Tunable impurity states in the unconventional superconductor FeTe_0.55Se_0.45

When a single impurity interacts with an unconventional superconductor, localized bound states appear inside its superconducting gap. Interestingly, these states can be used as a tool to bring insight into the electronic states of the material at stake. In this study, we use a low temperature scanning tunneling microscope operating at ultra-high vacuum, to find impurity states in the iron-based unconventional superconductor FeTe_0.55Se_0.45 (T_c=14.5 K). In order to enhance the energy resolution, a superconducting tip (Pb-coated) was employed. We find that the energy of the states shifts spatially and can be tuned by changing the distance between the sample and our scanning probe at the impurity site. In this talk I will discuss possible mechanisms that can lead to this dispersive behavior of the impurity energy state in space.