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Nontrivial band topology can combine with magnetic order in a to produce exotic states of matter such as quantum anomalous Hall (QAH) insulators and . An aim of condensed matter physics is to find new materials with useful properties and apply quantum mechanics to study them. The field has allowed physicists to better understand the uses of magnets for hard disk data storage, computer displays and other technologies. The recent discovery of topological insulators have attracted broad interest and researchers predict that the interplay between ferromagnetism and the topological insulator state can realize a range of exotic quantum magnetic phenomena of interest in fundamental physics and device applications.

In a new report, Yujun Deng and a research team at the departments of physics and quantum matter physics in China, probed quantum transport in a thin flake MnBi₂Te₄ topological insulator, with intrinsic magnetic order. The ferromagnetic layers coupled anti-parallelly to each other in the atomically thin MnBi₂Te₄ layered van der Waals crystal. However, the sample became ferromagnetic when it contained an odd number of septuple layers. The research team observed the zero-field QAH effect in a five-septuple-layer specimen at 1.4 Kelvin. The results established MnBi₂Te₄ as an ideal platform to explore exotic topological phenomena with spontaneously broken . The work is now published on Science.

Topological materials distinctly contain that are robust against local distresses. For instance, in a topological insulator (TI) such as (Bi₂Te₃), the bulk band topology can guarantee the existence of two-dimensional (2-D) surface states with . By introducing magnetism into the initially time-reversal invariant topological insulators (TIs), scientists can induce profound changes in their electronic structure. For example, to experimentally observe the QAH effect in , physicists had to precisely control the ratio of multiple elements in a non-stoichiometric material. Fine-tuning the material required reconciling conflicting demands and therefore, researchers had to precisely quantize the anomalous Hall effect only at temperatures up to T = 2 K, far below the and . To further explore the rich topological phenomena and their potential applications, researchers must use intrinsic magnetic TIs (topological insulators) with an innate magnetic order to study their topological effects in pristine crystals.

In this work, Deng et al. probed quantum transport in atomically thin flakes of intrinsic magnetic topological insulator MnBi₂Te_4. The material contained a layered compound containing septuple layers (Te-Bi-Te-Mn-Te-Bi-Te). The resulting MnBi₂Te₄ crystal was intrinsically magnetic and the magnetism originated from Mn²⁺ ions in the crystal. They studied thin flakes of MnBi₂Te_4.to minimize parallel bulk conduction and focused on MnBi₂Te₄ flakes containing an odd number of layers.

The team started with high quality MnBi₂Te₄ crystals grown using to obtain atomically thin MnBi₂Te₄ via . To accomplish this, they thermally evaporated Al₂O₃ thin film onto a freshly prepared surface of the bulk crystal, lifted the bulk using a thermal release tape then released the combined Al₂O₃/MnBi₂Te₄ stack on to a piece of transparent (PDMS) for microscopic inspection. Thereafter, they stamped the thin flakes onto a silicon wafer covered with SiO₂, followed by deposition of Cr/Au contacts for transport measurements. The team completed the process in an airtight box to prevent sample exposure to oxygen (O₂) and water (H₂O) to mitigate sample degradation. They then extensively studied the rich set of magnetic states for the few-layered samples.

Deng et al. observed a well-developed QAH effect at zero magnetic field in a five-layer MnBi₂Te₄ of much improved sample quality. They noted that an external magnetic field further improved the quantization by aligning the ferromagnetic layers. The ferromagnetic alignment also improved the robustness of the QAH effect against thermal fluctuations. At zero magnetic field, they obtained an energy gap that exceeded the value in , although still much smaller than for MnBi₂Te₄.

The energy gap did not directly measure the bandgap of the surface states in the crystal, but characterized the minimum energy required to excite an electron from the valence to the conduction band. For instance, a large difference between the energy gap and the predicted bandgap implied various disorders in the sample. As a result, there is much room to further increase the energy scale of the QAH effect in pristine, high-quality MnBi₂Te₄ samples.

After the applied external magnetic field fully polarized the five-layer sample, the energy gap diminished with increasing magnetic field. The QAH states gradually evolved in the experimental setup providing a peek into the electronic structure of the surface bands outside the bandgap. Deng et al. understood all states observed in the study from a unified view. The Hall measurements near zero magnetic field yield a gate efficiency of 5 x 10¹⁰ cm^-2/V, which agreed well with the efficiency estimated from the device geometry. Since MnBi₂Te₄ is a layered material, the team expect the techniques developed for 2-D materials to be applicable to MnBi₂Te₄. In this way, Yujun Deng and colleagues anticipate that van der Waals heterostructures integrating MnBi₂Te₄ with other magnetic/superconducting 2-D materials will provide fertile ground to further explore exotic topological quantum phenomena.

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