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 Ba1-xKxBiO3 (BKBO) single crystals in the doping range 0≤x≤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 x0.40 (superconductor phase), and a local excitation for x≤0.3 (insulator-semiconductor phase). The oscillator strength of the excitation in the low-loss region increases substantially and abruptly between x=0.30 and 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.