Potential antiferromagnetic fluctuations in hole-doped iron-pnictide superconductor Ba_{1-x}K_{x}Fe_{2}As_{2} studied by ^{75}As nuclear magnetic

We have performed ^{75}As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) on single crystalline Ba_{1-x}K_{x}Fe_{2}As_{2} for x = 0.27-1. ^{75}As nuclear quadruple resonance frequency ({ u}_{Q}) increases linearly with increasing x. The Knight shift K in normal state shows Pauli paramagnetic behavior with slight temperature T dependence. The value of K increases gradually with increasing x. By contrast, nuclear spin- lattice relaxation rate 1/T_{1} in normal state has a large T-dependence, which indicates existence of large antiferomagnetic (AF) spin fluctuations for all x. The T-dependence of 1/T_{1} shows a gap-like behavior below approximately 100 K for 0.6 < x < 0.9. These behaviors are well explained by the change of band structure with expansion of hole Fermi surfaces and shrink and disappearance of electron Fermi surfaces at Brillouin zone (BZ) with increasing x. The anisotropy of 1/T_{1}, represented by a ratio of 1/T_{1ab} to 1/T_{1c}, is always larger than 1 for all x, which indicates that the stripe-type AF fluctuations is dominant in this system. The K in superconducting (SC) state decreases, which corresponds to appearance of spin-singlet superconductivity. The T dependence of 1/T_{1} in SC state indicates 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 smaller gap decreases rapidly and nearly vanishes for x > 0.6 where the electron pockets in BZ disappear.

Single Crystal Growth and Characterization of the Iron-Based Superconductor KFe2As2 Synthesized by KAs Flux Method

Centimeter sized platelet single crystals of KFe2As2 were grown using a self-flux method. An encapsulation technique using commercial stainless steel container allowed the stable crystal growth lasting for more than 2 weeks. Ternary K-Fe-As systems with various starting compositions were examined to determine the optimal growth conditions. Employment of KAs flux led to the growth of large single crystals with the typical size of as large as 15 mm x 10 mm x 0.4 mm. The grown crystals exhibit sharp superconducting transition at 3.4 K with the transition width 0.2 K, as well as the very large residual resistivity ratio exceeding 450, evidencing the good sample quality.