Vortex core of topological superconductors is one of the promising platforms that can host the Majorana zero mode. Recently, topological superconductivity has been suggested at the surface of Fe(Se,Te)  and STM have been utilized to search for the zero-energy vortex bound state (ZVBS) associated with the Majorana zero mode [2,3]. However, it has been difficult to clearly distinguish the ZVBS from the lowest trivial vortex bound state that may appear at ~100 µeV. We have developed an ultra-low temperature (~ 85 mK) STM that enables us to perform local spectroscopy with high energy resolution of ~20 µeV . We systematically investigated many vortices in Fe(Se,Te) and 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 . These results are well reproduced by a simulation that takes Majorana-Majorana interaction and disorder in the vortex-lattice structure into account .
This work has been done in collaboration with T. Machida, Y. Sun, S. Pyon, S. Takeda, Y. Kohsaka, T. Sasagawa, T. Tamegai, and C. -K. Chiu.
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Tetsuo Hanaguri received his PhD in Applied Physics at Tohoku University in 1993. Currently, Tetsuo is a Team Leader of Emergent Phenomena Measurement Research Team, RIKEN Center for Emergent Matter Science. His research involves the experimental study of electronic states related to emergent phenomena in electron systems, such as high-temperature superconductivity and topological quantum phenomena.