Electronic states and magnetic properties of edge-sharing Cu-O chains

The electronic states and magnetic properties for the copper oxides containing edge-sharing Cu-O chains such as Li2CuO2, La6Ca8Cu24O41, and CuGeO3 are systematically studied. The optical conductivity sigma(omega) and the temperature dependence of the magnetic susceptibility chi(T) for single-crystalline samples Li2CuO2 are measured as a reference system and analyzed by using the exact diagonalization method for small Cu-O clusters. It is shown that the spectral distribution of sigma(omega) is different between edge-sharing and corner-sharing Cu-O-Cu bonds. The charge transfer gap in edge-sharing chains is larger than that of high-T-c cuprates. The exchange interaction between nearest-neighbor copper ions in edge-sharing chains J(1) depends sensitively on the Cu-O-Cu bond angles. In addition to J(1), the exchange interaction between next-nearest-neighbor copper ions J(2) has sufficient contribution to the magnetic properties. We calculate J(1) and J(2) for all the copper oxides containing edge-sharing Cu-O chains and discuss the magnetic properties.