[论文解读] Helical Chern insulator phase with broken time-reversal symmetry in MnBi$_2$Te$_4$

Chang Liu, Yongchao Wang+12 more|arXiv (Cornell University)|Jan 23, 2020

Topological Materials and Phenomena参考文献 31被引用 8

一句话总结

本研究在MnBi₂Te₄中发现了一种植入手性陈绝缘体相，该相具有时间反演对称性破缺，由高磁场和栅压调控稳定。该相表现出一种由反向传播陈能带（C = -1 和 C = +1）共存导致的鲁棒C = 0态，通过非局域输运和陈数从-3到+2的霍尔平台得到证实，揭示了通过磁场工程调控拓扑能带结构的新途径。

ABSTRACT

A perpetual quest in the field of topological quantum matter is to search for electronic phases with unprecedented band topology and transport phenomena. The most prominent example is the discovery of topological insulators, in which band inversion leads to topologically nontrivial bulk electronic structure and metallic boundary states. In two-dimensional topological insulators with time-reversal symmetry, a pair of helical edge states gives rise to the quantum spin Hall effect. When the time-reversal symmetry is broken by magnetic order, only one chiral edge mode remains and the quantum anomalous Hall effect emerges in zero magnetic field. This quantum Hall phase without Landau levels, first observed in magnetically doped topological insulators, is now called the Chern insulator. The recently discovered MnBi2Te4 combines intrinsic magnetism and nontrivial topology in one material, providing an ideal platform for exploring novel topological phases. Here, we investigate the transport properties of exfoliated MnBi2Te4 in exceedingly high magnetic fields up to 60 T. By varying the gate voltage, we observe systematic and yet uniquely complex evolution of quantized Hall plateaus with Chern numbers from C = -3 to +2. More surprisingly, a novel phase characterized by an extremely broad zero Hall plateau emerges as the most robust ground state in the high field limit. Theoretical calculations reveal that this C = 0 phase arises from the coexistence of a connate Chern band with C = -1 and a Zeeman-effect-induced Chern band with C = +1, as corroborated by nonlocal transport measurements. This helical Chern insulator phase with broken time-reversal symmetry represents an unexpected new member of the quantum Hall family, and manifests a new route to change the band topology by using magnetic field.

研究动机与目标

• 探索MnBi₂Te₄等本征磁性拓扑绝缘体中的新型拓扑量子相。
• 研究强磁场和静电栅压如何调控能带拓扑并诱导奇异的量子霍尔态。
• 识别并表征一种具有时间反演对称性破缺且具有独特宽广零霍尔平台的新量子相。
• 通过输运测量与理论分析，确定高磁场下鲁棒C = 0基态的起源。

提出的方法

• 将机械剥离的MnBi₂Te₄薄片置于高达60 T的强磁场中，以探测极端条件下的量子输运行为。
• 系统性地调节栅压以调控载流子密度，控制电子态，从而实现对不同陈数平台的调控。
• 通过量子化霍尔电阻测量识别出陈数C = -3至+2的平台，重点关注零霍尔平台的演化与稳定性。
• 采用非局域输运测量以确认手性边缘态的存在，并区分体相与边缘贡献。
• 利用理论建模解释C = 0相为本征C = -1陈能带与塞曼分裂的C = +1能带共存的结果。
• 分析本征磁性、能带反转与磁场诱导能级分裂之间的相互作用，以解释手性陈绝缘体的出现。

实验结果

研究问题

• RQ1 在高磁场和栅压调控下，MnBi₂Te₄中会涌现何种拓扑相？其与传统陈绝缘体有何不同？
• RQ2 为何在高场极限下，尽管净陈数为零，仍会出现宽广且鲁棒的零霍尔平台作为基态？
• RQ3 本征陈能带与塞曼诱导陈能带的竞争贡献如何导致具有拓扑保护的C = 0态？
• RQ4 非局域输运在区分该体系中边缘态与体相输运中起到何种作用？
• RQ5 C = -1与C = +1陈能带的共存是否可解释观测到的拓扑相变与稳定性？

主要发现

• 在高磁场（高达60 T）下，一种全新的C = 0相作为最鲁棒的基态出现，其零霍尔平台极为宽广。
• 通过栅压调控，观察到陈数从C = -3到C = +2的量化霍尔平台系统性演化。
• C = 0相由本征C = -1陈能带与塞曼分裂的C = +1能带共存稳定，理论建模已证实此机制。
• 非局域输运测量为C = 0相中手性边缘态的存在提供了直接证据，表明即使净陈数为零，该态仍具有拓扑保护。
• 手性陈绝缘体相具有时间反演对称性破缺特征，是量子霍尔家族的新成员。
• 磁场作为可调参数，可用于调控能带拓扑，通过场诱导能级交叉实现新拓扑态的涌现。