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Spatially controlling the Fermi level of topological insulators and keeping their electronic states stable are indispensable processes to put this material into practical use for semiconductor spintronics devices. So far, however, such a method has not been established yet. Here we show a novel method for doping a hole into n-type topological insulators Bi2X3 (X= Se, Te) that overcomes the shortcomings of the previous reported methods. The key of this doping is to adsorb H2O on Bi2X3 decorated with a small amount of carbon, and its trigger is the irradiation of a photon with sufficient energy to excite the core electrons of the outermost layer atoms. This method allows controlling the doping amount by the irradiation time and acts as photolithography. Such a tunable doping makes it possible to design the electronic states at the nanometer scale and, thus, paves a promising avenue toward the realization of novel spintronics devices based on topological insulators.

In the field of topological materials, the interaction between band topology and magnetism remains a current frontier for the advancement of new topological states and spintronic functionalities. D...

We investigate the coupled spin and orbital textures of the topological surface state in ${\mathrm{Bi}}_{2}$(Te,${\mathrm{Se})}_{3}$(0001) across full momentum space using spin- and angle-resolved photoelectron spectroscopy and relativistic one-step photoemission theory. For an approximately isotropic Fermi surface in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$, the measured intensity and spin momentum distributions, obtained with linearly polarized light, qualitatively reflect the orbital composition and the orbital-projected in-plane spin polarization, respectively. In ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, the in-plane lattice potential induces a hexagonal anisotropy of the Fermi surface, which manifests in an out-of-plane photoelectron spin polarization with a strong dependence on light polarization, excitation energy, and crystallographic direction.

The combination of x-ray magnetic circular dichroism with multiplet ligand field theory is a powerful method to study magnetic ground state properties. Here, the authors use this approach to reveal a strongly covalent ferromagnetic ground state of V and Cr impurities embedded in Bi${}_{x}$Sb${}_{2\ensuremath{-}x}$Te${}_{3}$ topological-insulator thin films. They find a substantial charge transfer from the Te ligands to the impurities and a surprisingly negligible contribution from ionic V${}^{3+}$ and Cr${}^{3+}$. The authors also succeed in quantifying the magnetic moments and orbital occupations, which are key parameters for theories of the magnetic coupling mechanism.

Physical review / B 102(18), 184401 (1-13) (2020). doi:10.1103/PhysRevB.102.184401, Rare-earth ions typically exhibit larger magnetic moments than transition-metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of topological insulators. Yet in a recent photoemission study of Eu-doped Bi$_2$Te$_3$ films, the spectra remained gapless down to T=20 K. Here we scrutinize whether the conditions for a substantial gap formation in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the M$_{4,5}$ x-ray absorption and magnetic circular dichroism spectra reveals a Eu$^{2+}$ valence and confirms a large magnetic moment, consistent with a 4$f^7$$^8S$$_{7/2}$ ground state. At temperatures below 10 K, bulk magnetometry indicates the onset of antiferromagnetic (AFM) ordering. This is in good agreement with density functional theory, which predicts AFM interactions between the Eu impurities. Our results support the notion that antiferromagnetism can coexist with topological surface states in rare-earth-doped Bi$_2$Te$_3$ and call for spectroscopic studies in the Kelvin range to look for novel quantum phenomena such as the quantum anomalous Hall effect., Published by Inst., Woodbury, NY

The layered van der Waals antiferromagnet MnBi$_2$Te$_4$ has been predicted to combine the band ordering of archetypical topological insulators such as Bi$_2$Te$_3$ with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi$_2$Te$_4$(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure., Revised version

