Band-dependent superconducting gap in SrFe$_{2}$(As$_{0.65}$P$_{0.35}$)$_{2}$ studied by angle-resolved photoemission spectroscopy

TL;DR

This study uses angle-resolved photoemission spectroscopy to investigate the three-dimensional superconducting gap structure in SrFe$_{2}$(As$_{0.65}$P$_{0.35}$)$_{2}$, revealing band-dependent gap magnitudes and sign changes, which shed light on the nodal superconductivity in this material.

Contribution

The paper provides the first detailed three-dimensional mapping of superconducting gaps in SrFe$_{2}$(As$_{0.65}$P$_{0.35}$)$_{2}$, highlighting band-dependent gap variations and sign changes.

Findings

Different magnitudes of SC gaps on three hole Fermi surfaces.

Almost isotropic and $k_{z}$-independent gaps on electron Fermi surfaces.

Sign change of the outer hole Fermi surface gap around the Z-X direction.

Abstract

The isovalent-substituted iron pnictide compound SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ 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 SrFe$_{2}$(As$_{0.65}$P$_{0.35}$)$_{2}$ 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 $k_{z}$-independent. We propose that the SC gap of the outer hole FS changes sign around the Z-X [($0, 0, 2\pi$)-($\pi,\pi, 2\pi$)] direction.