Optical conductivity studies of La3/2Sr1/2NiO4: Lattice effect on charge ordering

J. H. Jung, D. W. Kim, T. W. Noh, H. C. Kim, H. C. Ri, S. J. Levett, M. R. Lees, D. Mc K. Paul, G. Balakrishnan

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Optical conductivity spectra σ(ω) of a La3/2Sr1/2NiO4 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 eg 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 La3/2Sr1/2NiO4.

Original languageEnglish
Article number165106
Pages (from-to)1651061-1651067
Number of pages7
JournalPhysical Review B - Condensed Matter and Materials Physics
Issue number16
StatePublished - 15 Oct 2001

Bibliographical note

Funding Information:
This work was supported by grant No. 2000-2-30400-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation.


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