E. V. Chulkov

Bio: E. V. Chulkov is an academic researcher from Donostia International Physics Center. The author has contributed to research in topics: Topological insulator & Band gap. The author has an hindex of 9, co-authored 15 publications receiving 363 citations. Previous affiliations of E. V. Chulkov include University of the Basque Country & Saint Petersburg State University.

Highly-ordered wide bandgap materials for quantized anomalous Hall and magnetoelectric effects

Abstract: We acknowledge support by the University of the Basque Country (Grant Nos. GIC07IT36607 and IT-756-13), the Spanish Ministry of Science and Innovation (Grant Nos. FIS2013-48286-C02-02-P, FIS2013-48286-C02-01-P, and FIS2016-75862-P) and Tomsk State University Academic DI Mendeleev Fund Program in 2015 (research grant N 8.1.05.2015). Partial support by the Saint Petersburg State University project No. 15.61.202.2015 is also acknowledged. AE acknowledges financial support from DFG through priority program SPP1666 (Topological Insulators).

Magnetic extension as an efficient method for realizing the quantum anomalous hall state in topological insulators

Abstract: A new efficient method is proposed for inducing magnetism on the surface of a topological insulator through the deposition of a thin film of an isostructural magnetic insulator whose atomic composition is maximally close to that of the topological material Such a design prevents the formation of a strong interface potential between subsystems As a result, the topological state freely penetrates into the magnetic region, where it interacts with the exchange field and gets significantly split at the Dirac point It is shown that the application of this approach to thin films of a tetradymite-like topological insulator allows realizing the quantum anomalous Hall state with a band gap of several tens of meV

Rashba split surface states in BiTeBr

Abstract: Within density functional theory, we study the bulk band structure and surface states of BiTeBr. We consider both ordered and disordered phases, which differ in atomic order in the Te–Br sublattice. On the basis of relativistic ab initio calculations, we show that the ordered BiTeBr is energetically preferable as compared with the disordered one. We demonstrate that both Te- and Br-terminated surfaces of the ordered BiTeBr hold surface states with a giant spin–orbit splitting. The Te-terminated surface-state spin splitting has Rashba-type behavior with the coupling parameter αR ~ 2 eVA.

Many-body effects on the Rashba-type spin splitting in bulk bismuth tellurohalides

Abstract: We report on many-body corrections to one-electron energy spectra of bulk bismuth tellurohalides—materials that exhibit a giant Rashba-type spin splitting of the band-gap edge states. We show that the corrections obtained in the one-shot GW approximation noticeably modify the spin-orbit-induced spin splitting evaluated within density functional theory. We demonstrate that taking into account many-body effects is crucial to interpret the available experimental data.

Pressure effects on crystal and electronic structure of bismuth tellurohalides

Abstract: We study the possibility of pressure-induced transitions from a normal semiconductor to a topological insulator (TI) in bismuth tellurohalides using density functional theory and tight-binding method. In BiTeI this transition is realized through the formation of an intermediate phase, a Weyl semimetal, that leads to modification of surface state dispersions. In the topologically trivial phase, the surface states exhibit a Bychkov–Rashba type dispersion. The Weyl semimetal phase exists in a narrow pressure interval of 0.2 GPa. After the Weyl semimetal–TI transition occurs, the surface electronic structure is characterized by gapless states with linear dispersion. The peculiarities of the surface states modification under pressure depend on the band-bending effect. We have also calculated the frequencies of Raman active modes for BiTeI in the proposed high-pressure crystal phases in order to compare them with available experimental data. Unlike BiTeI, in BiTeBr and BiTeCl the topological phase transition does not occur. In BiTeBr, the crystal structure changes with pressure but the phase remains a trivial one. However, the transition appears to be possible if the low-pressure crystal structure is retained. In BiTeCl under pressure, the topological phase does not appear up to 18 GPa due to a relatively large band gap width in this compound.

Quantum anomalous Hall effect in intrinsic magnetic topological insulator MnBi2Te4

Abstract: In a magnetic topological insulator, nontrivial band topology combines with magnetic order to produce exotic states of matter, such as quantum anomalous Hall (QAH) insulators and axion insulators. In this work, we probe quantum transport in MnBi2Te4 thin flakes-a topological insulator with intrinsic magnetic order. In this layered van der Waals crystal, the ferromagnetic layers couple antiparallel to each other; atomically thin MnBi2Te4, however, becomes ferromagnetic when the sample has an odd number of septuple layers. We observe a zero-field QAH effect in a five-septuple-layer specimen at 1.4 kelvin, and an external magnetic field further raises the quantization temperature to 6.5 kelvin by aligning all layers ferromagnetically. The results establish MnBi2Te4 as an ideal arena for further exploring various topological phenomena with a spontaneously broken time-reversal symmetry.

Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor

Abstract: A possible sighting of Majorana states Nearly 80 years ago, the Italian physicist Ettore Majorana proposed the existence of an unusual type of particle that is its own antiparticle, the so-called Majorana fermion. The search for a free Majorana fermion has so far been unsuccessful, but bound Majorana-like collective excitations may exist in certain exotic superconductors. Nadj-Perge et al. created such a topological superconductor by depositing iron atoms onto the surface of superconducting lead, forming atomic chains (see the Perspective by Lee). They then used a scanning tunneling microscope to observe enhanced conductance at the ends of these chains at zero energy, where theory predicts Majorana states should appear. Science, this issue p. 602; see also p. 547 Scanning tunneling microscopy is used to observe signatures of Majorana states at the ends of iron atom chains. [Also see Perspective by Lee] Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

Magnetic topological insulators

Abstract: The importance of global band topology is unequivocally recognized in condensed matter physics, and new states of matter, such as topological insulators, have been discovered. Owing to their bulk band topology, 3D topological insulators possess a massless Dirac dispersion with spin–momentum locking at the surface. Although 3D topological insulators were originally proposed in time-reversal invariant systems, the onset of a spontaneous magnetization or, equivalently, a broken time-reversal symmetry leads to the formation of an exchange gap in the Dirac band dispersion. In such magnetic topological insulators, tuning of the Fermi level in the exchange gap results in the emergence of a quantum Hall effect at zero magnetic field, that is, of a quantum anomalous Hall effect. Here, we review the basic concepts of magnetic topological insulators and their experimental realization, together with the discovery and verification of their emergent properties. In particular, we discuss how the development of tailored materials through heterostructure engineering has made it possible to access the quantum anomalous Hall effect, the topological magnetoelectric effect, the physics related to the chiral edge states that appear in these materials and various spintronic phenomena. Further theoretical and experimental research on magnetic topological insulators will provide fertile ground for the development of new concepts for next-generation electronic devices for applications such as spintronics with low energy consumption, dissipationless topological electronics and topological quantum computation. Magnetic topological insulators enable the investigation of the interplay between magnetism and topological electronic states. This Review summarizes the basic notions of magnetic topological insulators and the progress in the experimental realization of exotic topological phenomena.

Topological Axion States in the Magnetic Insulator MnBi 2 Te 4 with the Quantized Magnetoelectric Effect

Abstract: Topological states of quantum matter have attracted great attention in condensed matter physics and materials science. The study of time-reversal-invariant topological states in quantum materials has made tremendous progress. However, the study of magnetic topological states falls much behind due to the complex magnetic structures. Here, we predict the tetradymite-type compound ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ and its related materials host topologically nontrivial magnetic states. The magnetic ground state of ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ is an antiferromagetic topological insulator state with a large topologically nontrivial energy gap ($\ensuremath{\sim}0.2\text{ }\text{ }\mathrm{eV}$). It presents the axion state, which has gapped bulk and surface states, and the quantized topological magnetoelectric effect. The ferromagnetic phase of ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ might lead to a minimal ideal Weyl semimetal.

Robust axion insulator and Chern insulator phases in a two-dimensional antiferromagnetic topological insulator

Abstract: The intricate interplay between non-trivial topology and magnetism in two-dimensional materials can lead to the emergence of interesting phenomena such as the quantum anomalous Hall effect. Here we investigate the quantum transport of both bulk crystal and exfoliated MnBi2Te4 flakes in a field-effect transistor geometry. For the six septuple-layer device tuned into the insulating regime, we observe a large longitudinal resistance and zero Hall plateau, which are characteristics of an axion insulator state. The robust axion insulator state occurs in zero magnetic field, over a wide magnetic-field range and at relatively high temperatures. Moreover, a moderate magnetic field drives a quantum phase transition from the axion insulator phase to a Chern insulator phase with zero longitudinal resistance and quantized Hall resistance h/e2, where h is Planck’s constant and e is electron charge. Our results pave the way for using even-number septuple-layer MnBi2Te4 to realize the quantized topological magnetoelectric effect and axion electrodynamics in condensed matter systems. A large longitudinal resistance and zero Hall plateau—hallmarks of an axion insulator—are found in MnBi2Te4. Moreover, a moderate magnetic field drives a quantum phase transition to a Chern insulator phase with zero longitudinal resistance and quantized Hall resistance h/e2.