Multi-atom quasiparticle scattering interference for superconductor energy-gap symmetry determination

Complete theoretical understanding of the most complex superconductors requires a detailed knowledge of the symmetry of the superconducting energy-gap Delta(a)(k), for all momenta k on the Fermi surface of every band a. While there are a variety of techniques for determining vertical bar Delta(a)(k)vertical bar, no general method existed to measure the signed values of Delta(a)(k). Recently, however, a technique based on phaseresolved visualization of superconducting quasiparticle interference (QPI) patterns, centered on a single non-magnetic impurity atom, was introduced. In principle, energy-resolved and phase-resolved Fourier analysis of these images identifies wavevectors connecting all k-space regions where Delta(a)(k) has the same or opposite sign. But use of a single isolated impurity atom, from whose precise location the spatial phase of the scattering interference pattern must be measured, is technically difficult. Here we introduce a generalization of this approach for use with multiple impurity atoms, and demonstrate its validity by comparing the Delta(a)(k) it generates to the Delta(a)(k) determined from single-atom scattering in FeSe where s +/- energy-gap symmetry is established. Finally, to exemplify utility, we use the multi-atom technique on LiFeAs and find scattering interference between the hole-like and electronlike pockets as predicted for Delta(a)(k) of opposite sign.

Automated summary

Understanding the symmetry of superconducting energy-gap in the most complex superconductors requires detailed knowledge of the gap Δ(a)(k) at all momenta on the Fermi surface. While techniques exist to determine the magnitude of the energy gap, measuring the signed values remains a challenge. A recent technique using phaseresolved visualization of superconducting quasiparticle interference patterns around a single non-magnetic impurity atom has shown promise in identifying wavevectors connecting regions of the same or opposite sign of the energy gap.