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Zero-energy bound states have been detected at interstitial excess irons in the iron-based superconductor Fe(Se,Te) by using a scanning tunneling microscope (STM) [1], and its relationship to the Majorana quasiparticles (QP) has been discussed [2]. However, higher energy resolution of the order of ~10 μeV is necessary to make clear the origin of the bound states at the excess irons.
Here, we performed high-energy resolution (~20 μeV) tunneling spectroscopy around excess irons, using a dilution-refrigerator STM [3]. We found that most of the excess irons indicate only finite energy peaks. We also found excess irons with the zero energy peak, but it splits into a pair of finite energy peaks with increasing the distance from the excess irons and with changing the set-point conductance. These findings suggest that the bound states at the excess irons are not Majorana QP in origin but associated with the conventional Yu-Shiba-Russinov state.

[1] J-X. Yin et al., Nat. Phys. 11, 543 (2015).
[2] K. Jiang et al., Phys. Rev. X 9, 011033 (2019)
[3] T. Machida et al., Rev. Sci. Instrum. 89, 0930707 (2018).