Year: 2011
Type: article
Abstract: We report comprehensive $^{77}\mathrm{Se}$ NMR measurements on a single crystalline sample of the recently discovered FeSe-based high-temperature superconductor ${\mathrm{K}}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ (${T}_{c}=33$ K) in a broad temperature range up to 290 K. Despite deviations from the stoichiometric KFe${}_{2}$Se${}_{2}$ composition, we observed $^{77}\mathrm{Se}$ 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 $^{77}\mathrm{Se}$ NMR line shape splits into two peaks with equal intensities at all temperatures when we apply the magnetic field along the $\mathit{ab}$ 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$${}_{\ifmmode\pm\else\textpm\fi{}}$ 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 ${\ensuremath{\lambda}}_{\mathit{ab}}~290$ nm and the carrier concentration ${n}_{e}~1\ifmmode\times\else\texttimes\fi{}{10}^{+21}$ cm${}^{\ensuremath{-}3}$. Our Knight shift ${}^{77}K$ data indicate that the uniform spin susceptibility decreases progressively with temperature, in analogy with the case of FeSe (${T}_{c}~9$ K) as well as other FeAs high-${T}_{c}$ systems. The strong suppression of ${}^{77}K$ 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 ${T}_{c}$, 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 ${T}_{c}$ in $1/{T}_{1}T$. Instead, $1/{T}_{1}T$ exhibits qualitatively the same behavior as overdoped non-superconducting Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ with $x~0.14$ 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.
Source: Physical Review B
Authors: D. A. Torchetti, M. Fu, D. C. Christensen, K. J. Nelson, T. Imai, H. C. Lei, C. Petrovic
Language: Latvian
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
Funder: U.S. Department of Energy
OpenAlex ID: W1660143301