Chemistry of materials 31(8), 2795 - 2806 (2019). doi:10.1021/acs.chemmater.8b05017, High-quality single crystals of MnBi2Te4 are grown for the first time by slow cooling within a narrow range between the melting points of Bi$_2$Te$_3$ (586 °C) and MnBi$_2$Te$_4$ (600 °C). Single-crystal X-ray diffraction and electron microscopy reveal ubiquitous antisite defects in both cation sites and, possibly, Mn vacancies (Mn$_{0.85(3)}$Bi$_{2.10(3)}$Te$_4$). Thermochemical studies complemented with high-temperature X-ray diffraction establish a limited high-temperature range of phase stability and metastability at room temperature. Nevertheless, the synthesis of MnBi$_2$Te$_4$ can be scaled-up as powders can be obtained at subsolidus temperatures and quenched at room temperature. Bulk samples exhibit long-range antiferromagnetic ordering below 24 K. The Mn(II) out-of-plane magnetic state is confirmed by the magnetization, X-ray photoemission, X-ray absorption, and linear dichroism measurements. The compound shows a metallic type of resistivity in the range 4.5–300 K and is an n-type conductor that reaches a thermoelectric figure of merit up to ZT = 0.17. Angle-resolved photoemission experiments show a surface state forming a gapped Dirac cone, thus strengthening MnBi$_2$Te$_4$ as a promising candidate for the intrinsic magnetic topological insulator, in accordance with theoretical predictions. The developed synthetic protocols enable further experimental studies of a crossover between magnetic ordering and nontrivial topology in bulk MnBi$_2$Te$_4$., Published by American Chemical Society, Washington, DC

The layered van der Waals compounds ( MnBi 2 Te 4)( Bi 2 Te 3) were recently established as the first intrinsic magnetic topological insulators. We report a study on the epitaxial growth of ( MnBi 2 Te 4 ) m ( Bi 2 Te 3 ) n films based on the co-deposition of MnTe and Bi 2 Te 3 on BaF 2 (111) substrates. X-ray diffraction and scanning transmission electron microscopy evidence the formation of multilayers of stacked MnBi 2 Te 4 septuple layers and Bi 2 Te 3 quintuple layers with a predominance of MnBi 2 Te 4. The elemental composition and morphology of the films is further characterized by x-ray photoemission spectroscopy and atomic force microscopy. X-ray magnetic circular and linear dichroism spectra are comparable to those obtained for MnBi 2 Te 4 single crystals and confirm antiferromagnetic order in the films.

BaTiO${}_{3}$ (BTO) is one of the most important ferroelectric oxides. Its most common source of uncontrolled doping, the oxygen vacancy, releases free electrons that confine in a two-dimensional electron gas (2DEG) at the surface. This 2DEG can be readily monitored by angle-resolved photoemission, and be used to measure the defect dynamics in ultrathin BTO films. The authors reveal two remarkable properties: first, oxygen defects close to the thin film surface are preferably in a charge state of 2+, which limits charge carrier trapping. Second, these defects migrate even below room temperature, fostering significant surface charge carrier density fluctuations that ultimately quench the 2DEG.

The layered polar semiconductor BiTeI exhibits large Rashba-type spin–orbit splittings in its bulk and surface electronic structure. Here we report an artificial structural modification near the surface of BiTeI(0001) induced by annealing in vacuum. Using scanning tunneling microscopy we show that the annealing-induced change in the near-surface stoichiometry results in a structural change from a non-centrosymmetric triple-layered to a quintuple-layered structure. The structural change gives rise to the emergence of topological surface states with helical spin texture as demonstrated by angle-resolved photoemission experiments and relativistic first-principles calculations. The results provide a way to modify the electronic structure of layered materials by a controlled manipulation of the atomic stacking, This work was supported by the Deutsche Forschungsgemeinschaft through SFB1170 (project A01), and partly by the Tomsk State University competitiveness improvement programme (project No. 8.1.01.201 7), by the Russian Science Foundation (Grants No. 17-12-01047, in part of crystal growth and ARPES measurements (figure 4) and 18-12-00169, in part of DFT calculations), Russian Foundation for Basic Research (Grant No. 17- 02-00729) and Saint Petersburg State University (project 15.61.202.201 5).

Paramagnetic heavy fermion insulators consist of fully occupied quasiparticle bands inherent to Fermi liquid theory. The gap emergence below a characteristic temperature is the ultimate sign of coherence for a many-body system, which in addition can induce a non-trivial band topology. Here, we demonstrate a simple and efficient method to compare a model study and an experimental result for heavy fermion insulators. The temperature dependence of the gap formation in both local moment and mixed valence regimes is captured within the dynamical mean field (DMFT) approximation to the periodic Anderson model (PAM). Using the topological coherence temperature as the scaling factor and choosing the input parameter set within the mixed valence regime, we can unambiguously link the theoretical energy scales to the experimental ones. As a particularly important result, we find improved consistency between the scaled DMFT density of states and the photoemission near-gap spectra of samarium hexaboride (SmB6).

