¹⁷O NMR probe of spin fluctuations in triplet superconductor Sr₂RuO₄

From the ¹⁷O Knight shift measurements on O(1) in RuO₂ planes and O(2) in SrO layers in Sr₂RuO₄ with T_c = 1.5 K, it is shown that the distribution of the spin density at the O(1) sites reflects two-dimensional electronic structure with most of its fraction on the in-plane p_π orbitals. From systematic measurements of the nuclear spin-lattice relaxation rate, T₁^-1, it is found that dynamical susceptibility is highly anisotropic in contrast to the uniform susceptibility being isotropic. Their in-plane components for the O(1) sites are almost independent of wavenumber although they are exchange-enhanced, whereas it is unexpected that the out-of-plane component being q dependent is pronouncedly enhanced by interlayer antiferromagnetic spin fluctuations below T* ∼ 130 K where the c-axis resistivity shows a metallic behavior. These novel characters of spin fluctuations are suggested to lead to discussions about a possible mechanism of spin-triplet p-wave superconductivity in Sr₂RuO₄.