梯子型格子を持つ系の超伝導-その物性の解明と新しい系の探索

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Recently, the ladder system which consists of two coupled spin-1/2 chains was proposed as an interesting intermediate step between one-and two-dimensional systems. Dagotto et al. and Rice et. Al. predicted that hole-doping in a spin-ladder system might produce the metallic state with a spin gap and give rise to novel superconductivity.To examine the theoretical prediction, carrier doping to SrィイD214xィエD2CaィイD2xィエD2CuィイD224ィエD2OィイD241ィエD2, being composed of CuィイD22ィエD2OィイD23ィエD2 planes with two-leg ladders, has been performed. We discovered that heavily Ca-substituted compound (x=13.6) showed superconductivity under pressure (TィイD2cィエD2(onset)〜l2K at 3.5 Gpa). To discuss the superconductivity further, the single crystal measurements have been made. Superconductivity was observed between 3.5 Gpa and 8 Gpa (TィイD2cィエD2 (onset)〜9K at 4.5GPa), accompanied by metallic resistivity between the ladders. This results strongly suggests that the application of pressure brings about a crossover from … More one-to two-dimensional and the system is anisotropic two-dimensional at the optimum pressure for the superconducting transition.To discuss the relationship between spin gap and the superconductivity, we carried out the inelastic neutron scattering experiment for SrィイD22.5ィエD2CaィイD211.5ィエD2CuィイD224ィエD2OィイD241ィエD2 single crystal. The spin gap energy for the ladder is found to be 32.1meV and the exchange coupling was estimated to be 90 meV (along the ladders) and 65 meV (along the rung).Specific heat, magnetic susceptibility and neutron scattering measurements were performed on SrィイD214xィエD2CaィイD2xィエD2CuィイD224ィエD2OィイD241ィエD2 single crystals. The specific heat of Ca-substituted samples SrィイD214xィエD2CaィイD2xィエD2CuィイD224ィエD2OィイD241ィエD2 showed a sharp phase transition at about 3 K. By the measurements of the magnetic susceptibility, it was confirmed that the phase transition was originated from antiferromagnetic long-range order. Besides, the several magnetic Bragg peaks were observed by neutron scattering measurement, and a possible magnetic structure on the chain and ladder sites were determined. In this system, the singlet state and antiferromagnetic order induced by appropriate hole-doping coexist at ambient pressure. Less