Strong-Coupling Spin-Singlet Superconductivity with Multiple Full Gaps in Hole-Doped Ba_0.6K_0.4Fe₂As₂Probed by⁵⁷Fe-NMR

Abstract

We present57 Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba 0:6 K 0:4 Fe 2 As 2 (T c ¼ 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1=T 1 ) demonstrate the development of wave-number (q)-dependent spin fluctuations, except at q ¼ 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57 Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The 57 Fe-1=T 1 results are totally consistent with a s AE -wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective 1=T 1 data for Ba 0:6 K 0:4 Fe 2 As 2 and LaFeAsO 0:7 , which seemingly follow a T 5 -and a T 3 -like behaviors below T c , are consistently explained in terms of this model only by changing the size of the superconducting gap. The recent discovery of superconductivity in the iron (Fe)-based oxypnictide LaFeAsO 1Àx F x at the superconducting (SC) transition temperature T c ¼ 26 K has provided a new route toward the realization of high-T c superconductivity.1) The mother material, LaFeAsO, exhibits a structural phase transition from tetragonal (P4=nmm) to orthorhombic (Cmma) form at T $ 155 K and then exhibits a striped antiferromagnetic (AFM) order with Q ¼ ð0; Þ or ð; 0Þ and T N $ 140 K.2) The calculated Fermi surfaces (FSs) for undoped LaFeAsO consist of two small electron cylinders around the tetragonal M point and two hole cylinders, plus a heavy 3D hole pocket, around the À point.3) Measurements of the nuclear spin-lattice relaxation rate (1=T 1 ) for the LaFeAsO system in the SC state revealed the lack of a coherence peak below T c and the presence of T 3 -like behavior, suggesting an unconventional SC nature. [4][5][6]

NMR Study of Superconductivity and Spin Fluctuations in Hole-Doped Superconductor Ca 1− x Na x Fe 2 As 2 ( T

“…. Previous NMR studies on Fe superconductors [42,43] show that strong stripe AFM spin fluctuations lead Our experimental evidences for singlet pairing and spin fluctuations are consistent with many electron and the hole-doped iron pnictides with lower dopings [6,7,42,[44][45][46][47] . It is surprising, however, to see prominent low-energy spin fluctuations under such a large nominal doping (0.33 hole/Fe).…”

Muon spin rotation study of magnetism and superconductivity in Ba(Fe1−xCox)2Bernhard1, Wang2, Nuccio3 et al. 2012Phys. Rev. B5422871View full textAdd to dashboardCite

“…69 However, such a spin-triplet state does not seem to be realized in Ba(Fe 1−x Co x ) 2 As 2 for which NMR Knight shift measurements revealed a pronounced superconductivityinduced reduction of the Knight shift that is a hallmark of a spin-singlet superconducting state. 70,71 Therefore, it appears that alternative explanations for the enhanced spin correlations in the superconducting state of optimally doped and overdoped Ba(Fe 1−x Co x ) 2 As 2 need to be explored. This issue is beyond the scope of our present μSR study.…”

Unusual disorder effects in superconductingLaFeAs1−δO0.9F0.1as revealed byHammerath1, Drechsler2, Grafe3 et al. 2010Phys. Rev. B

“…To the best of our knowledge, no exponential but powerlaw dependencies ∝ T n have been observed 15,29,[33][34][35][36][37][38][39][40][41][42][43] for all Fe-based pnictide superconductors, with n in between 1.5 and 6, indicating unconventional superconductivity. These power law dependencies have been discussed within different models, such as s ± -and dwave symmetries, including the possibility of multiple SC gaps.…”

“…These power law dependencies have been discussed within different models, such as s ± -and dwave symmetries, including the possibility of multiple SC gaps. [38][39][40][41] Within the 122 family heavily overdoped compounds such as KFe 2 As 2 exhibit the lowest value observed so far whereas optimally or slightly underdoped compounds show the largest values. Recently, it has been suggested, 15,33 that the frequently observed T 3 powerlaw in the NSLRR should not be considered as an intrinsic effect but instead be attributed to some unspecified inhomogeneities in view of the missing correlation between the T c -value and the NSLRR exponent, while higher exponents would occur for cleaner samples.…”