Raman and infrared studies of Sr2TiO4: A material isomorphic to (La,Sr)2CuO4 superconductors

Abstract

The high-temperature superconductor (${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$)${\mathrm{CuO}}_4$ has the ${\mathrm{K}}_2$ ${\mathrm{NiF}}_4$ structure, as does ${\mathrm{Sr}}_2$ ${\mathrm{TiO}}_4$. In order to understand better the phonon spectra obtained from ceramic (${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$)${\mathrm{CuO}}_4$, we have performed Raman and infrared measurements on ceramic, insulating ${\mathrm{Sr}}_2$ ${\mathrm{TiO}}_4$. All four Raman-allowed phonons are observed and found to be quite sharp, because in this centrosymmetric structure the Raman-mode frequencies are isotropic. The infrared reflectivity spectra are more difficult to interpret because reststrahlen bands are measured, which extend from the transverse-optic (TO) frequency to the longitudinal-optic (LO) frequency. Symmetry forbids overlap of the 3$A_{2u}$ bands as well as the 4$E_u$ bands. The interpretation, nevertheless, is difficult because the $A_{2u}$ and $E_u$ bands can overlap one another. By comparison with single-crystal results in fluorides and chlorides having the same structure, we have been able to identify many of the infrared modes in our spectra. This knowledge permits a determination of most of the infrared active modes in (${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$)${\mathrm{CuO}}_4$. In the superconductor, two other effects aid our understanding of the spectra. (a) The effective charges are smaller than in ${\mathrm{Sr}}_2$ ${\mathrm{TiO}}_4$, reducing the TO-LO splitting, and thus the overlap of the reflectivity bands. (b) The electron plasma is confined to the xy plane, and consequently it interacts strongly only with the $E_u$ phonons, and leaves the $A_{2u}$ phonons largely unaffected.