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 congruent-to 250 to 25000 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 x congruent-to 0.4 this material is a superconductor with T(c) congruent-to 30 K. Excitations across the CDW energy gap produce a large peak in the optical conductivity at about 16000 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 congruent-to 3000 cm-1 to 10000 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.