Weak Spin Fluctuation with Finite Wave Vector and Superconducting Gap Symmetry in KxFe2-ySe2: 77Se Nuclear Magnetic Resonance
We report 77 Se-nuclear magnetic resonance (NMR) results down to sufficiently low temperatures under magnetic fields parallel to both the a b -plane and the c -axis in a paramagnetic/superconducting (PM/SC) phase of K x Fe 2- y Se 2 . The observation of anisotropy in the orbital part of the Knight shift results in the anisotropy of its spin part increasing on approaching the transition temperature. The anisotropy of the Korringa relation suggests the presence of the weak spin fluctuations with a finite wave vector q , which induce the magnetic fluctuations along the a b -plane at the Se site. Such fluctuations do not correspond to the stripe (π,0) correlation of the Fe moment observed in many Fe-based superconductors, and are not contradictory to weak (π,π) correlations. The nuclear spin–lattice relaxation rate 1/ T 1 shows a field-independent T 1 T ∼const. behavior at low temperatures for H ∥ a b , which is attributed to the nonzero density of states at the Fermi level and can be explained by the sign-ch...