Quasiparticle Scattering Interference in (K,Tl)FexSe2 Superconductors

TL;DR

This paper models quasiparticle interference patterns in (K,Tl)Fe_xSe2 superconductors, predicting distinct features for different pairing symmetries to help identify the pairing mechanism via STM experiments.

Contribution

It provides the first detailed theoretical prediction of QPI patterns for different pairing symmetries in (K,Tl)Fe_xSe2 superconductors, highlighting how impurity scattering affects observable features.

Findings

QPI occurs around specific momentum transfers due to electron pockets.

Distinct QPI features differentiate d_{x^2-y^2}-wave and s-wave pairing.

Unique QPI patterns are predicted for local pair-potential impurities.

Abstract

We model the quasiparticle interference (QPI) pattern in the recently discovered (K,Tl)Fe_xSe2 superconductors. We show in the superconducting state that, due to the absence of hole pockets at the Brillouin zone center, the quasiparticle scattering occurs around the momentum transfer q=(0,0) and (\pm \pi, \pm \pi) between electron pockets located at the zone boundary. More importantly, although both d_{x^2-y^2}-wave and s-wave pairing symmetry lead to nodeless quasiparticle excitations, distinct QPI features are predicted between both types of pairing symmetry. In the presence of a nonmagnetic impurity scattering, the QPI exhibits strongest scattering with q=(\pm \pi, \pm \pi) for the d_{x^2-y^2}-wave pairing symmetry; while the strongest scattering exhibits a ring-like structure centered around both q=(0,0) and (\pm \pi, \pm \pi) for the isotropic s-wave pairing symmetry. A unique QPI pattern has also been predicted due to a local pair-potential-type impurity scattering. The significant contrast in the QPI pattern between the d_{x^2-y^2}-wave and the isotropic s-wave pairing symmetry can be used to probe the pairing symmetry within the Fourier-transform STM technique.