Quasiparticle interference and symmetry of superconducting order parameter in strongly electron-doped iron-based superconductors

Abstract

Motivated by recent experimental reports of significant spin–orbit coupling (SOC) and a sign-changing order-parameter in the Li _1− _x Fe _x (OHFe) _1− _y Zn _y Se superconductor with only electron pockets present, we study the possible Cooper-pairing symmetries and their quasiparticle interference (QPI) signatures. We find that each of the resulting states— s -wave, d -wave and helical p -wave—can have a fully gapped density of states consistent with angle-resolved photoemission spectroscopy experiments and, due to SOC, are a mixture of spin singlet and triplet components leading to intra- and inter-band features in the QPI signal. Analyzing predicted QPI patterns we find that only the spin-triplet dominated even parity A _1 _g ( s -wave) and B _2 _g ( d -wave) pairing states are consistent with the experimental data. Additionally, we show that these states can indeed be realized in a microscopic model with atomic-like interactions and study their possible signatures in spin-resolved STM experiments.

Keywords

• unconventional superconductivity
• iron-based systems
• scanning tunneling spectroscopy
• superconducting order parameter
• spin–orbit coupling
• phase sensitive probes