Direct visualization of sign reversal s^+- superconducting gaps in FeTe0.55Se0.45

TL;DR

This study uses quasiparticle interference imaging to directly visualize the sign reversal of superconducting gaps in FeTe0.55Se0.45, confirming the s^+- pairing symmetry in this iron-based superconductor.

Contribution

It applies a defect-bound-state QPI technique to directly observe sign reversal gaps, providing experimental evidence for s^+- pairing symmetry in FeTe0.55Se0.45.

Findings

Negative phase reference quantity near the gap indicates sign reversal.

Enhanced scattering at q_2 consistent with s^+- pairing.

No sign change observed at q_3, supporting theoretical models.

Abstract

In many unconventional superconductors, the pairing of electrons is driven by the repulsive interaction, which leads to the sign reversal of superconducting gaps along the Fermi surfaces (FS) or between them. However, to measure this sign change is not easy and straightforward. It is known that, in superconductors with sign reversal gaps, non-magnetic impurities can break Cooper pairs leading to the quasiparticle density of states in the superconducting state. The standing waves of these quasiparticles will interfere each other leading to the quasiparticle interference (QPI) pattern which carries the phase message reflecting also the superconducting gap structure. Based on the recently proposed defect-bound-state QPI technique, we explore the applicability of this technique to a typical iron based superconductor FeTe$_{0.55}$Se$_{0.45}$ with roughly equivalent gap values on the electron and hole pockets connected by the wave vector q_2=(0,\pi). It is found that, on the negative energy side, with the energy slightly below the gap value, the phase reference quantity $|g(q,-E)|\cos(\theta_{q,+E}-\theta_{q,-E}) becomes negative and the amplitude is strongly enhanced with the scattering vector q_2, but that corresponding to the scattering between the electron-electron pockets, namely q_3=(\pi,\pi), keeps all positive. This is well consistent with the theoretical expectation of the s^+- pairing gap and thus serves as a direct visualization of the sign reversal gaps. This experimental observation is also supported by the theoretical calculations with the Fermi surface structure and s^+- pairing gap.