skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: 77Se NMR Investigation of the KxFe2−ySe2 high-Tc Superconductor (Tc = 33 K)

Abstract

We report comprehensive {sup 77}Se NMR measurements on a single crystalline sample of the recently discovered FeSe-based high-temperature superconductor K{sub x}Fe{sub 2-y}Se{sub 2} (T{sub c} = 33 K) in a broad temperature range up to 290 K. Despite deviations from the stoichiometric KFe{sub 2}Se{sub 2} composition, we observed {sup 77}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 {sup 77}Se 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{sub {+-}} symmetry of superconductivity in the absence of the hole bands at the Brillouin zone center. From the increase of NMR linewidth below T{sub c} induced by the Abrikosov lattice of superconducting vortices, we estimate the in-plane penetration depth {lambda}{sub ab} {approx} 290 nm and the carrier concentration n{sub e} {approx} 1more » x 10{sup +21} cm{sup -3}. Our Knight shift {sup 77}K data indicate that the uniform spin susceptibility decreases progressively with temperature, in analogy with the case of FeSe (T{sub c} {approx} 9 K) as well as other FeAs high-T{sub c} systems. The strong suppression of {sup 77}K observed immediately below T{sub 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/T1 immediately below T{sub 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{sub c} in 1/T{sub 1}T. Instead, 1/T{sub 1}T exhibits qualitatively the same behavior as overdoped non-superconducting Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2} with x {approx} 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.« less

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
Brookhaven National Lab. (BNL), Upton, NY (United States)
Sponsoring Org.:
DOE - OFFICE OF SCIENCE
OSTI Identifier:
1025442
Report Number(s):
BNL-95217-2011-JA
Journal ID: ISSN 1098-0121; R&D Project: PO-013; KC0202010; TRN: US1104882
DOE Contract Number:  
DE-AC02-98CH10886
Resource Type:
Journal Article
Journal Name:
Physical Review. B, Condensed Matter and Materials Physics
Additional Journal Information:
Journal Volume: 83; Journal Issue: 10; Journal ID: ISSN 1098-0121
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; BRILLOUIN ZONES; COOPER PAIRS; FLUCTUATIONS; KNIGHT SHIFT; MAGNETIC FIELDS; PENETRATION DEPTH; SHAPE; SPIN; SPIN-LATTICE RELAXATION; SUPERCONDUCTIVITY; SUPERCONDUCTORS; SYMMETRY; VACANCIES; VORTICES; high-temperature superconductivity; angle resolved photoemission spectroscopy

Citation Formats

Petrovic, C, Torchetti, D.A. Fu, M., Christensen, D C, Nelson, K J, Imai, T, and Lei, H C. 77Se NMR Investigation of the KxFe2−ySe2 high-Tc Superconductor (Tc = 33 K). United States: N. p., 2011. Web.
Petrovic, C, Torchetti, D.A. Fu, M., Christensen, D C, Nelson, K J, Imai, T, & Lei, H C. 77Se NMR Investigation of the KxFe2−ySe2 high-Tc Superconductor (Tc = 33 K). United States.
Petrovic, C, Torchetti, D.A. Fu, M., Christensen, D C, Nelson, K J, Imai, T, and Lei, H C. Fri . "77Se NMR Investigation of the KxFe2−ySe2 high-Tc Superconductor (Tc = 33 K)". United States.
@article{osti_1025442,
title = {77Se NMR Investigation of the KxFe2−ySe2 high-Tc Superconductor (Tc = 33 K)},
author = {Petrovic, C and Torchetti, D.A. Fu, M. and Christensen, D C and Nelson, K J and Imai, T and Lei, H C},
abstractNote = {We report comprehensive {sup 77}Se NMR measurements on a single crystalline sample of the recently discovered FeSe-based high-temperature superconductor K{sub x}Fe{sub 2-y}Se{sub 2} (T{sub c} = 33 K) in a broad temperature range up to 290 K. Despite deviations from the stoichiometric KFe{sub 2}Se{sub 2} composition, we observed {sup 77}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 {sup 77}Se 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{sub {+-}} symmetry of superconductivity in the absence of the hole bands at the Brillouin zone center. From the increase of NMR linewidth below T{sub c} induced by the Abrikosov lattice of superconducting vortices, we estimate the in-plane penetration depth {lambda}{sub ab} {approx} 290 nm and the carrier concentration n{sub e} {approx} 1 x 10{sup +21} cm{sup -3}. Our Knight shift {sup 77}K data indicate that the uniform spin susceptibility decreases progressively with temperature, in analogy with the case of FeSe (T{sub c} {approx} 9 K) as well as other FeAs high-T{sub c} systems. The strong suppression of {sup 77}K observed immediately below T{sub 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/T1 immediately below T{sub 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{sub c} in 1/T{sub 1}T. Instead, 1/T{sub 1}T exhibits qualitatively the same behavior as overdoped non-superconducting Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2} with x {approx} 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.},
doi = {},
url = {https://www.osti.gov/biblio/1025442}, journal = {Physical Review. B, Condensed Matter and Materials Physics},
issn = {1098-0121},
number = 10,
volume = 83,
place = {United States},
year = {2011},
month = {3}
}