Low Temperature Structure of Infinite-Layered Superconductor Sr_1-xLa_xCuO₂

Abstract

Local structure of infinite layered superconductor Sr₁-xLa_xCuO₂ in low temperature was investigated by EXAFS, X-ray diffraction and Raman spectroscopy. EXAFS showed that the Cu-O bond distance slightly expanded in a range <0.1Å with lowering temperature down to 20K.
The expansion was remarkable below Tc(onset)=43K. Tetragonal lattice parameters shrank with lowering temperature but the shrinkage of a-parameter seemed to be almost saturated below Tc(onset). Oxygen atoms may slightly shift their positions above and below the CuO₂ plane. Raman spectrum of Sr₁-xLa_xCuO₂ superconductors were Raman active although infinite layered structure should be Raman inactive from its symmetry. Resonance Raman peaks were observed at 151cm_-1 and 304cm_-1 in a usage of 4880Å Ar ion laser. Its energy of 2.54eV almost corresponds to that of the exitation between lower and upper Hubard bands. The peak intensities increased with lowering temperature even above Tc(onset). The intensity change may also be related to the change observed with both X-ray absorption and diffraction in local structure with temperature. The CuO₂ plane may buckle as reported in atomic pair distribution function refinement of neutron diffraction data on (Nd.Ce)₂CuO₄ in low temperature.