{1-x}$K$_x$BiO$_3$ with $x=0.25, 0.40$, and $0.50$ is studied. An increase in the laser wavelength from 488 to 750 nm scarcely affects the amplitudes and frequencies of the spectral lines in the Raman spectra of the nonsuperconducting compound with $x=0.25$. For the optimally doped ($x=0.40$) and overdoped ($x=0.50$) superconducting compounds, a substantial increase in the line intensity and a considerable shift of the characteristic frequencies are observed. This result suggests that, in the whole range of superconducting compositions $0.37\leq x\leq0.50$, the local symmetry of the Ba$_{1-x}$K$_x$BiO$_3$ crystal lattice differs from the perfect cubic symmetry, which should take place according to the literature data. The fact that resonance phenomena are observed when the laser photon energy is shifted toward the optical gap testifies to the presence of local electron pairs in the whole range of superconducting compositions $0.37\leq x\leq0.50$ and is evidence in favor of the superconductivity mechanism proposed for Ba$_{1-x}$K$_x$BiO$_3$ on the basis of the X-ray absorption studies in our previous paper.