Intrinsic magnetic topological insulators in van der Waals layered MnBi ₂ Te ₄ -family materials

Abstract

The interplay of magnetism and topology is a key research subject in condensed matter physics, which offers great opportunities to explore emerging new physics, such as the quantum anomalous Hall (QAH) effect, axion electrodynamics, and Majorana fermions. However, these exotic physical effects have rarely been realized experimentally because of the lack of suitable working materials. Here, we predict a series of van der Waals layered MnBi2Te4-related materials that show intralayer ferromagnetic and interlayer antiferromagnetic exchange interactions. We find extremely rich topological quantum states with outstanding characteristics in MnBi2Te4, including an antiferromagnetic topological insulator with the long-sought topological axion states on the surface, a type II magnetic Weyl semimetal with one pair of Weyl points, as well as a collection of intrinsic axion insulators and QAH insulators in even- and odd-layer films, respectively. These notable predictions, if proven experimentally, could profoundly change future research and technology of topological quantum physics.

Emergence of Nontrivial Low‐Energy Dirac Fermions in Antiferromagnetic EuCd2As2

“…As a consequence, the ground state of EuCd 2 As 2 becomes a novel axion insulator state which is not studied in previous theoretical paper 14. This ground state is similar to that reported recently in the MnBi 2 Te 4 family 17,18. But for EuCd 2 As 2 , massless Dirac surface states appear on some specifically oriented surfaces which are protected by the mirror or symmetries.…”

“…Furthermore, it was shown that for other surfaces, there are gaped surface states contributing to half‐quantized anomalous Hall conductivity. This class of AFM materials was coined as AFM TIs, and as a possible candidate for an axion insulator, as reported recently for MnBi 2 Te 4 17–21. However, the topological nature of the surface of MnBi 2 Te 4 remains puzzling due to disagreements between experimental results and theoretical predictions 19–21.…”

Electronic States and Magnetic Response of MnBi2Te4 by Scanning Tunneling Microscopy and Spectroscopy

“…Generally, after sample cleavage, the new surface of bulk MnBi 2 Te 4 is divided into two parts (layers) according to the depth from the topmost layer (see schematic in Fig. 4D): (i) the top layer with the thickness of few nanometers, which hosts an embedded Dirac cone structure due to the unclear magnetic order (26,27,38); (ii) the deep layer with the predicted out-of-plane antiferromagnetic order (18,19). Depending on the detection depths of different techniques, the top layer is detectable to surface sensitive techniques, such as STM and ARPES, while the deep intrinsic layer is detectable to bulk sensitive techniques such as transport measurements.…”

“…For example, the observations of magnetism-induced gap opening at the Dirac point are either too large or too small (absent) (17)(18)(19)(23)(24)(25)(26)(27)(28) and the assignment of the topological surface states are still controversial (26)(27)(28). Here we employ scanning tunneling microscopy (STM) to systematically study the electronic structure of MnBi 2 Te 4 in both real and momentum spaces, and the complementary information helps us to have a better understanding of the material.…”

Intrinsic quantum anomalous Hall phase induced by proximity in the van der Waals heterostructure germanene/ Cr2Ge2Te6

“…Beyond MnBi 2 Te 4 , other vdW superlattice-like Mn-Bi-Te structures with tunable chemical formulas as (MnBi 2 Te 4 ) m (Bi 2 Te 3 ) n (where m and n are integer) have also been proposed [18,[24][25][26]. Through tuning numbers of layers, stacking configurations, or compositions, these vdW heterostructures have been further proposed to investigate rich topological quantum phenomena between magnetism and topology, such as QAH phases [18,20,23], axion insulator states [22], antiferromagnetic TIs [21,22], and their phase transitions [18]. These advances mentioned above forward a strategy for investigating magnetic topological phases and especially QAH effects in vdW layered materials.…”