A comprehensive understanding of spin-polarized photoemission is crucial for accessing the electronic structure of spin-orbit coupled materials. Yet, the impact of the final state in the photoemission process on the photoelectron spin has been difficult to assess in these systems. We present experiments for the spin-orbit split states in a Bi-Ag surface alloy showing that the alteration of the final state with energy may cause a complete reversal of the photoelectron spin polarization. We explain the effect on the basis of ab initio one-step photoemission theory and describe how it originates from linear dichroism in the angular distribution of photoelectrons. Our analysis shows that the modulated photoelectron spin polarization reflects the intrinsic spin density of the surface state being sampled differently depending on the final state, and it indicates linear dichroism as a natural probe of spin-orbit coupling at surfaces., New manuscript with a changed interpretation and theoretical description of the experimental results

As an avenue to spintronics, controlling spin-polarized electronic states in low-dimensional heterostructures is at the frontier of physics and nanotechnology. Manipulating these states via ferroelectric polarization seems promising, but actually making high-quality interfaces, to realize states with large spin splitting upon a ferroelectric substrate, remains tricky. The authors rise to the challenge, directly demonstrating metallic states with large spin splitting in the wide band gap of BaTiO${}_{3}$, with evidence for considerable coupling between overlayer and substrate.

The ferromagnetic topological insulator $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_{2}{\mathrm{Te}}_{3}$ has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_{2}{\mathrm{Te}}_{3}$ thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V $3d$ impurity band was extracted and exhibits a high density of states near the Fermi level, in agreement with spin-polarized first-principles calculations. Our results indicate the occurrence of a ferromagnetic superexchange interaction mediated by the observed impurity band, contributing to the ferromagnetism in this system.

The photoelectron spin polarization in angle-resolved photoemission from the topological surface state in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$(0001) has been investigated in a combined experimental and theoretical study. Our measurements show significant photon-energy-dependent deviations in the three-dimensional spin polarization of the photoelectron when compared to the expected intrinsic spin polarization of the surface state. The experimental observations are in line with relativistic one-step photoemission calculations. Our theoretical analysis confirms that spin-orbit coupling in the initial-state wave functions in combination with the dipole selection rules strongly influences the photoelectron spin polarization. Furthermore, spin-dependent final-state effects are found to influence the spin polarization significantly. A quantitative access to the three-dimensional spin-polarization vector in topological insulators is thus challenged by a complex interplay of initial- and final-state effects in the photoemission process.

Ex situ analyses on topological insulator films require protection against surface contamination during air exposure. This work reports on a technique that combines deposition of protective capping just after epitaxial growth and its mechanical removal inside ultra-high vacuum systems. This method was applied to Bi2Te3 films with thickness varying from 8 to 170 nm. Contrarily to other methods, this technique does not require any sputtering or thermal annealing setups installed inside the analyzing system and preserves both film thickness and surface characteristics. These results suggest that the technique presented here can be expanded to other topological insulator materials.

Semiconductors with strong spin–orbit interaction as the underlying mechanism for the generation of spin-polarized electrons are showing potential for applications in spintronic devices. Unveiling the full spin texture in momentum space for such materials and its relation to the microscopic structure of the electronic wave functions is experimentally challenging and yet essential for exploiting spin–orbit effects for spin manipulation. Here we employ a state-of-the-art photoelectron momentum microscope with a multichannel spin filter to directly image the spin texture of the layered polar semiconductor BiTeI within the full two-dimensional momentum plane. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the valence and conduction band electrons in BiTeI have spin textures of opposite chirality and of pronounced orbital dependence beyond the standard Rashba model, the latter giving rise to strong optical selection-rule effects on the photoelectron spin polarization. These observations open avenues for spin-texture manipulation by atomic-layer and charge carrier control in polar semiconductors., This work was supported by DFG (through SFB 1170 ‘ToCoTronics’) and through FOR1162 (P3). We acknowledge the support by the Basque Departamento de Educacion, UPV/EHU (Grant Number IT-756-13), Spanish Ministerio de Economia y Competitividad (MINECO Grant Number FIS2013-48286-C2-2-P), Tomsk State University Academic D.I. Mendeleev Fund Program in 2015 (Research Grant Number 8.1.05.2015), the Russian Foundation for Basic Research (Grant Numbers 15-02-01797 and 15-02-589 02717). Partial support by the Saint Petersburg State University (Grant Number 15.61.202.2015) is also acknowledged.