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Jian Wang’s group publishes an article in Nano Letters reporting quantum transport and modulation in topological insulator/normal insulator superlattices
《纳米快报》报道北大王健等人在拓扑绝缘体超晶格量子输运和调控方面的重要进展

Topological insulators, as new quantum materials, have attracted great attention in condensed matter physics due to their exotic surface property. Recently, theoretical studies have demonstrated that the topological protected surface states are 2D version of Weyl electron. As for 3D, Weyl semimetal possessing “3D surface states” can be achieved in superlattices (SLs) where topological insulators (TIs) are interlaced with normal insulators (NIs). The TI/NI SLs with multiple Dirac channels are predicted to offer great opportunity to design novel materials and investigate new quantum phenomena. More importantly, TI/NI SLs can make the modulation of properties of topological materials become possible.

 

Prof. Jian Wang at Peking University (PKU), in collaboration with Prof. Xin-Cheng Xie at PKU (theory), Prof. Maohai Xie at HKU (MBE sample growth) and Prof. Yong Wang at ZJU (TEM measurements), firstly and systematically studied the transport property of the artificial TI/NI SL systems. They investigated the quantum transport of SL heterostructure consisted of different thickness of TI layersand found that tuning the thickness of TI Bi2Se3 layers may completely change the transport dimensionality from 3D to 2D in Bi2Se3/In2Se3 SLs. The discovery demonstrated the feasibility of modulation of topological material property by using TI/NI SLs. This work may stimulate the research on exploring exotic quantum state and potential magneto-conductance, thermoelectric and spintronics applications in TI/NI SLs. The results were published online in Nano Letters (DOI: 10.1021/nl502220p, 2014) with a title of “Crossover from 3D to 2D Quantum Transport in Bi2Se3/In2Se3 Superlattices”. Jian Wang, Xin-Cheng Xie and Mao-Hai Xie are corresponding authors of this paper. Yanfei Zhao and Haiwen Liu contributed equally to this work.

 

Jian Wang’s group at ICQM, School of Physics, PKU has focused on topological insulators for several years and made a series of achievements in this field with collaborators. Such as electron-electron quantum correction in topological insulators (Physical Review B 83, 245438 (2011)); superconductor-topological insulator interaction (Physical Review B 85, 045415 (2012)); negative magnetoresistance in parallel magnetic field in topological insulator films (Nano Research 5, 739 (2012)); p type and n type topological insulators grown on GaAs substrates (AIP Advances 3, 072112 (2013)); topological insulator heterostructures (Scientific Reports 3, 3060 (2013)); crossover between weak antilocalization and weak localization of bulk states in ultrathin topological insulator films (Scientific Reports 4, 5817 (2014)), and so on.

 

The work was supported by National Basic Research Programs of China, National Natural Science Foundation of China, 1000 Talents Program for Young Scientists of China, the Research Fund for the Doctoral Program of Higher Education (RFDP) of China, and Collaborative Innovation Center of Quantum Matter, China.

Figure 1. Transmission electron microscopy (TEM) and EDX map of (Bi2Se3)6/(In2Se3)6 SL structure and the schematic structure for the transport measurements of the TI / NI SLs.

Figure 2. Transport properties of (Bi2Se3)12/(In2Se3)6 SLs and (Bi2Se3)6/(In2Se3)6 SLs.

 拓扑绝缘体体内是有能隙的绝缘体,而表面是无能隙的自旋轨道耦合的金属态。作为一种新的量子材料,拓扑绝缘体已成为当前凝聚态物理最重要的研究领域之一。超晶格是指在纳米尺度可人工调制周期的晶体结构。最新的理论研究表明,拓扑绝缘体的特殊表面态是Weyl电子的二维体现,而拓扑绝缘体/普通绝缘体形成的超晶格可以构成理论预言的三维Weyl半金属,从而观测到真正的三维表面态。更为重要的是,拓扑绝缘体超晶格结构,有望实现对拓扑材料物性的人工调制。因此拓扑绝缘体超晶格的研究对于发现新的量子现象以及探索新的量子材料都具有重要科学意义。然而,拓扑绝缘体/普通绝缘体超晶格的电输运实验研究及可调控特性一直未见报道。

 

最近,北京大学物理学院量子材料科学中心王健研究员,与中心谢心澄教授、香港大学谢茂海教授、浙江大学王勇教授等人合作,首次对于拓扑绝缘体/普通绝缘体(Bi2Se3/In2Se3)超晶格的量子输运特性展开系统研究。在低温强磁场下对不同拓扑绝缘体层厚的Bi2Se3/In2Se3超晶格的电输运测量发现:改变其中拓扑绝缘体层Bi2Se3的厚度会导致体系的量子输运维度从三维转变为二维。该结果证实了人工调控拓扑材料物性的可行性,是拓扑绝缘体超晶格量子输运特性的首次报道。这一工作不仅为新量子态的探索,也为研发人工调制的拓扑材料及其在磁电、热电和自旋电子学等方面的潜在应用奠定了基础。相关工作以“Crossover from 3D to 2D Quantum Transport in Bi2Se3/In2Se3 Superlattices”为题,在线发表在《纳米快报》(Nano Letters DOI: 10.1021/nl502220p (2014))上,北京大学王健研究员、谢心澄教授和香港大学谢茂海教授为该论文共同通讯作者,北京大学博士生赵弇斐和博士后刘海文为论文的共同第一作者。

 

量子材料科学中心王健研究组与合作者近年来在拓扑绝缘体薄膜及其异质结构的电输运特性领域取得了一系列进展。例如:王健等人在拓扑绝缘体的研究中首次定量引入电子-电子相互作用的量子修正,已成为相关方向的奠基性论文(Physical Review B 83, 245438 (2011));系统研究了三种超导体与拓扑绝缘体薄膜的相互作用(Physical Review B 85, 045415 (2012));发现拓扑绝缘体薄膜在平行磁场下的负磁阻行为及其与电流方向的相关性(Nano Research 5, 739 (2012));在半导体砷化镓衬底上成功制备出p型和n型的拓扑绝缘体薄膜,为拓扑绝缘体的p-n型器件研发奠定了基础(AIP Advances 3, 072112 (2013)),相关工作得到国际媒体的关注和报道;首次对拓扑绝缘体异质结构进行了电输运实验研究,证实了表面在量子输运中的关键性作用(Scientific Reports 3, 3060 (2013));在平行场下对拓扑绝缘体超薄膜进行了50特斯拉以上的强磁场电输运测量,观测到拓扑绝缘体体态引起的弱反局域化到弱局域化的转变(Scientific Reports 4, 5817 (2014)),等等。

 

上述研究得到了国家重大科学研究计划、国家自然科学基金、中组部“青年千人”计划、高等学校博士学科点专项科研基金以及量子物质科学协同创新中心的支持。

1(Bi2Se3)6/(In2Se3)6 超晶格结构的透射电镜图,元素分辨图以及电输运测试示意图。

2  (Bi2Se3)12/(In2Se3)6 (Bi2Se3)6/(In2Se3)6超晶格的量子输运特性