查看更多>>摘要：The topological magnetoelectric effect(TME)is a hallmark response of the topological field theory,which provides a paradigm shift in the study of emergent topological phenomena.However,its direct observation is yet to be realized due to the demanding magnetic configuration required to gap all surface states.Here,we theoretically propose that axion insulators with a simple ferromagnetic configuration,such as the MnBi2Te4/(Bi2Te3)n family,provide an ideal playground to realize the TME.In the designed triangular prism geometry,all the surface states are magnetically gapped.Under a vertical electric field,the surface Hall currents give rise to a nearly half-quantized orbital moment,accompanied by a gapless chiral hinge mode circulating in parallel.Thus,the orbital magnetization from the two topological origins can be easily distinguished by reversing the electric field.Our work paves the way for direct observation of the TME in realistic axion-insulator materials.
查看更多>>摘要：The layer Hall effect describes electrons spontaneously deflected to opposite sides at different layers,which has been experimentally reported in the MnBi2Te4 thin films under perpendicular electric fields.Here,we reveal a universal origin of the layer Hall effect in terms of the so-called hidden Berry curvature,as well as material design principles.Hence,it gives rise to zero Berry curvature in momentum space but non-zero layer-locked hidden Berry curvature in real space.We show that,compared to that of a trivial insulator,the layer Hall effect is significantly enhanced in antiferromagnetic topological insulators.Our universal picture provides a paradigm for revealing the hidden physics as a result of the interplay between the global and local symmetries,and can be generalized in various scenarios.
查看更多>>摘要：The intrinsic magnetic topological insulator MnBi2Te4 provides a feasible pathway to the high-temperature quantum anomalous Hall(QAH)effect as well as various novel topological quantum phases.Although quantized transport properties have been observed in exfoliated MnBi2Te4 thin flakes,it remains a big challenge to achieve molecular beam epitaxy(MBE)-grown MnBi2Te4 thin films even close to the quantized regime.In this work,we report the realization of quantized anomalous Hall resistivity in MBE-grown MnBi2Te4 thin films with the chemical potential tuned by both controlled in situ oxygen exposure and top gating.We find that elongated post-annealing obviously elevates the temperature to achieve quantization of the Hall resistivity,but also increases the residual longitudinal resistivity,indicating a picture of high-quality QAH puddles weakly coupled by tunnel barriers.These results help to clarify the puzzles in previous experimental studies on MnBi2Te4 and to find a way out of the big difficulty in obtaining MnBi2Te4 samples showing quantized transport properties.
查看更多>>摘要：Topological materials,which feature robust surface and/or edge states,have now been a research focus in condensed matter physics.They represent a new class of materials exhibiting nontrivial topological phases,and provide a platform for exploring exotic transport phenomena,such as the quantum anomalous Hall effect and the quantum spin Hall effect.Recently,magnetic topological materials have attracted considerable interests due to the possibility to study the interplay between topological and magnetic orders.In particular,the quantum anomalous Hall and axion insulator phases can be realized in topological insulators with magnetic order.MnBi2Te4,as the first intrinsic antiferromagnetic topological insulator discovered,allows the examination of existing theoretical predictions;it has been extensively studied,and many new discoveries have been made.Here we review the progress made on MnBi2Te4 from both experimental and theoretical aspects.The bulk crystal and magnetic structures are surveyed first,followed by a review of theoretical calculations and experimental probes on the band structure and surface states,and a discussion of various exotic phases that can be realized in MnBi2Te4.The properties of MnBi2Te4 thin films and the corresponding transport studies are then reviewed,with an emphasis on the edge state transport.Possible future research directions in this field are also discussed.
查看更多>>摘要：We review recent progress in the electronic structure study of intrinsic magnetic topological insulators(MnBi2Te4).(Bi2Te3)n(n=0,1,2,3)family.Specifically,we focus on the ubiquitously(nearly)gapless behavior of the topological Dirac surface state observed by photoemission spectroscopy,even though a large Dirac gap is expected because of surface ferromagnetic order.The dichotomy between experiment and theory concerning this gap behavior is perhaps the most critical and puzzling question in this frontier.We discuss various proposals accounting for the lack of magnetic effect on the topological Dirac surface state,which are mainly categorized into two pictures,magnetic reconfiguration and topological surface state redistribution.Band engineering towards opening a magnetic gap of topological surface states provides great opportunities to realize quantized topological transport and axion electrodynamics at higher temperatures.
查看更多>>摘要：The search for magnetic topological materials has been at the forefront of condensed matter research for their potential to host exotic states such as axion insulators,magnetic Weyl semimetals,Chern insulators,etc.To date,the MnBi2nTe3n+1 family is the only group of materials showcasing van der Waals-layered structures,intrinsic magnetism and non-trivial band topology without trivial bands at the Fermi level.The interplay between magnetism and band topology in this family has led to the proposal of various topological phenomena,including the quantum anomalous Hall effect,quantum spin Hall effect and quantum magnetoelectric effect.Among these,the quantum anomalous Hall effect has been experimentally observed at record-high temperatures,highlighting the unprecedented potential of this family of materials in fundamental science and technological innovation.In this paper,we provide a comprehensive review of the research progress in this intrinsic magnetic topological insulator family,with a focus on single-crystal growth,characterization of chemical disorder,manipulation of magnetism through chemical substitution and external pressure,and important questions that remain to be conclusively answered.
查看更多>>摘要：In the past 5 years,there has been significant research interest in the intrinsic magnetic topological insulator family compounds MnBi2+2nTe4+3n(where n=0,1,2...).In particular,exfoliated thin films of MnBi2Te4 have led to numerous experimental breakthroughs,such as the quantum anomalous Hall effect,axion insulator phase and high-Chern number quantum Hall effect without Landau levels.However,despite extensive efforts,the energy gap of the topological surface states due to exchange magnetic coupling,which is a key feature of the characteristic band structure of the system,remains experimentally elusive.The electronic structure measured by using angle-resolved photoemission(ARPES)shows significant deviation from ab initio prediction and scanning tunneling spectroscopy measurements,making it challenging to understand the transport results based on the electronic structure.This paper reviews the measurements of the band structure ofMnBi2+2nTe4+3n magnetic topological insulators using ARPES,focusing on the evolution of their electronic structures with temperature,surface and bulk doping and film thickness.The aim of the review is to construct a unified picture of the electronic structure of MnBi2+2nTe4+3n compounds and explore possible control of their topological properties.
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