Superconducting gaps of iron-based superconductors investigated by STM/STS

Tetsuo Hanaguri

 Mechanisms of superconductivity in iron-based superconductors have been hotly debated. Experimentally, it is highly demanding to distinguish so-called s+- gap, which is expected from the nesting-driven spin fluctuation model, from conventional s++ gap; we have to determine the relative phase of the gap opening on disconnected Fermi-surface pocket.
 STM/STS can be a powerful tool to tackle this issue. By analyzing the magnetic-field dependence of amplitudes of quasi-particle interference patterns, which can be imaged by spectroscopic imaging STM, the phase information becomes available through the coherence factor of the quasi-particle scattering processes. Our results on Fe(Se,Te) suggest the s+- gap. We also discuss defect states in LiFeAs, which can also be a test for the gap structure.