Novel Character of Spin Fluctuations in Spin-Triplet Superconductor Sr 2RuO 4: 17O-NMR Study

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Abstract
We report 17 O-NMR studies in the normal state of high-quality single-crystal Sr 2 RuO 4 with T c ∼1.5 K, which has been identified as spin-triplet p -wave superconductor. From systematic measurements of the Knight shift and the nuclear spin-lattice relaxation time T 1 on the O(1) sites in the RuO 2 planes and the O(2) sites in the SrO layers, microscopic distribution of the spin density on the p orbitals and the anisotropy of low-frequency components of dynamical susceptibility χ( q ,ω n ) have been unraveled as follows; (1) The spin susceptibility at the O(1) sites is much larger than that at the O(2) sites. (2) From the anisotropy in the Knight shift at the O(1) sites, it is shown that the spin density resides predominantly on the in-plane p π orbitals hybridized with the 4 d x y orbitals. (3) From the pronounced anisotropy in (1/ T 1 T ) at both sites developed below T * ∼130 K where the c -axis resistivity shows a metallic behavior, we have found that the low-frequency components of χ( q ,ω n ) is highly anisotropic in contrast to the uniform spin susceptibility being isotropic. The in-plane components of χ( q ,ω n ) at the O(1) sites are almost independent of wave number although they are exchange-enhanced, whereas its out-of-plane component is markedly enhanced by interlayer antiferromagnetic (AF) spin fluctuations below T * ∼130 K. (4) For the apical O(2) sites on which the p orbitals are hybridized with the 4 d x z , y z and p c orbitals on the O(1) site, the in-plane components of χ( q ,ω n ) are enhanced by AF-spin fluctuations as well. The AF-spin fluctuations develop as quasiparticles hop coherently along the c -axis below T * . These characteristics of spin fluctuations are suggested to provide a new scenario based on the Hund's coupling as a possible mechanism of spin-triplet p -wave superconductivity in Sr 2 RuO 4 , not on analogy with 3 He.