最近，量子物质科学协同创新中心、北京大学量子材料科学中心杜瑞瑞教授、林熙研究员与美国普渡大学GáBor Csáthy教授合作，在InAs/GaSb量子自旋霍尔绝缘体的边缘态中发现了一种新的电子物质态：螺旋拉廷格液体（Helical Luttinger liquid），这是实验上首次发现此类受拓扑保护的电子关联态。
拉廷格液体是指由于电子-电子相互作用产生的一维电子关联体系。有关拉廷格液体的理论和实验研究在过去20年来是凝聚态物理的前沿之一，单壁碳纳米管的电子态、分数量子霍尔效应边缘态都先后在实验上被证实可以用拉廷格液体来描述。拓扑材料是近年来凝聚态物理研究的热点，量子自旋霍尔绝缘体是其家族成员之一，杜瑞瑞教授组于2011年首先在实验上证实了InAs/GaSb半导体量子阱体系是一种量子自旋霍尔绝缘体（PRL 107, 136603）。最近，通过极低温条件下的电输运测量，他们发现InAs/GaSb边缘态中的电子行为具有拉廷格液体的特征：边缘态电阻随温度的降低和测量偏压的减小呈指数规律的增加。由于量子自旋霍尔绝缘体边缘态电子本身具有螺旋的性质（即特定自旋的电子只能向特定方向运动），这使得其不同于以往发现的拉廷格液体，故被称为“螺旋拉廷格液体”。
这一实验首次证实了较强的电子-电子相互作用在受到对称性保护的拓扑材料中仍起到重要作用，并能从本质上改变其电子性质。这一研究为包括e/2分数电荷激发等在内的一系列新奇量子关联现象的研究提供了平台。更为重要的是，含有电子-电子相互作用的螺旋边缘态与超导体结合可能产生奇特类型的费米子元激发(parafermions)，从而为可扩展的拓扑量子计算提供了创新的方案。
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| InAs/GaSb边缘态螺旋拉廷格液体呈现的孤子型元激发的示意图。 |
实验上测得的边缘态电导随温度和测量偏压的变化。 |
相关文章于2015年9月25日在《物理评论快报》上发表（Physical Review Letters 115, 136804），并在其主页以编辑推荐的形式报道（平均每六篇PRL文章才会选出一篇作为编辑推荐文章）。北大杜瑞瑞教授为文章通讯作者，北大博士生李听昕为第一作者。
上述研究得到国家重点基础科学研究计划（973计划）、国家自然科学基金等项目经费的资助。
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ICQM member Prof. Rui-Rui Du and his collaborators publish a paper on Physical Review Letters reporting the observation of a helical Luttinger-liquid state.
Recently, Prof. Rui-Rui Du, and Prof. Xi Lin of ICQM, in collaboration with Prof. GáBor Csáthy of Purdue University, discovered a new electronic state called ‘Helical Luttinger liquid’ in InAs/GaSb quantum spin Hall edges. This is the first experimental observations of a Luttinger -liquid state in topological materials.
The Luttinger liquid describes one-dimensional (1D) interacting electron system. During the past 20 years, theoretical and experimental studies of Luttinger liquid have become a frontier of condensed matter physics. Carbon nanotubes, as well as fractional quantum Hall edge states, have been experimentally studied as spinful Luttinger liquid and chiral Luttinger liquid, respectively. Topological materials have recently attracted a lot of interest in condensed matter physics, and the quantum spin Hall insulator (QSHI) is one of them. In 2011, Prof. Rui-Rui Du group first experimentally proved that InAs/GaSb semiconductor quantum wells system can form a new type of QSHI (PRL 107, 136603). Recently, by ultra-low temperature transport measurements, they found that electrons in InAs/GaSb edge states exhibit Luttinger-liquid behavior, where the resistance of the edge states increases following a power-law with the decreasing of temperature and bias voltage. The underlying physics, often referred as ‘helical Luttinger liquid’, is quite distinct from the known types of Luttinger liquids due to the helical nature (i.e. spin-momentum locking) of the QSHI edge states.
The results indicate that strong electron-electron interaction effects could play important roles in symmetry-protected topological matters, influencing their electronic properties in a fundamental way. This finding offers a platform for studying novel quantum correlation effects, such as e/2 charge excitations. More importantly, interacting helical edge states coupled with superconductors can support parafermionic modes, which are promising for topological quantum computation.
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| Schematic drawing of soliton-like excitations in the edge of InAs/GaSb QSHI. |
The temperature and bias voltage dependence of the measured helical edge conductance. |
The results will be published in Physical Review Letters (PRL 115, 136804) as an Editor’s Suggestion paper. Prof. Rui-Rui Du is the corresponding author, and Tingxin Li, his Ph. D. student at ICQM, is the first author of this paper.
The work in Peking University was supported by National Basic Research Programs of China, and National Natural Science Foundation of China.
