75As Nuclear Magnetic Resonance Measurements Research Articles

In-plane anisotropy of the magnetic and the electric properties of the Fe pnictide Ba(Fe1−x Co x )2As2

The anisotropy of the electronic and the magnetic properties within the FeAs planes of Co(2%)-doped BaFe2As2 has been investigated by using 75As nuclear magnetic resonance (NMR) measurements on the As (As0) sites with all four nearest neighbor sites occupied by Fe. Even in the tetragonal phase at temperatures above 100 K, two sets of NMR spectra with twofold symmetry of in-plane Knight shifts and electronic nuclear quadrupole frequencies are observed. They originate from two domains with their symmetry axes lying at right angles to each other. The anisotropies of the electronic and the magnetic properties become pronounced at ∼140 K, below which an in-plane anisotropy is reported to appear in electrical resistivity under a pressure along one of Fe-Fe directions. 75As NMR measurements have also been carried out for the As (As1) sites surrounded by one Co and three Fe. From the data, we conclude that the spin susceptibility of As1 is ∼1/3 that of As0 and that the antiferromagnetic spin fluctuation of As1 is fairly suppressed as compared with that of As0. We discuss these behaviors of the FeAs layer in connection with the impurity-induced local orbital order model proposed by theoretical studies.

NMR study of hole-doped iron-pnictide superconductor Ba1−xKxFe2As2 (x = 0.27−1)

We performed the 75As nuclear magnetic resonance measurements of Ba1−xKxFe2As2 (BKFA) for x = 0.27, 0.39, 0.58, 0.64, 0.69 and 1 to understand gap structure in this system. Temperature dependence of spin lattice relaxation rate (1/T1) below superconducting (SC) transition temperature Tc changes gradually from x = 0.39 to 1.0. This result indicates that the SC gap structure changes smoothly from full gap state with maximum Tc into nodal-line structure state for x = 1 (KFe2As2). BKFA does not have distinct SC symmetry change. The nodal gap structure appears continuously and develops gradually with increasing x.

75As NMR studies of superconducting Ba0.72K0.28Fe2As2

We have performed 75As nuclear magnetic resonance measurements on a single crystal of the hole-doped superconductor Ba0.72K0.28Fe2As2 (Tc=31.5K). The spin–lattice relaxation rate 1/T1 decreases below Tc with no coherence peak and shows a step-wise variation at low temperatures, which is indicative of multiple superconducting gaps. In the normal state, we find that Fe antiferromagnetic spin fluctuations are anisotropic.