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

Abstract

The in-plane thermal conductivity κ of overdoped FeAs-based superconductor BaFe1.73Co0.27As2 (Tc = 8.1 K) single crystal was measured down to 80 mK. In zero field, the residual linear term κ0/T is negligible, suggesting a nodeless superconducting gap in the ab-plane. In magnetic field, κ0/T increases rapidly, very different from that of conventional s-wave superconductors. This anomalous κ0/T (H) may reveal an exotic superconducting gap structure in overdoped BaFe1.73Co0.27As2: the vanishing hole (β) pocket has a much larger gap than the electron (γ and δ) pockets which contain most of the carriers. Such an exotic gap structure is an evidence for superconducting state induced by interband interactions, in which the band with the smaller density of states has a larger gap.

Superconducting properties of thes±-wave state: Fe-based superconductors

“…The normal DOSs N a (a = S, L) for each band were assumed equal and the unitary impurity (c = 0) with the concentration Γ/∆ L = 0.05 was included. The overall behavior of this theoretical result is very similar to the experimental measurements of κ(H)/T for Ba(Fe 1−x Co x ) 2 As 2 [124,125] shown in Fig.30. In particular, the systematic increase of the slope of κ(H)/T vs H with increasing Co doping ("x") and the evolution of the overall shape of the field dependence from a flat (concave up) for a smaller "x" to a steep (concave down, hence looks almost ∼ √ H) for a larger "x" is exactly captured by this simple two band s ± -wave pairing model.…”

Robust Nodal Superconductivity Induced by Isovalent Doping inBa(Fe1−xRux)2As2andQiu1, Zhou2, Zhang3 et al. 2012Phys. Rev. XSelf Cite431170View full textAdd to dashboardCite

“…Figure 3 shows the temperature dependence of the inplane thermal conductivity for OP20K, UD17K, and UD4K in zero and finite magnetic fields, plotted as =T vs T. All the curves are roughly linear, as previously observed in BaFe 1:9 Ni 0:1 As 2 [8], KFe 2 As 2 [10], and overdoped BaðFe 1Àx Co x Þ 2 As 2 single crystals [9,12]. Therefore, we fit all the curves to =T ¼ a þ bT À1 , with fixed at 2.…”

“…Those experiments have been further supported by measurements of bulk properties such as thermal conductivity [7][8][9]. On the overdoped (with respect to the optimal doping) side, nodal superconductivity has been found in the extremely hole-doped KFe 2 As 2 [10,11], while a strongly anisotropic nodeless gap [9] or isotropic nodeless gaps with significantly different magnitudes [12,13] have been suggested in heavily electron-doped BaðFe 1Àx Co x Þ 2 As 2 . On the underdoped side, recent heat-transport measurements have claimed possible nodes in the superconducting gap of hole-doped Ba 1Àx K x Fe 2 As 2 with x < 0:16 [14], in contrast to the nodeless gaps that have been found in electron-doped BaðFe 1Àx Co x Þ 2 As 2 [9].…”

“…In iron pnictides, the strong Fermi-surface evolution upon hole and electron doping has given clues to understand the complex superconducting gap structures [10,12]. However, the Fermi-surface evolution upon isovalent P and Ru doping is rather delicate, mainly caused by abovedescribed crystal structure change.…”

Calorimetric evidence for nodes in the overdoped Ba(Fe0.9Co0.1)2As2

“…Conflicting interpretations of the SC order parameter have also been reported in Co-doped BaFe 2 As 2 (Ba122 phase). Angle-resolved photoemission spectroscopy (ARPES) [4] and scanning tunneling microscopy (STM) [5] suggest an isotropic gap, whereas Raman scattering [6], nuclear magnetic resonance (NMR) [7], muon spin rotation (µSR) [8], thermal conductivity [9,10], specific heat [11] and penetration depth [12,13] measurements all indicate an anisotropic gap on one or more Fermi surface sheets. Strong doping dependence of SC properties and/or specific features of the measurement techniques that probe preferentially a fraction of the total Fermi surface could be behind these seemingly contradictory results [14,15].…”