Local surface electronic response of Bi2Te3 topological insulator upon europium doping

The most relevant characteristic of a topological insulator material is the presence of edge/surface states that are protected by the bulk topology, and therefore, insensitive to nonmagnetic disorder. However, if such disorder is induced by magnetic atoms or the topological insulator is subjected to an external magnetic field, the time-reversal symmetry is expected to break down, affecting the robustness of the edge/surface states. In this work, europium (Eu)-doped bismuth telluride thin films were grown by molecular beam epitaxy in order to analyze the effect of a small fraction of atoms with magnetic properties on topologically protected surface states. For films with different Eu concentrations, morphological and electronic characterizations were carried out using atomic force microscopy, scanning tunnelling microscopy, and scanning tunnelling spectroscopy (STS) techniques. The results show that, regardless of the Eu concentration, the layered structure characteristic of the ${\mathrm{Bi}}₂{\mathrm{Te}}₃$ phase is maintained. However, large $(>2%)$ Eu concentrations induce the appearance of protrusions and clusters on the surface of the films. The STS measurements show the presence of surface states for pure and low-content Eu:${\mathrm{Bi}}₂{\mathrm{Te}}₃$. The suppression of surface states is indicated by STS spectra in regions with well-defined gaps for some locally limited regions of our samples with large concentration of Eu atoms. From density functional theory we are able to show that the Eu substitutional impurity at the Bi site is not the main mechanism responsible for the observed changes in the topological insulator band structure. Furthermore, the magnetic properties of europium are not the key factor dictating the different ${\mathrm{Bi}}₂{\mathrm{Te}}₃$ local surface electronic properties experimentally observed by STS, which are mostly affected by alloying and atom replacement, which induce a chemical modification of the surface potential.