Potential Antiferromagnetic Fluctuations in Hole-Doped Iron-Pnictide Superconductor Ba1-xKxFe2As2 Studied by 75As Nuclear Magnetic Resonance Measurement

Abstract

We have performed 75 As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on single-crystalline Ba 1- x K x Fe 2 As 2 for x = 0.27–1. 75 As nuclear quadruple resonance frequency (ν Q ) increases linearly with increasing x . The Knight shift K in the normal state shows Pauli paramagnetic behavior with a weak temperature T dependence. K increases gradually with increasing x . By contrast, the nuclear spin–lattice relaxation rate 1/ T 1 in the normal state has a strong T dependence, which indicates the existence of large antiferomagnetic (AF) spin fluctuations for all x 's. The T dependence of 1/ T 1 shows a gaplike behavior below approximately 100 K for 0.6 < x < 0.9. This behaviors is well explained by the change in the band structure with the expansion of hole Fermi surfaces and the shrinkage and disappearance of electron Fermi surfaces at the Brillouin zone (BZ) with increasing x . The anisotropy of 1/ T 1 , represented by the ratio of 1/ T 1 a b to 1/ T 1 c , is always larger than 1 for all x 's, which indicates that stripe-type AF fluctuations are dominant in this system. The K in the superconducting (SC) state decreases, which corresponds to the appearance of spin-singlet superconductivity. The T dependence of 1/ T 1 in the SC state indicates a multiple-SC-gap feature. A simple two-gap model analysis shows that the larger superconducting gap gradually decreases with increasing x from 0.27 to 1 and a smaller gap decreases rapidly and nearly vanishes for x > 0.6 where electron pockets in BZ disappear.