ANISOTROPIC DIELECTRIC FUNCTION AND ELECTRONIC-STRUCTURE OF THE INFINITE-LAYER COMPOUND (SR1-XCAX)YCUO2

The electronic structure and dielectric functions of single crystallit es of the infinite-layer compound (Sr1-xCax)yCuO2 (y = 0. 90, x = 0.3) are investigated by transmission-electron-energy-loss spectroscopy wi th a cold-field emission-gun TEM. The band gap along the c axis is est imated to be significantly greater than in the aa plane, which suggest s anisotropic electronic properties. Unlike other cuprates, oxygen cor e-loss spectra of this compound display two distinct preedge peaks. A close correlation was observed between the valence-band excitations be low 10 eV in the aa plane and the oxygen K core-loss fine structure. T he approximately 2 eV excitation in the valence-band spectrum is assoc iated with a transition from the lower Hubbard band to the upper Hubba rd band and is detected only in the aa plane. This excitation coincide s with the first preedge peak in O K core-loss spectra, which is assig ned to a transition from the O 1s state to the upper Hubbard band. The results are consistent with the band-structure calculations, for the infinite-layer compound, which indicate strong Cu 3d-O 2p hybridizatio n within the aa plane, but little overlap of the wave function along t he c axis.