Local surface electronic response of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ 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.

Figure 1

(Left panels) Large scale AFM topographic images for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ samples with different Eu concentrations: (a) pure ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, (b) 2% Eu-doped sample, and (c) 11% Eu-doped sample. (Right panels) Small scale STM topographic images on the surface of pure ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (d), 2% Eu-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (e), and 11% Eu-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (f). Dashed arrows highlight the segregation region (blue) and small protrusions (white) distributed along the QL terraces in the Eu-doped samples. STM images were acquired using $I=400\mathrm{pA}$ and $V=1\mathrm{V}$.

Figure 2

AFM topographic image showing protrusions or possible clusters associated with the europium deposition process distributed along the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ surface for the sample grown with 2% Eu (a,b) and for the sample grown with 11% Eu (c,d). An extreme case where the region is characterized by an all-covered surface is shown in (d). White dashed circles represent clusters, blue dashed arrow represents regions of segregation, and white dashed arrows represent small protrusions.

Figure 3

(a) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ Raman spectra as a function of Eu concentration. In addition to the three characteristic peaks of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ phase $({\mathit{A}}_{\mathit{1}\mathit{g}}^{\mathit{1}},{\mathit{E}}_{\mathit{g}}^{\mathit{2}}$, and ${\mathit{A}}_{\mathit{1}\mathit{g}}^{\mathit{2}})$, it is possible to observe three additional peaks in the doped samples that matched with the EuTe phase indicating the coexistence of a secondary phase for some regions of the doped samples. (b) The full width at half maximum (FWHM) of the ${\mathit{E}}_{\mathit{g}}^{\mathit{2}}$ mode as a function of Eu concentration.

Figure 4

(Left panels) A series of representative STS spectra taken at points along the sample surface where the signature of the surface states appears as a function of Eu concentrations. (a) Undoped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, (b) 2% Eu-doped, and (c) 11% Eu-doped. The Dirac point energy $\left(E_D\right)$ is represented by the red dashed line and obtained by extrapolating the region of the curve in which the differential conductance $(dI/dV)$ is linear. The edges of BVB and BCB determining the band gap are marked with vertical gray dashed lines. (Right panels) A series of representative STS spectra taken at points along the sample surface where the signature of the surface states disappears exhibiting a variable energy gap (region of zero differential conductance) for (d) 2% Eu-doped sample and (e) 11% Eu-doped sample. In all panels, the Fermi level is located at zero energy. It is important to note that the graphs are not represented with the $x$ axis on the same scale and are displaced vertically with systematically increasing energy gap for better visualization.

Figure 5

(a) Histogram of Dirac energy $\left(E_D\right)$ obtained from STS results on undoped (pristine) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, 2% Eu-doped sample and, 11% Eu-doped sample. (b) Histogram of energy gap $\left(E_{\mathrm{g}}\right)$ obtained from STS results on 2% Eu-doped sample and, 11% Eu-doped sample. A Gaussian function has been fitted to each histogram to quantify the mean Dirac energy value and mean energy gap value.

Figure 6

(a) Detailed constant-current STM image in the vicinity of a cluster. Image acquired at $I=500\mathrm{pA}$ and $V=1\mathrm{V}$. (b) LDOS as a function of cluster distance measured by STS at the points indicated by colored dots. The curves are offset vertically for clarity. Inset shows the histogram that accounts for the energy gap values over the cluster.

Figure 7

Calculated band structure of (a) undoped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and (b) 4% Eu-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ along the $M\text{−}\mathrm{\Gamma }\text{−}K$ directions of the surface Brillouin zone. Undoped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ band structure exhibits a Dirac cone (solid red line) connecting the valence and conduction bands (shaded red areas). On the other hand, the Dirac cone–like structure at the $\mathrm{\Gamma }$ point is lifted as well as the bands split due to the Eu incorporation process. Here the energy scale has been shifted so that the Fermi level lies at zero.

Figure 8

(Left panels) DFT-based calculations of the total density of states (DOS) for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ doped with different Eu concentrations. (a) Undoped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, (b) 4% Eu-doped sample, and (c) 8% Eu-doped sample. In all panels, the Fermi level is located at zero energy. The right panels represent a close-up view in the vicinity of the Fermi level delimited by the dashed rectangle in the left panels. A gaplike structure is obtained by increasing the nominal Eu doping concentration to 8%.