Quantum Hall effect (QHE) plays an important role in materials science and precision measurement. Due to the dissipationless edge states, QHE exhibits exotic transport properties with quantized Hall resistance of h/νe2 and vanishing longitudinal resistance. Here, h is Planck's constant, ν is Landau filling factor and e is electron charge. QHE usually originates from the formation of remarkable energy gap and the broken time-reversal-symmetry, which requires materials with high mobility, high magnetic field and ultralow temperature. These rigorous conditions greatly limit deep exploration and wide applications of QHE. Therefore, seeking QHE at weak or zero magnetic field and higher temperatures has become an important topic in physics and materials science. In 1988, Haldane theoretically proposed that QHE can be realized without applying external magnetic field, i.e. Chern insulator state or quantum anomalous Hall effect (QAHE). In 2013, QAHE with C=1 was experimentally observed in thin films of chromium-doped (Bi,Sb)2Te3 at temperature down to 30 mK. QAHE is also contributed by dissipationless chiral edge states while the realization of QAHE does not require external magnetic field. Thus, QAHE is more suitable for application in low-consumption electronics compared with QHE. However, only one dissipationless edge state can be realized at ultralow temperatures in the previous studies. Therefore, realizing multiple dissipationless edge states and increasing the working temperature of QAHE are not only the most important research topics in physical sciences, but also expected to promote the development of low-consumption electronics and integrated circuits.
Recently, a research collaboration led by Professor Jian Wang at Peking University, Professor Yong Xu and Professor Yang Wu at Tsinghua University has discovered high-Chern-number and high-temperature Chern insulator state in MnBi2Te4 devices, representing a great breakthrough in Chern insulators and topological quantum states. The paper entitled "High-Chern-Number and High-Temperature Quantum Hall Effect without Landau Levels" was published online in National Science Review. Professor Jian Wang at Peking University and Professor Yong Xu at Tsinghua University are corresponding authors of this paper. Jun Ge, Yanzhao Liu at Peking University and Jiaheng Li, Hao Li at Tsinghua University contributed equally to this work. The research team discovered Chern insulators with two dissipationless edge states (high-Chern-number Chern insulator) above 10 K in intrinsic magnetic topological material MnBi2Te4 devices. Furthermore, by reducing the thickness of MnBi2Te4 devices, they also discovered Chern insulator state with one dissipationless edge state at the temperature as high as 45 K (high-temperature Chern insulator state), much higher than the antiferromagnetic transition temperature (Néel temperature) of MnBi2Te4 devices. These discoveries indicate the exciting possibility that the Chern insulator state or QAHE may be possibly realized at room temperature if appropriate materials and parameters are selected, consequently leading to a new generation of low-consumption or dissipationless information highway and bringing about the information technology revolution.
MnBi2Te4 is a layered magnetic topological material. As shown in subfigure (a), monolayer MnBi2Te4 includes seven atomic layers in a unit cell, forming a Te-Bi-Te-Mn-Te-Bi-Te septuple layer (SL), which can be viewed as intercalating a Mn-Te bilayer into the center of a Bi2Te3 quintuple layer. The researchers fabricated several MnBi2Te4 devices with different thickness. In 9-SL and 10-SL MnBi2Te4 devices, a Hall resistance plateau with height of h/2e2 accompanied by nearly vanishing longitudinal resistance is observed under applying an external magnetic field of 5 T, which is characteristic of the Chern insulator with two dissipationless edge states (C=2) (subfigures (b), (c)). More interestingly, the C=2 Chern insulator state in 10-SL MnBi2Te4 device can sustain above 10 K. The researchers further studied the influence of thickness of MnBi2Te4 devices on Chern number. In 7-SL and 8-SL MnBi2Te4 devices, a quantized Hall resistance plateau h/e2 accompanying with nearly vanishing longitudinal resistance, i.e. Chern insulator state with C=1 is observed. More importantly, the Hall plateau shows nearly quantized resistance even at 45 K in 7-SL MnBi2Te4 device (subfigures (d), (e), (f)) and above 30 K in 8-SL MnBi2Te4 device, which are obviously higher than the Néel temperature (about 22 K) of MnBi2Te4 devices.
The observed high-Chern-number and high-temperature Chern insulator states require weak magnetic field due to the antiferromagnetic nature of MnBi2Te4 at zero magnetic field. As the ordinary QHE can also give rise to quantized Hall resistance plateau and vanishing longitudinal resistance, it is necessary to exclude the influence of Landau levels (LLs) induced by external magnetic field on the findings. The researchers firstly estimated the mobility of MnBi2Te4 devices, which is found to be ranging from 100-300 cm2 V-1 s-1. Such low mobility require an external magnetic field higher than 30 T for QHE with LLs to be observed, which is much higher than the quantization magnetic field in all MnBi2Te4 devices. The researchers further demonstrated that the sign of Chern number remains unchanged with the carrier type when applying back gate voltages, unambiguously excluding the possibility of the ordinary QHE with LLs.
The origin of the observed Chern insulator states is revealed by theoretical calculations. Ferromagnetic MnBi2Te4 is predicted to be the simplest magnetic Weyl semimetal, which possesses only one pair of Weyl points (WPs) near the Fermi level. Quantum confinement leads to the Chern insulator state and layer-dependent Chern number in few-layer MnBi2Te4, permitting the existence of multiple dissipationless edge states in the bulk band gap, which is consistent with the experimental findings. The discovery of high-Chern-number Chern insulator state also provides experimental evidences for the magnetic Weyl semimetal state in MnBi2Te4.
The high-Chern-number and high-temperature Chern insulator states discovered in the intrinsic magnetic topological materials will stimulate the exploration on room-temperature QAHE, and pave the way for great breakthroughs in physics, materials science and information technology.
See the article:
Jun Ge, Yanzhao Liu, Jiaheng Li, Hao Li, Tianchuang Luo, Yang Wu, Yong Xu, Jian Wang
High-Chern-Number and High-Temperature Quantum Hall Effect without Landau Levels