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The Hall effect of quantum anomaly in the insulator with intrinsic magnetic topology

2020-02-14 3889609

The irregular band topology can be combined with the magnetic order in the magnetic topological insulator to produce heterogeneous materials, such as quantum abnormal Holzer (QAH) insulator and axon insulator. The purpose of condensed matter physics is to find new materials with useful properties and to study them with quantum mechanics. This field enables physicists to better understand the use of magnets in hard disk data storage, computer displays, and other technologies. The latest discovery of topological insulator has aroused wide interest. Researchers predict that the interaction between ferromagnetism and topological insulator state can realize a series of strange quantum magnetic phenomena of interest in basic physics and equipment applications.
In a new report, Deng Yujun and a research team from the Department of physics and quantum matter physics of China have detected quantum transport in thin sheet MnBi 2Te 4 topological insulator with inherent magnetic sequence. The ferromagnetic layers are antiparallel coupled to each other in the atomic thin MnBi 2Te 4 layered van der Waals crystal. However, when the sample contains an odd number of sample layers, the sample becomes ferromagnetic. The team observed the zero field qah effect of five and seven layer specimens at 1.4 Kelvin. The results make MnBi 2Te 4 an ideal platform time inversion symmetry for exploring the external topological phenomena with spontaneous fracture. The work is now published in science.
Topological materials obviously contain topological protected quantum states, which are robust to local embarrassment. For example, in a topological insulator (TI) such as bismuth telluride (BI 2Te 3), the body band topology can guarantee the existence of a two-dimensional (2-D) surface state with interstitial free Dirac dispersion. By introducing magnetism into the original Ti, scientists can induce profound changes in its electronic structure. For example, to observe the qah effect in chromium doped (Bi, sb) 2te3 through experiments, physicists must precisely control the proportion of various elements in non stoichiometric materials. Fine tuning of materials requires coordination of conflicting requirements, so researchers must quantify the anomalous Hall effect precisely only at temperatures up to t = 2K, well below the Curie temperature and exchange gap of materials. In order to further explore the rich topological phenomena and their potential applications, researchers must use intrinsic magnetism TI company (topological insulator) to have a kind of inherent magnetism in order to study the influence on the original crystal and its topology.
In this work, Deng et al. Quantum transport is detected in the atomic sheet of the intrinsic magnetic topological insulator MnBi 2Te 4. The material consists of a layered ternary tetragonal compound with a four layered structure (TE Bi te Mn te Bi TE). The obtained MnBi 2Te 4 crystal is inherently magnetic, and the magnetic properties are derived from Mn 2 + ions in the crystal. They studied thin MnBi 2Te 4 slices to minimize parallel body conduction and thin MnBi 2Te 4 slices with odd layers.
The team first used flux method to grow high-quality MnBi 2Te 4 crystal, and obtained atomic thin MnBi 2Te 4 through Al 2O 3 assisted stripping. For this reason, they evaporated the Al 2O 3 thin film onto the surface of the newly prepared block crystal, lifted the block with a thermal strip, and then released the combined Al 2O 3 / MnBi 2Te 4 stack to a transparent PDMS for microscopic examination. After that, they imprinted a thin sheet on a silicon chip covered with SiO2, and then deposited Cr / Au contacts for transmission measurement. The team completed the process in a closed box to prevent samples from being exposed to oxygen (O2) and water (H2O), thereby reducing sample degradation. Then they extensively studied the rich set of magnetic states of several layers of samples.
Deng et al. He observed that the five layer MnBi 2Te 4 had a good qah effect under zero magnetic field, and the quality of the sample was greatly improved. They point out that the external magnetic field further improves quantification by aligning the ferromagnetic layer. Ferromagnetic alignment also improves the robustness of qah effect against thermal fluctuations. At zero magnetic field, their energy gap is larger than that in the magnetically Doped Ti films, although it is still much smaller than the expected exchange energy gap of MnBi 2Te 4.
The energy gap does not directly measure the band gap of the surface state in the crystal, but represents the minimum energy needed to excite the electron from the valence state to the conduction band. For example, the huge difference between energy gap and predicted band gap means various anomalies in the sample. As a result, there is still much room for further improvement of the energy level of qah effect in the original high-quality MnBi 2Te 4 samples.
When the external magnetic field is applied to polarize the five layer sample, the energy gap decreases with the increase of the magnetic field. The state of qah evolves gradually in the experimental setup, so that the electronic structure of the surface band outside the band gap can be seen. Deng et al. Understand all states observed in the study from a unified perspective. The gate efficiency measured by hall with near zero magnetic field is 5 x 10 10 cm-2 / V, which is very consistent with the efficiency estimated according to the device geometry. Since MnBi 2Te 4 is a layered material, the research team hopes that the technology developed for two-dimensional materials can be applied to MnBi 2Te 4. In this way, Deng Yujun and his colleagues predict that the van der Waals heterostructure, which combines MnBi 2Te 4 with other two-dimensional magnetic / superconducting materials, will provide fertile ground for further exploration of strange topological quantum phenomena.

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