Optical conductivity studies of La3/2Sr1/2NiO4: Lattice effect on charge ordering - art. no. 165106

Optical conductivity spectra sigma(omega) of a La3/2Sr1/2NiO4 single crysta l were investigated over a wide photon energy range with variations of temp erature and polarization. Strong anisotropies in phonon modes and electroni c structures are observed between the ab plane (E\\ab) and c axis (E\\c). I n the midinfrared region, sigma(omega) for E\\ab show several peaks due to small polaron and optical transitions between neighboring Ni sites; however , those for E\\c show negligible spectral weights. By assigning proper opti cal transitions, the crystal field splitting energy between e(g) orbitals a nd Hund's rule exchange energy are estimated to be around 0.7 eV and 1.4 eV , respectively. With decreasing temperature, there are large changes in the phonon modes and the spectral weights are transferred to higher energy. Be low the charge ordering temperature, the polaron absorption is suppressed a nd an optical gap starts to appear. The optical gap initially increases wit h decreasing temperature; however, it starts to decrease near 120 K. Our x- ray diffraction measurements show an increase of the a axis lattice constan t below 120 K. These results suggest the importance of the lattice degrees of freedom for stabilizing the charge ordering in La3/2Sr1/2NiO4.