Evolution of the low-energy excitations and dielectric function of Ba1-xKxBiO3 (0 <= x <= 0.50).

Evolution of the low-energy excitations and dielectric function of Ba[sub 1[minus][ital x]]K[sub [ital x]]BiO[sub 3] (BKBO) single crystals in the doping range 0[le][ital x][le]0.50, i.e., spanning the semiconductor to superconductor regimes, is investigated by electron-energy-loss spectroscopy (EELS) with a cold-field-emission-gun transmission-electron microscope (cFEG TEM). A free-carrier plasmon in BKBO is observed by transmission EELS for [ital x][ge]0.40 (superconductor phase), and a local excitation for [ital x][le]0.3 (insulator-semiconductor phase). The oscillator strength of the excitation in the low-loss region increases substantially and abruptly between [ital x]=0.30 and [ital x]=0.40, signaling the semiconductor-metal transition. Subtle but important changes are observed in the higher-loss regions (bulk plasma) with K doping. The evolution of the low-energy excitations and dielectric function of BKBO is compared to that of cuprate superconductors.