Optical conductivity studies of La_3/2Sr_1/2NiO₄: Lattice effect on charge ordering

Optical conductivity spectra σ(ω) of a La_3/2Sr_1/2NiO₄ single crystal were investigated over a wide photon energy range with variations of temperature and polarization. Strong anisotropies in phonon modes and electronic structures are observed between the ab plane (E∥ab) and c axis (E∥c). In the midinfrared region, σ(ω) 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 optical transitions, the crystal field splitting energy between e_g orbitals and 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. Below the charge ordering temperature, the polaron absorption is suppressed and an optical gap starts to appear. The optical gap initially increases with decreasing temperature; however, it starts to decrease near 120 K. Our x-ray diffraction measurements show an increase of the a axis lattice constant below 120 K. These results suggest the importance of the lattice degrees of freedom for stabilizing the charge ordering in La_3/2Sr_1/2NiO₄.