Physics of Spin-Triplet Superconductivity

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We firmly established for the first time that a superconductor with its electrons paired in a spin-triplet state actually exists. Through extensive investigations focusing on the ruthenate superconductor Sr_2RuO_4,we obtained a number of new key concepts in the study of superconductivity :(1) Determination of the superconducting multi-gaps : We devised a method of determining the superconducting gap on each of the Fermi surface. For Sr_2RuO_4,we understand the physical origin of its multi-gap structure based on the spin-triplet symmetry and the spin fluctuations.(2) Determination of the Cooper-pair spin state : By nuclear magnetic resonance (NMR), we determined that the triplet spins lie in the RuO_2 plane, but that they can be rotated by weak magnetic fields.(3) Confirmation of odd parity : We fabricated a pi-junction SQUID (Superconducting QUantum Interference Device) consisting of Sr_2RuO_4 and an s-wave superconductor AuIn, and demonstrated that the parity of the superconducting wave function of Sr_2RuO_4 is odd, a requirement for the spin-triplet superconductivity.(4) Construction of the theoretical framework of superconducting mechanisms : We have succeeded in constructing a general theoretical framework of superconducting mechanism based on Coulomb repulsion among the strongly correlated electrons. By applying it to a detailed realistic model of Sr_2RuO_4,we are able to derive that the spin-triplet pairing is more stable than other states.We also discovered superconductivity in silver-lead oxide, Ag_5Pb_2O_6,clarified the mechanism of the Mott transition and orbital ordering in (Ca,Sr)_2RuO_4,and revealed the metamagnetic quantum critical phenomenon in Sr_3Ru_2O_7. We sponsored the international symposium on "Spin-Triplet Superconductivity and Ruthenate Physics", attended by most of the active researchers in this field, including over 20 researches from 8 abroad.