Evolution of the low-energy excitations and dielectric function of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$ (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 x≥0.40 (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.

• Received 23 February 1993

DOI:https://doi.org/10.1103/PhysRevB.47.14503