OPTICAL STUDY OF THE LA2-XSRXNIO4 SYSTEM - EFFECT OF HOLE DOPING ON THE ELECTRONIC-STRUCTURE OF THE NIO2 PLANE

The optical spectra of La2-xSrxNiO4 are systematically investigated from the infrared to ultraviolet energy range and over a wide compositional range, 0 less-than-or-equal-to x less-than-or-equal-to 0.5. La2NiO4 is a charge-transfer (CT) insulator with a CT energy gap of about 4 eV, twice as large as that in the isostructural La2CuO4. Introduction of holes into the NiO2 plane induces broad absorption in the infrared region below the CT gap energy, similar to the case of La2-xSrxCuO4. The induced absorption consists of two distinct bands centered at 0.6 and 1.5 eV. The total spectral weight of the two absorption bands increases proportionately to the dopant concentration x. Notable differences from the cuprates are that these bands do not shift with increasing x and that a Drude-type absorption centered at omega = 0 does not appear up to x = 0.5 in the case of the nickelate. They might arise from different symmetries of the doped holes and/or different spin quantum number of the Ni2+ ion.