Anisotropic dielectric function and electronic structure of the infinite-layer compound (Sr1-xCax)yCuO2

The electronic structure and dielectric functions of single crystallites of the infinite-layer compound (Sr1-xCax)yCuO2 (y=0.90, x=0.3) are investigated by transmission-electron-energy-loss spectroscopy with a cold-field emission-gun TEM. The band gap along the c axis is estimated to be significantly greater than in the aa plane, which suggests anisotropic electronic properties. Unlike other cuprates, oxygen core-loss spectra of this compound display two distinct preedge peaks. A close correlation was observed between the valence-band excitations below 10 eV in the aa plane and the oxygen K core-loss fine structure. The ∼2 eV excitation in the valence-band spectrum is associated with a transition from the lower Hubbard band to the upper Hubbard band and is detected only in the aa plane. This excitation coincides with the first preedge peak in O K core-loss spectra, which is assigned 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 hybridization within the aa plane, but little overlap of the wave function along the c axis.