Spatially dispersing impurity-driven states in the iron-based superconductor FeTe_0.55Se_0.45

An impurity in superconductors can break the Cooper pairs into quasiparticles, having quasiparticle states within the superconducting gap. The in-gap states are of importance because their characteristics manifest the superconducting ground state. For example, the impurity-induced in-gap states can be used to reveal the superconducting gap symmetry [1], and when they especially appear at the zero energy, they can indicate a topological superconductivity as Majorana bound states [2,3]. From this perspective, the iron-based superconductor FeTe_0.55Se_0.45 has been investigated by scanning tunneling microscopy with particular interest in spatially varying Yu-Shiba-Rusinov type impurity states [4] and zero-energy Majorana bound states at magnetic impurities [5] or in vortex cores [6]. In this talk, I will present Josephson scanning tunneling microscopy experiments with high energy resolution, showing another type of the spatially dispersing impurity states in FeTe_0.55Se_0.45, which can be connected to the surface topological nature of this material.

[1] A. V. Balatsky, I. Vekhter, and J.-X. Zhu, Rev. Mod. Phys. 78, 373 (2006).

[2] L. Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008).

[3] J. Linder et al., Phys. Rev. Lett. 104, 067001 (2010).

[4] D. Chatzopoulos et al., Nat. Commun. 12, 298 (2021).

[5] J.-X. Yin et al., Nat. Phys. 11, 543 (2015).

[6] T. Machida et al., Nat. Mater. 18, 811 (2019).