三维拓扑绝缘体的反常负磁阻效应研究

In general, magnetic fields tend to localize electrons and lead to positive magnetoresistance. Therefore, large negative magnetoresistance is unusual and always represents some peculiar physics. Negative magnetoresistance was often associated with magnetic materials. Recently, anomalous negative magnetoresistance was observed in non-magnetic Dirac and Weyl semimetal, which is attributed to the chiral anomaly of Weyl fermions. This negative magnetoresistance is very sensitive to the angle between magnetic field and current, and the maximum value is obtained when magnetic field is parallel to the current. However, a similar negative magnetoresistance was also discovered in non-magnetic 3D topological insulator and topological crystal insulator. So far, their physical mechanisms have not yet been clearly defined. The interpretations of different experimental mechanisms have their own limitations and does not apply to each other. This project plans to determine the relationship between the anomalous negative magnetoresistance and topological non-trivial state, and research how the disorder and carrier concentration affect the negative magnetoresistance, through a series of special comparative experiments. It will provide useful experimental evidence for revealing hidden intrinsic physics and establishing a unified theory.

一般，磁场使传导电子运动局域化，引起正的磁电阻。大的负磁阻并不常见，往往代表着奇异的物理现象。负磁阻常与磁性材料联系在一起。最近在非磁性的狄拉克半金属和外尔半金属中，发现了手征反常引起的负磁阻效应。负磁阻对磁场和电场角度非常敏感，磁场平行于电场时最大。然而，在非磁性的三维拓扑绝缘体和拓扑晶体绝缘体中，也发现了类似的反常负磁阻效应和无序引起的各向同性负磁阻效应。目前，它们的物理机制还没有明确统一建立。不同实验的机理解释都存在各自的局限性，并不适用与彼此。本项目拟通过一系列对照实验研究，探明负磁阻效应与拓扑非平庸态的相关性；研究无序、载流子浓度等因素对拓扑绝缘体负磁阻的影响，为揭示隐藏的本征物理机制和建立统一理论提供实验依据。

在经典物理中，磁场总是阻碍电子沿电场方向运动，使其做局域回旋运动，从而引起正的磁电阻效应。负的磁电阻效应通常与磁性材料的磁有序态相关联，而在非磁性材料中则代表着新奇的物理现象。当外磁场平行于测试电流方向时，在非磁性的狄拉克半金属和外尔半金属中发现了负的纵向磁电阻，这被认为是其拓扑电子能带结构中手性相反狄拉克（外尔）点引起手征反常的直接证据。然而，在没有手征反常的三维拓扑绝缘体和拓扑晶体绝缘体中也观测到了类似的反常负磁阻效应，从而导致了相关理论解释的巨大混乱。本项目详细研究了拓扑表面态、载流子浓度、迁移率以及无序等参数对负磁阻效应的影响。拓扑绝缘体Bi2Se3和拓扑超导体候选者SrxBi2Se3单晶的各项异性负磁阻在2-300 K温度区间和0-35 T磁场区间都稳定存在，并且低载流子浓度和高迁移率的样品更容易观测到负磁阻；在拓扑平庸同构体Sb2Se3的单晶和薄膜样品中均未观测到类似的负磁阻，表明拓扑表面态与负的纵向磁电阻效应具有较大相关性。受本项目研究经费支持，我们也详细调控研究了MnBi2Te4(Bi2Te3)n体系磁性拓扑绝缘体的磁结构、层间和层内磁耦合竞争以及不同解理面的拓扑表面态电子结构。我们成功合成了铁磁拓扑绝缘体MnBi8Te13，并将MnBi6Te10从层间反铁磁调控为准二维铁磁体，而且揭示了拓扑表面态能带与体Rashba劈裂能带的杂化可以显著影响该体系Bi2Te3层解离面的拓扑表面态能隙。.本项目共发表研究论文5篇，其中Physical Review X 2篇，Nano Letters 1 篇，Physical Review B 2篇；另有一篇论文在Physical Review B审稿中。