無限層超伝導体Sr1-xLaxCuO2の低温構造 (酸化物超伝導体の合成と特性<特集>)

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Local structure of infinite layered superconductor Sr1-xLaxCuO2 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-xLaxCuO2 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.
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