Unusual nodal behaviors of the superconducting gap in the iron-based superconductor Ba(Fe0.65Ru0.35)

Abstract

We have investigated the superconducting gap of optimally doped Ba(Fe0.65Ru0.35)2As2 by angle-resolved photoemission spectroscopy (APRES) using bulk-sensitive 7 eV laser and synchrotron radiation. It was found that the gap is isotropic in the kx-ky plane both on the electron and hole Fermi surfaces (FSs). The gap magnitudes of two resolved hole FSs show similar kz dependences and decrease as kz approaches ~2c (i.e., around the Z point) unlike the other Fe-based superconductors reported so far, where the superconducting gap of only one hole FS shows a strong kz dependence. This unique gap structure can be understood in the scenario that the 2 orbital character is mixed into both hole FSs due to the finite spin-orbit coupling between almost degenerate FSs and is reproduced by calculations within the random phase approximation including the spin-orbit coupling.The momentum dependence of the superconducting (SC) order parameter in the Fe-based superconductors (FeSCs) has been intensively investigated by angleresolved photoemission spectroscopy (ARPES) due to their close relationship with paring mechanisms. A variety of superconducting gap symmetries in FeSCs, such as s±-wave, nodeless s++ and nodal s-wave, have been discussed from the theoretical point of view [1,2]. Up to now, even for the most intensively studied 122 systems, the situation is still complicated. As for the isovalently substituted BaFe2(As1-xPx)2 compound, it was revealed by the penetration depth and thermal conductivity measurements that there exist line nodes in the SC gap (for x = 0.33) [3]. It has been pointed out based on the random phase approximation (RPA) calculation that, three-dimensional (3D) horizontal line nodes may appear around the Z point [k = (0, 0, 2/c)] on the outermost strongly warped hole Fermi surface (FS) which has dominantly 2 orbital character [4]. However, ARPES experiments have not come to agreement on the existence of the horizontal line nodes on the 3D hole FS of BaFe2(As0.7P0.3)2 [5, 6].Ba(Fe1-xRux)2As2, another isovalently substituted system, in which doping is done at the pivotal Fe site [7,8], also exhibits a highly warped hole FS along the kz direction [9, 10]. Recent thermal conductivity measurements suggest that Ba(Fe1-xRux)2As2 has also SC gap line nodes for a wide doping range [11]. In order to clarify the SC gap structure and the location of the suggested line nodes in momentum space, we performed an ARPES study of optimally doped Ba(Fe0.65Ru0.35)2As2.In this study, two kinds of APRES apparatus were employed: one is bulk-sensitive ARPES using a vacuum ultraviolet laser; the other is based on synchrotron radiation, which allows us to trace the kz dependence of the SC gap by tuning the incident photon energy. We show that the SC gaps both on the hole FSs and electron FSs are isotropic in the kx-ky plane, while the SC gaps on the hole FSs are strongly kz dependent. Interestingly, both of the two resolved hole FSs exhibit SC gaps with almost the same kz dependence and magnitudes. We attribute this unique observat...

Influence of Spin-Orbit Coupling in Iron-Based Superconductors

“…Recent experimental observations contest this simplification as the breaking of spin-rotational invariance measured via inelastic neutron scattering (INS) [5][6][7], anisotropies in the superconducting gap parameter [8,9], and topologically non-trivial surface states [10][11][12][13], all suggest the importance of SOC in the physics of these materials. This has been corroborated by observation of energy splittings in angle-resolved photoemission spectroscopy (ARPES) measurements on a variety of FeSCs consistent with SOC [14][15][16][17][18][19]. Interpretation of these splittings is however complicated by significant band and orbital-dependent renormalizations in ARPES on FeSCs, as well as the remarkably similar influence of nematic or orbital order on the dispersion near the Brillouin zone centre [20].…”

Effects of spin-orbit coupling on spin-fluctuation induced pairing in iron-based superconductors

“…Such studies concluded that SOC is important for explaining e.g. the observed k z -dependence of the superconducting gap structure 42,43 . In addition, there exists a number of theoretical works applying effective models starting from projected band structures that point out interesting possible consequences of SOC on the superconducting pairing of FeSCs [44][45][46][47][48] .…”