Abstract
We report comprehensive NMR measurements on a single crystalline sample of the recently discovered FeSe-based high-temperature superconductor FeSe ( K) in a broad temperature range up to 290 K. Despite deviations from the stoichiometric KFeSe composition, we observed NMR line shapes as narrow as 4.5 kHz under a magnetic field applied along the crystal axis, and found no evidence for co-existence of magnetic order with superconductivity. On the other hand, the NMR line shape splits into two peaks with equal intensities at all temperatures when we apply the magnetic field along the 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 symmetry of superconductivity in the absence of the hole bands at the Brillouin zone center. From the increase of NMR linewidth below induced by the Abrikosov lattice of superconducting vortices, we estimate the in-plane penetration depth nm and the carrier concentration cm. Our Knight shift data indicate that the uniform spin susceptibility decreases progressively with temperature, in analogy with the case of FeSe ( K) as well as other FeAs high- systems. The strong suppression of observed immediately below 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 immediately below , 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 in . Instead, exhibits qualitatively the same behavior as overdoped non-superconducting Ba(FeCo)As with 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.
1 More- Received 25 January 2011
DOI:https://doi.org/10.1103/PhysRevB.83.104508
©2011 American Physical Society
Synopsis
NMR uncovers missing magnetic fluctuations
Published 18 March 2011
Magnetic resonance measurements probe the “insides” of an iron-based superconductor, raising questions about the origins of superconductivity in these materials.
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