Pressure Dependence of Superconducting Transition Temperature on Perovskite-Type Fe-Based Superconductors and NMR Study of Sr2VFeAsO3

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We report the pressure dependences of the superconducting transition temperature (T_{\text{c}}) in several perovskite-type Fe-based superconductors through the resistivity measurements up to {\sim}4 GPa. In Ca4(Mg,Ti)3Fe2As2Oy with the highest T_{\text{c}} of 47 K in the present study, the T_{\text{c}} keeps almost constant up to {\sim}1 GPa, and starts to decrease above it. From the comparison among several systems, we obtained a tendency that low T_{\text{c}} with the longer a-axis length at ambient pressure increases under pressure, but high T_{\text{c}} with the shorter a-axis length at ambient pressure hardly increases. We also report the 75As-NMR results on Sr2VFeAsO3. NMR spectrum suggests that the magnetic ordering occurs at low temperatures accompanied by some inhomogeneity. In the superconducting state, we confirmed the anomaly by the occurrence of superconductivity in the nuclear spin lattice relaxation rate 1/T_{1}, but the spin fluctuations unrelated with the superconductivity are dominant. It is conjectured that the localized V-3d moments are magnetically ordered and their electrons do not contribute largely to the Fermi surface and the superconductivity in Sr2VFeAsO3.
2011-01-15