77Se NMR investigation of the KxFe2-ySe2 high-Tc superconductor (Tc=33 K)

We report comprehensive Se-77 NMR measurements on a single crystalline sample of the recently discovered FeSe-based high-temperature superconductor K x Fe2-ySe2 (T-c = 33 K) in a broad temperature range up to 290 K. Despite deviations from the stoichiometric KFe2Se2 composition, we observed Se-77 NMR line shapes as narrow as 4.5 kHz under a magnetic field applied along the crystal c axis, and found no evidence for co-existence of magnetic order with superconductivity. On the other hand, the Se-77 NMR line shape splits into two peaks with equal intensities at all temperatures when we apply the magnetic field along the ab plane. This suggests that K vacancies may have a superstructure and that the local symmetry of the Se sites is lower than the tetragonal fourfold symmetry of the average structure. This effect might be a prerequisite for stabilizing the s(+/-) symmetry of superconductivity in the absence of the hole bands at the Brillouin zone center. From the increase of NMR linewidth below T-c induced by the Abrikosov lattice of superconducting vortices, we estimate the in-plane penetration depth lambda(ab) similar to 290 nm and the carrier concentration n(e) similar to 1 x 10(+21) cm(-3). Our Knight shift K-77 data indicate that the uniform spin susceptibility decreases progressively with temperature, in analogy with the case of FeSe (T-c similar to 9 K) as well as other FeAs high-T-c systems. The strong suppression of K-77 observed immediately below T-c for all crystal orientations is consistent with a singlet pairing of Cooper pairs. We do not however observe the Hebel-Slichter coherence peak of the nuclear spin-lattice relaxation rate 1/T-1 immediately below Tc, expected for conventional BCS s-wave superconductors. In contrast with the case of FeSe, we do not observe evidence for an enhancement of low-frequency antiferromagnetic spin fluctuations near Tc in 1/T1T. Instead, 1/T1T exhibits qualitatively the same behavior as overdoped non-superconducting Ba(Fe1-xCox)(2)As-2 with x similar to 0.14 or greater, where hole bands are missing in the Brillouin zone center. We will discuss the implications of our results on the unknown mechanism of high-temperature superconductivity in FeSe and FeAs systems.