From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe₂As₂

Abstract

Abstract. The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe 2 As 2 , there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature T c . We then compare KFe 2 As 2 to Ba 0.6 K 0.4 Fe 2 As 2 , obtained by Ba substitution, where the pairing symmetry is s-wave and the T c is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe 2 As 2 to the nodal ironbased superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.

Angular-dependent upper critical field of overdoped Ba(Fe1−xNix)2Murphy1, Tanatar2, Graf3 et al. 2013Phys. Rev. B245150View full textAdd to dashboardCite

“…For pure NbSe 2 the H −2 c2 (sin θ) plot shows most clear deviations from linearity among all materials, with an upward curvature towards θ = 90 • , of opposite trend to pure MgB 2 and BaNi122. The two angular data sets for profoundly multi-band iron pnictide superconductors, slightly underdoped, BaK122, 60 and heavily overdoped K122, 65 generally follow linear dependence despite profound difference in the superconducting gap structure, nodeless in the former case 69 and with vertical line nodes in the latter. 70 Considering that among all the materials for which we were able to find published H c2 (θ), only pure multi-band MgB 2 and NbSe 2 reveal clear deviations from Eq.…”

Multiband Superconductivity in KFe2As2: Evidence for One Isotropic and Several Lilliputian Energy Gaps

“…In Refs. 9 and, 49 thermal-conductivity measurements κ(T)/T, performed for T > 0.1 K, were found to extrapolate at T → 0 to a finite residual term κ(0)/T independent of sample purity. This was interpreted as a signature of universal heat transport, a property of superconductors with symmetry-imposed line nodes such as d-wave states.…”

s+isstate with broken time-reversal symmetry in Fe-based superconductors

“…(i) Near optimal doping, x ∼ 0.4, ARPES 25,26 , neutron scattering 27 , penetration depth 28 and thermal conductivity 29,30 measurements give strong evidence for nodeless, near-constant s± gap, which changes sign between hole and electron pockets. This is consistent with theoretical calculations [5][6][7][8]11,31 .…”