Infrared study of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$ from charge-density-wave insulator to superconductor

Measurements of the reflectivity of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$, for x=0 to 0.4, in the frequency range from ≃250 to 25 000 ${\mathrm{cm}}^{\mathrm{-}1}$ are used to obtain the conductivity as a function of frequency and composition. The end-member compound, ${\mathrm{BaBiO}}_3$, has a half-filled electronic band, and a charge-density-wave (CDW) ground state, while at x≃0.4 this material is a superconductor with ${\mathit{T}}_{\mathit{c}}$≃30 K. Excitations across the CDW energy gap produce a large peak in the optical conductivity at about 16 000 ${\mathrm{cm}}^{\mathrm{-}1}$ (2 eV), which shifts to lower energy and broadens as the band filling is reduced by doping (x>0). For compositions near x=0.4 a broad contribution to the conductivity extending from ≃3000 ${\mathrm{cm}}^{\mathrm{-}1}$ to 10 000 ${\mathrm{cm}}^{\mathrm{-}1}$, which appears to have evolved from the CDW peak, is still observed. These observations suggest that a remnant CDW-like order may still be present even in the composition range where superconductivity occurs. They also highlight fundamental differences in the electronic properties of bismuthate and cuprate materials.