Sr_2RuO_4の超伝導および常伝導状態の異方的特性

We have recently discovered superconductivity of Sr2RuO4 with Tc ≈ 1 K. This compound is the only layered perovskite superconductor without copper that is known so far: the same crystal structure as a high-Tc superconducting copper oxide. In this study, we have investigated the superconducting and normal-state properties of Sr2RuO4 by measuring the electrical resistivity, magnetic susceptibility and specific heat using single crystals. In the normal state we found the T2 dependence of the resistivity below 25 K, the enhanced paramagnetic spin susceptibility, and the enhanced Sommerfeld coefficient of the specific heat. These three results can consistently be described as a highly-anisotropic Fermi-liquid behavior of the normal state of Sr2RuO4. In addition, concerning the peculiar temperature dependence of the resistivity along the c axis, we present a systematic interpretation based on competition between the life time of the quasiparticles governed by a number of scattering processes and the time for the quasiparticles to travel the interlayer distance d. From the upper critical fields Hc2, we will show that the superconducting state of Sr2RuO4 is also highly anisotropic with the ratio of the coherence lengths Γs = ξab(0)/ξc(0) = 26 evaluated for zero temperature. Furthermore, we found peculiar anisotropic flux-pinning behavior: a second dissipation peak in the ac susceptibility prominently appears only when the direction of the flux lines, as well as of their induced motions, is parallel to the ab planes. The peak effect in Sr2RuO4 is attributed to the synchronization of the flux lines to the pinning centers as the flux-line lattice softens with increasing magnetic field and temperature. In this study we clarify that Sr2RuO4 serves as an ideal reference material for investigation of the limits of applicability of the Fermi liquid theory to other highly correlated layered compounds, including high-Tc cuprates. In addition, the comparison with these physical properties of Sr2RuO4 and those of high-Tc superconductors are very important to clarify the mechanism of the high Tc, which still remains unclear at present.Abstract / p1 Contents / p2 1.Introduction / p3 2.Experimental / p6 2.1 Sample preparation / p6 2.2 Measurements / p6 3.Normal-State Properties / p7 3.1 Experimental results / p7 3.2 Highly anisotropic Fermi liquid state / p9 3.3 Anomalous temperature dependence of resistivity along the c axis / p10 4.Superconducting Properties / p15 4.1 Anisotropic superconducting state / p15 4.2 Peculiar flux pinning in the mixed state / p17 5.Conclusions / p23 Acknowledgments / p25 References / p26広島大学(Hiroshima University)博士(理学)Physical Sciencedoctora