Majorana quasiparticles (MQPs) in condensed matter may play an important role in future fault-tolerant quantum computing but their implementations in real systems are elusive. Vortex core of topological superconductors is one of the possible platforms that accommodates the MQP. Recently, it has been shown that the surface of the iron-based superconductor Fe(Se,Te) hosts topological superconductivity [1] and scanning tunneling spectroscopies have been utilized to detect the zero-energy vortex bound state (ZVBS) associated with the MQP [2,3]. However, the results are controversial [2,3]. Here, we performed ultra-low temperature (~ 85 mK) spectroscopic-imaging scanning tunneling microscopy with unprecedented high energy resolution (~ 20 eV) to clarify the nature of the ZVBS in Fe(Se,Te) [4]. Our comprehensive investigation of many vortices at different magnetic fields revealed that there are two kinds of vortices with and without the ZVBS. We found that chemical and electronic quenched disorders are apparently unrelated to the ZVBS formation, whereas increasing magnetic field suppresses the fraction of vortices with the ZVBS. We argue the relation between the ZVBS and the MQP by taking the vortex-vortex interaction and the disorder in the vortex lattice into consideration.

[1] P. Zhang et al., Science 360, 182 (2018).
[2] D. Wang et ?al., Science 362, 333 (2018).
[3] M. Chen et al., Nature Commun. 9, 970 (2018).
[4] T. Machida et al., arXiv:1812.08995.

Bio：Tetsuo Hanaguri is a team leader at RIKEN Center for Emergent Matter Science. He obtained his PhD from Tohoku University in 1993. He worked as a research associate and an associate professor at the University of Tokyo before moving to RIKEN in 2004. He has been working in the field of low-temperature experiments and has transferred >30,000 liters of liquid helium to the cryostats during his career. He is currently interested in spectroscopic-imaging scanning tunneling microscopy on unconventional superconductors and topological materials.