Electron-phonon coupling in superconducting ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$: A Raman scattering study

Superconducting and nonsuperconducting phases in the ${\mathrm{Ba}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{K}}_{\mathrm{x}}$ ${\mathrm{BiO}}_3$ system have been studied at temperatures down to 4 K by use of Raman spectroscopy. For superconducting ${\mathrm{Ba}}_{0.6}$ ${\mathrm{K}}_{0.4}$ ${\mathrm{BiO}}_3$, an optical phonon at 348 ${\mathrm{cm}}^{\mathrm{-}1}$ (43 meV) has the distinctive Fano line shape indicative of significnat coupling between the phonon and a broad electronic continuum. For nonsuperconducting ${\mathrm{Ba}}_{0.8}$ ${\mathrm{K}}_{0.2}$ ${\mathrm{BiO}}_3$, the same phonon is not observed to be coupled to the electronic states. Assignment of the 348-${\mathrm{cm}}^{\mathrm{-}1}$ peak to specific normal modes of vibration is discussed as is the similarity to the phonon at 340 ${\mathrm{cm}}^{\mathrm{-}1}$ in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ that also exhibits electron-phonon coupling.