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Register a new userElectronic Structure of Fe$_{1.08}$Te bulk crystals and epitaxial FeTe thin films on Bi$_2$Te$_3$
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The electronic structure of thin films of FeTe grown on Bi$_2$Te$_3$ is investigated using angle-resolved photoemission spectroscopy, scanning tunneling microscopy and first principles calculations. As a comparison, data from cleaved bulk FeTe taken under the same experimental conditions is also presented. Due to the substrate and thin film symmetry, FeTe thin films grow on Bi$_2$Te$_3$ in three domains, rotated by 0$^{circ}$, 120$^{circ}$, and 240$^{circ}$. This results in a superposition of photoemission intensity from the domains, complicating the analysis. However, by combining bulk and thin film data, it is possible to partly disentangle the contributions from three domains. We find a close similarity between thin film and bulk electronic structure and an overall good agreement with first principles calculations, assuming a p-doping shift of 65~meV for the bulk and a renormalization factor of around 2. By tracking the change of substrate electronic structure upon film growth, we find indications of an electron transfer from the FeTe film to the substrate. No significant change of the films electronic structure or doping is observed when alkali atoms are dosed onto the surface. This is ascribed to the films high density of states at the Fermi energy. This behavior is also supported by the ab-initio calculations.
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The bulk band structure of Bi$_2$Te$_3$ has been determined by angle-resolved photoemission spectroscopy and compared to first-principles calculations. We have performed calculations using the local density approximation (LDA) of density functional theory and the one-shot $GW$ approximation within the all-electron full-potential linearized augmented-plane-wave (FLAPW) formalism, fully taking into account spin-orbit coupling. Quasiparticle effects produce significant changes in the band structure of bite~when compared to LDA. Experimental and calculated results are compared in the spectral regions where distinct differences between the LDA and $GW$ results are present. Overall a superior agreement with $GW$ is found, highlighting the importance of many-body effects in the band structure of this family of topological insulators.
An important challenge in the field of topological materials is to carefully disentangle the electronic transport contribution of the topological surface states from that of the bulk. For Bi$_2$Te$_3$ topological insulator samples, bulk single crystals and thin films exposed to air during fabrication processes are known to be bulk conducting, with the chemical potential in the bulk conduction band. For Bi$_2$Te$_3$ thin films grown by molecular beam epitaxy, we combine structural characterization (transmission electron microscopy), chemical surface analysis as function of time (x-ray photoelectron spectroscopy) and magnetotransport analysis to understand the low defect density and record high bulk electron mobility once charge is doped into the bulk by surface degradation. Carrier densities and electronic mobilities extracted from the Hall effect and the quantum oscillations are consistent and reveal a large bulk carrier mobility. Because of the cylindrical shape of the bulk Fermi surface, the angle dependence of the bulk magnetoresistance oscillations is two-dimensional in nature.
We electrically detect charge current induced spin polarization on the surface of molecular beam epitaxy grown Bi$_2$Te$_3$ thin film in a two-terminal device with a ferromagnetic MgO/Fe and a nonmagnetic Ti/Au contact. The two-point resistance, measured in an applied magnetic field, shows a hysteresis tracking the magnetization of the Fe. A theoretical estimate is obtained for the change in resistance on reversing the magnetization direction of Fe from coupled spin-charge transport equations based on quantum kinetic theory. The order of magnitude and the sign of the hysteresis is consistent with spin-polarized surface state of Bi$_2$Te$_3$.
The electronic and magnetic properties of individual Fe atoms adsorbed on the surface of the topological insulator Bi$_2$Te$_3$(111) are investigated. Scanning tunneling microscopy and spectroscopy prove the existence of two distinct types of Fe species, while our first-principles calculations assign them to Fe adatoms in the hcp and fcc hollow sites. The combination of x-ray magnetic circular dichroism measurements and angular dependent magnetization curves reveals out-of-plane anisotropies for both species with anisotropy constants of $K_{text{fcc}} = (10 pm 4)$ meV/atom and $K_{text{hcp}} = (8 pm 4)$ meV/atom. These values are well in line with the results of calculations.
Combining the ability to prepare high-quality, intrinsic Bi$_2$Te$_3$ topological insulator thin films of low carrier density with in-situ protective capping, we demonstrate a pronounced, gate-tunable change in transport properties of Bi$_2$Te$_3$ thin films. Using a back-gate, the carrier density is tuned by a factor of $sim 7$ in Al$_2$O$_3$ capped Bi$_2$Te$_3$ sample and by a factor of $sim 2$ in Te capped Bi$_2$Te$_3$ films. We achieve full depletion of bulk carriers, which allows us to access the topological transport regime dominated by surface state conduction. When the Fermi level is placed in the bulk band gap, we observe the presence of two coherent conduction channels associated with the two decoupled surfaces. Our magnetotransport results show that the combination of capping layers and electrostatic tuning of the Fermi level provide a technological platform to investigate the topological properties of surface states in transport experiments and pave the way towards the implementation of a variety of topological quantum devices.
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