Infrared study of Ba1-xKxBiO3 from charge-density-wave insulator to superconductor

Measurements of the reflectivity of Ba1-xKxBiO3, for x=0 to 0.4, in the frequency range from 250 to 25 000 cm-1 are used to obtain the conductivity as a function of frequency and composition. The end-member compound, BaBiO3, has a half-filled electronic band, and a charge-density-wave (CDW) ground state, while at x0.4 this material is a superconductor with Tc30 K. Excitations across the CDW energy gap produce a large peak in the optical conductivity at about 16 000 cm-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 cm-1 to 10 000 cm-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. © 1993 The American Physical Society.