STM studies of iron-based topological superconductors

FeSe_1-xTe_x (x ~ 0.5) is a unique superconductor that possesses a topological surface state. Self-proximity-induced superconductivity at the surface may effectively be of chiral p-wave where the Majorana quasiparticle is expected in the vortex core. In principle, the Majorana quasiparticle appears as a vortex bound state at precisely zero energy, so scanning tunneling microscopy/spectroscopy (STM/STS) can detect it. In reality, however, low-lying trivial vortex bound states at finite energies (~ 100 μeV) may overlap with the putative zero-energy state, making it difficult to obtain a convincing result. We developed an ultra-low temperature STM that enabled us to achieve high enough energy resolution (~ 20 μeV) to distinguish the Majorana bound state from the trivial ones [1]. We performed experiments on FeSe_0.4Te_0.6 and observed the zero-energy vortex bound state below 20 μeV, which indicates its Majorana-quasiparticle origin [2]. We found that some vortices do not host the zero-energy vortex bound state and argue that this is due to the Majorana-Majorana interaction in a disordered vortex lattice [3]. We also investigated the changes in the superconducting gap and the band structure upon Te substitution using high-resolution STS and quasiparticle-interference imaging on the single crystals in the low Te substitution regime. We found that the superconducting gap and the band structure change concomitantly inside the electronic nematic phase, suggesting that the electronic nematicity plays a minor role in the topological nature.