Possible Sign-Reversings-Wave Superconductivity in Co-Doped BaFe₂As₂Proved by Thermal Transport Measurements

Abstract

Thermal transport measurements have been performed on single-crystalline Co-doped BaFe 2 As 2 down to 0.1 K and under magnetic fields up to 7 T. Significant peak anomalies are observed in both thermal conductivity and thermal Hall conductivity below T c as an indication of the enhancement of the quasiparticle mean-free path. Moreover, we find a sizable residual T -linear term in thermal conductivity, possibly due to a finite quasiparticle density of states in the superconducting gap induced by impurity pair breaking. Our findings support a pairing symmetry compatible with the theoretically predicted sign-reversing s-wave state.KEYWORDS: iron pnictide superconductor, Co-doped BaFe 2 As 2 , thermal transport, signreversing s-wave stateThe symmetry of the order parameter is essential for identifying the superconducting pairing mechanism. In conventional superconductors (e.g., Al and Pb), the effective electron interaction is mediated by phonons, which gives rise to the isotropic s-wave pairing symmetry.On the other hand, electron pairs glued by magnetic interactions form unconventional pairing states: p-, d-wave states and so on. So far, such unconventional superconducting states have been found in a number of materials on the boarder between magnetism and superconductivity. 1 These findings suggest that a system close to magnetic instability provides a fertile field for unconventional superconductivity. A new family of superconductors containing layers of iron pnictides bear resemblance to unconventional superconductors such as high-T c cuprates with a two-dimensional electronic structure and a magnetic order proximity to the superconducting phase. 2, 3 Therefore, an exotic superconducting pairing state can be naively expected in this system. In fact, an intriguing pairing state of sign-reversing s-wave symmetry has been theoretically proposed. 4,5 Here, we report the first thermal transport evidence of a novel pairing state in Co-doped BaFe 2 As 2 . In particular, we find a sizable residual T -linear term of the thermal conductivity, possibly due to the impurity-induced in-gap state. In addition, significant peak anomalies are observed in both thermal conductivity and thermal Hall conductivity originating from

Doping Dependence of Heat Transport in the Iron-Arsenide SuperconductorBa(Fe1−xCox)2As2Tanatar1, Reid2, Shakeripour3 et al. 2010Phys. Rev. Lett.163361613View full textAdd to dashboardCite

“…In a recent study, a large κ 0 /T was reported [16] in a Co-Ba122 sample with T c and residual resistivity ratio similar to our x = 0.074 sample, amounting to 60 % of κ N /T , compared with less than 1-2 % in our sample. This stands in contradiction not only with our results on more than 10 crystals of Co-Ba122 at various dopings, but also with previous results on K-Ba122 [3] and Ni-doped BaFe 2 As 2 (Ni-Ba122) [17], where in all cases κ 0 /T ≃ 0 at H = 0.…”

Thermal conductivity of overdopedBaFe1.73Co0.27As2single crystal: Evidence for nodeless multiple superconducting gaps

“…By contrast, a large κ 0 /T was observed in BaFe 1.86 Co 0.14 As 2 . 30 For the prototype FeSe x (T c = 8.8 K) superconductor, the thermal conductivity shows clear behavior of multiple nodeless superconducting gaps. 31 In two superconductors with lower T c , κ 0 /T of BaNi 2 As 2 (T c = 0.7 K) is consistent with a dirty fully gapped superconductivity, 32 while LaFePO (T c = 7.4 K) appears to have a finite κ 0 /T , suggesting the gap on some band may have nodes.…”

“…30 It has been noted that the large κ 0 /T in zero field observed in BaFe 1.86 Co 0.14 As 2 by Machida et al 30 may be extrinsic.…”

“…[28][29][30] It will be very interesting to study highly underdoped and overdoped samples to demonstrate its superconducting gap structure over the whole doping range. Furthermore, due to the high T c and H c2 of optimally doped sampls, magnetic field can only be applied up to about 30% of their H c2 .…”

Evidence from anisotropic penetration depth for a three-dimensional nodal superconducting gap in single-crystallineBa(Fe1−xNixMartin1, Kim2, Gordon3 et al. 2010Phys. Rev. B578440View full textAdd to dashboardCite

“…Some early measurements, including angle -resolved electron spectroscopy [1][2][3][4], point contact spectroscopy [5], penetration depth [6,7] and specific heat [8] were interpreted in a framework of fully gapped superconductivity. Others, such as NMR [9], penetration depth [10][11][12][13] and thermal conductivity [14,15] have suggested a strongly anisotropic gap. More recent systematic measurements of thermal conductivity over the whole doping phase diagram in Ba(Fe 1−x Co x ) 2 As 2 (FeCo-122) single crystals have shown a clear evolution towards a more anisotropic gap in the overdoped regime [16].…”