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

Quasiparticle interference (QPI), by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and multiorbital tight-binding calculations, is used to investigate the band structure and superconducting order parameter of LiFeAs. Using this combination of techniques we identify intraband 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 demonstrate an s(+/-) gap structure, where a sign change occurs in the superconducting order parameter between the e and h bands.