Sign inversion in the superconducting order parameter of LiFeAs inferred from Bogoliubov quasiparticle interference

TL;DR

This study combines STM/STS, ARPES, and theoretical calculations to reveal a sign-changing s$_ ext{±}$ superconducting gap in LiFeAs, demonstrating the use of quasiparticle interference to probe order parameter symmetry.

Contribution

The paper introduces a phase-sensitive QPI method to directly detect the sign change in the superconducting order parameter of LiFeAs.

Findings

Identification of intra- and interband scattering vectors in QPI maps.

Reconciliation of band dispersion discrepancies between ARPES and STM/STS.

Evidence for an s$_ ext{±}$ gap structure with a sign change between hole and electron bands.

Abstract

Quasiparticle interference (QPI) by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES), and multi-orbital tight bind- ing calculations are used to investigate the band structure and superconducting order parameter of LiFeAs. Using this combination we identify intra- and interband scattering vectors between the hole (h) and electron (e) bands in the QPI maps. Discrepancies in the band dispersions inferred from previous ARPES and STM/STS are reconciled by recognizing a difference in the $k_z$ sensitivity for the two probes. The observation of both h-h and e-h scattering is exploited using phase-sensitive scattering selection rules for Bogoliubov quasiparticles. From this we infer an s$_\pm$ gap structure, where a sign change occurs in the superconducting order parameter between the e and h bands.