Abstract
The isovalent-substituted iron pnictide compound SrFe2(As1−xPx)2 exhibits multiple evidence for nodal superconductivity via various experimental probes, such as the penetration depth, nuclear magnetic resonance and specific heat measurements. The direct identification of the nodal superconducting (SC) gap structure is challenging, partly because the presence of nodes is not protected by symmetry but instead caused by an accidental sign change of the order parameter, and also because of the three-dimensionality of the electronic structure. We have studied the SC gaps of SrFe2(As0.65P0.35)2 in three-dimensional momentum space by synchrotron and laser-based angle-resolved photoemission spectroscopy. The three hole Fermi surfaces (FSs) at the zone center have SC gaps with different magnitudes, whereas the SC gaps of the electron FSs at the zone corner are almost isotropic and kz-independent. As a possible nodal SC gap structure, we propose that the SC gap of the outer hole FS changes sign around the Z-X [(0, 0, 2π) − (π, π, 2π)] direction.
Highlights
Since the discovery of high-temperature superconductivity in the iron pnictides and chalcogenides[1], the mechanism of Cooper pairing and the symmetry of the order parameter have been central issues of debate[2,3]
In-plane FS mapping taken with hν = 24 eV and 28 eV is shown in Superconducting gaps in the hole Fermi surfaces
The observation of three FSs and their radii indicate that hν = 7 eV measures a plane close to the Z point
Summary
Since the discovery of high-temperature superconductivity in the iron pnictides and chalcogenides[1], the mechanism of Cooper pairing and the symmetry of the order parameter have been central issues of debate[2,3]. Saito et al.[19] have shown that, by taking orbital flucutuations into account, a nodal s-wave state may appear with loop-shaped line nodes on the electron FSs. in stark contrast to the spin-fluctuation scenario, the gap function on the outer hole FS around the Z point may have a finite value that is comparable to those in the other two hole FSs, as a result of the interorbital correlations among the d orbitals. Yamashita et al.[22] observed a fourfold oscillation in angle-resolved thermal conductivity as a function of magnetic field direction within the basal plane They have concluded that loop-like line nodes appear on the outer electron FS23 to explain their observations. There is a discrepancy in the literature on the ARPES data utilizing variable energy photons from synchrotron[24,25]
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