Electronic processes underlie the operation of most devices in everyday life. In recent years, the concept of describing electronic states based on topology has become one of the dominant concepts in condensed matter physics, solid state chemistry, and quantum materials science. This “new quantum revolution” has led to a fundamental revision of the understanding of the material properties mainly related to relativistic effects.
Before the “topology era”, it was believed that the electronic properties of metals, insulators, and semiconductors were determined by their one-particle band structure (i.e., by the energy-momentum relation), which was directly related to the crystal lattice symmetry of the solid. In the mid-20th century, the inclusion of electron-electron interactions in the band structure picture made it possible to understand such purely quantum phenomena as superconductivity and magnetic ordering, and describe their occurrence in terms of the corresponding symmetry reduction.
However, recently it has become clear that such a coherent and productive description of electronic states is incomplete. In particular, within its framework, it was not possible to explain the effect of quantization of Hall conductivity in two-dimensional electron systems under a quantizing magnetic field or in thin films of compounds with strong spin-orbit coupling, but without an external magnetic field. In these examples, the transition to the “quantum Hall regime” is accompanied by a dramatic change in the electronic characteristics of the system without symmetry breaking.
It turned out that almost a fifth of all materials known today have a topologically nontrivial zone structure of Bloch states, which is determined by the Berry curvature. This quantum mechanical characteristic is related to the phase of the electron wave function in momentum space. Topological insulators (TI) constitute a special class of quantum materials, which, as a rule, are narrow-gap semiconductors with an inverted band gap. They are characterized by integer values of topological invariants obtained by integrating the Berry curvature over the Brillouin zone.
The central point of the topological band theory is the correspondence principle: A topological invariant in the volume of a material determines the presence and number of protected states on its boundary. The surface states are Dirac fermions, the spectral branches of which overlap the volume band gap, and the momentum and spin polarization are fixed orthogonally to each other. These features give special stability for unidirectional electron propagation at the TI boundary due to the prohibition of “backward” scattering in the case of non-magnetic defects.
However, the introduction of a magnetic order into TI significantly changes the spectrum of the surface state, opening an energy exchange gap in it, which makes it possible to control electronic processes and generate new properties in TI. Such unique phenomena as the quantum and semi-quantum anomalous Hall effect, the phase of the axion insulator, and other effects promising for future applications based on the control of spin-polarized currents have been predicted and experimentally confirmed in magnetic TIs.
Research directions
1. Theoretical search for new compounds with a topologically nontrivial band structure;
2. Investigation of bulk and surface electronic properties of magnetic TIs;
3. Study of the quantum anomalous Hall effect in thin films of intrinsic and impurity magnetic TIs;
4. Description of one-dimensional electronic states occurring at the edges and domain walls in thin films of antiferromagnetic TIs;
5. Investigation of the anomalous Hall effect on the surface and in thin films of a magnetic semiconductor with a strong Rashba effect.
Towards comprehension of the surface state properties in the intrinsic magnetic topological insulators
Spectroscopic studies (ARPES) of intrinsic antiferromagnetic TI MnBi2Te4 give very contradictory results. We propose a microscopic mechanism that relates the change in the magnitude and sign of the exchange gap of surface states to the variation of the electrostatic surface potential from sample to sample. In addition, we provide an explanation for the appearance of a finite density of states inside the exchange gap in the case of strong electrostatic fluctuations along the sample surface. The physical nature of such fluctuations is associated with an inhomogeneous concentration of antisite defects in the surface layers of MnBi2Te4.
V. N. Men’shov, I. A. Shvets, E. V. Chulkov, Phys. Rev. B 106, 205301 (2022)
Electronic Spectrum Features under the Transition from Axion Insulator Phase to Quantum Anomalous Hall Effect Phase in an Intrinsic Antiferromagnetic Topological Insulator Thin Film
The intrinsic antiferromagnetic TI MnBi2Te4 provides an attractive platform for realizing various magnetic and topological states. In the ground state, the thin films of the MnBi2Te4 compound with an even number of septuple-layer blocks are axial insulators, but with increasing external magnetic field, they exhibit a transition to the quantum anomalous Hall effect regime, which is accompanied by a conversion from collinear to non-collinear magnetization textures. A theoretical study of such a topological transition has been carried out and the energy spectrum, Berry curvature, and topological indices have been obtained depending on the degree of non-collinearity. For various topological regimes, boundary electronic states are described that occur on the film side face and on the domain wall separating regions with opposite canting angles. The obtained results significantly deepen the understanding of the relationship between band topology and magnetic ordering.
V. N. Men’shov & E. V. Chulkov, JETP Letters, 117, 147 (2023)
Electron states emerging at magnetic domain walls of magnetic semiconductors with strong Rashba effect
The band structure of the semiconductor surface with a strong Rashba effect is characterized by a non-trivial Berry curvature. The introduction of a magnetic impurity into such a material leads to the establishment of a ferromagnetic order and the appearance of a local exchange gap at the Kramers degeneracy point. It is shown that an electronic resonance state occurs on the magnetic domain wall. With relatively weak exchange splitting, the resonant state has linear dispersion in a local exchange gap with low spectral broadening and manifests the chirality property. Such states on magnetic domain walls can make a significant contribution to the longitudinal and transverse conductivity on the surface. As the exchange interaction increases, the spectral branch of the resonant state becomes blurred. The polar semiconductor BiTeI doped with transition metal atoms is a suitable material platform for detecting the predicted phenomena.
I. P. Rusinov, V. N. Men’shov, E. V. Chulkov, Phys. Rev. B 110, 195405 (2024)
Staff list
The group’s staff has published more than 150 articles on the subject of quantum materials in the best Russian and foreign scientific journals.
In their research, the group’s staff uses both classical methods of theoretical analysis of physical models and advanced numerical techniques based on density functional theory.
- V. N. Men’shov, I. P. Rusinov, E. V. Chulkov, Intrinsic anomalous Hall effect at the surface of magnetic semiconductor with strong Rashba effect, JETP Letters, 121, # 5 (2025).
- I. P. Rusinov, V. N. Men’shov, E. V. Chulkov, Electron states emerging at magnetic domain walls of magnetic semiconductors with strong Rashba effect, Phys. Rev. B 110, 195405 (2024). DOI: 10.1103/PhysRevB.110.195405
- V. N. Men’shov, E. V. Chulkov, Electronic Spectrum Features under the Transition from Axion Insulator Phase to Quantum Anomalous Hall Effect Phase in an Intrinsic Antiferromagnetic Topological Insulator Thin Film, JETP Letters, 117, 147 (2023). DOI: 10.1134/S0021364022602962
- V. N. Men’shov, E. V. Chulkov, Bound States of a Short-Range Defect on the Surface of an Intrinsic Antiferromagnetic Topological Insulator in a Noncollinear Phase, JETP Letters, 118, 837 (2023). DOI: 10.1134/S0021364023603408
- N. L. Zaitsev, I. P. Rusinov, T. V. Menshchikova, E. V. Chulkov, Interplay between exchange-split Dirac and Rashba-type surface states at the MnBi2Te4/BiTeI interface, Phys. Rev. B 107, 045402 (2023). DOI: 10.1103/PhysRevB.107.045402
- V. N. Men’shov, I. A. Shvets, E. V. Chulkov, Towards comprehension of the surface state properties in the intrinsic magnetic topological insulators, Phys. Rev. B 106, 205301 (2022). DOI: 10.1103/PhysRevB.106.205301