Robust Nodal Superconductivity Induced by Isovalent Doping inBa(Fe1−xRux)2As2and

Abstract

We present the ultra-low-temperature heat-transport study of iron-based superconductors BaðFe 1Àx Ru x Þ 2 As 2 and BaFe 2 ðAs 1Àx P x Þ 2 . For optimally doped BaðFe 0:64 Ru 0:36 Þ 2 As 2 , a large residual 0 =T at zero field and a ffiffiffiffi ffi H p dependence of 0 ðHÞ=T are observed, which provide strong evidences for nodes in the superconducting gap. This result demonstrates one more nodal superconductor in iron pnictides. The similarities between isovalent Fe and P dopings strongly suggest that the nodal superconductivity in Since the discovery of high-T c superconductivity in iron pnictides [1,2], the issue of what mechanisms underlie the electron pairing in these systems has been a central one [3]. One key step in resolving it is to clarify the symmetry and structure of the superconducting gap [4]. However, even for the most studied ðBa; Sr; Ca; EuÞFe 2 As 2 (122) system, the situation is still fairly complex [4].Near optimal doping where the T c is the highest, for both hole-and electron-doped 122 compounds, the angleresolved photon-emission-spectroscopy (ARPES) experiments have clearly demonstrated multiple nodeless superconducting gaps [5,6]. 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]. It is hard to get a simple pairing mechanism from such complex situation of the superconducting gap structure.More intriguingly, nodal superconductivity has also been found in optimally doped BaFe 2 ðAs 0:67 P 0:33 Þ 2 (T c ¼ 30K) [15,16], in which the superconductivity is induced by isovalent P doping. Moreover, LaFePO (T c $6K) was found to display clear nodal behavior [17][18][19], and recently evidence for nodes in the superconducting gap of LiFeP (T c $ 4:5 K) has emerged from measurements of magnetic penetration depth [20]. The nodal superconductivity in these P-doped compounds is very striking, which raises the puzzling questions of whether, and why, the P doping is unique among iron-based superconductors. Although various theoretical explanations have been offered for this puzzle, investigators are far from reaching consensus [21][22][23][24][25].A recent empirical proposal is that the nodal state in ironpnictide superconductors, except for KFe 2 As 2 , is induced when the pnictogen height h Pn from the iron plane decreases below a threshold value of approximately 1:33 A [20]. According to this proposal, there may exist a transition from...

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