最近，量子物質科學協同創新中心、beat365量子材料科學中心杜瑞瑞教授、林熙研究員與美國普渡大學GáBor Csáthy教授合作，在InAs/GaSb量子自旋霍爾絕緣體的邊緣态中發現了一種新的電子物質态：螺旋拉廷格液體（Helical Luttinger liquid），這是實驗上首次發現此類受拓撲保護的電子關聯态。
拉廷格液體是指由于電子-電子相互作用産生的一維電子關聯體系。有關拉廷格液體的理論和實驗研究在過去20年來是凝聚态物理的前沿之一，單壁碳納米管的電子态、分數量子霍爾效應邊緣态都先後在實驗上被證實可以用拉廷格液體來描述。拓撲材料是近年來凝聚态物理研究的熱點，量子自旋霍爾絕緣體是其家族成員之一，杜瑞瑞教授組于2011年首先在實驗上證實了InAs/GaSb半導體量子阱體系是一種量子自旋霍爾絕緣體（PRL 107, 136603）。最近，通過極低溫條件下的電輸運測量，他們發現InAs/GaSb邊緣态中的電子行為具有拉廷格液體的特征：邊緣态電阻随溫度的降低和測量偏壓的減小呈指數規律的增加。由于量子自旋霍爾絕緣體邊緣态電子本身具有螺旋的性質（即特定自旋的電子隻能向特定方向運動），這使得其不同于以往發現的拉廷格液體，故被稱為“螺旋拉廷格液體”。
這一實驗首次證實了較強的電子-電子相互作用在受到對稱性保護的拓撲材料中仍起到重要作用，并能從本質上改變其電子性質。這一研究為包括e/2分數電荷激發等在内的一系列新奇量子關聯現象的研究提供了平台。更為重要的是，含有電子-電子相互作用的螺旋邊緣态與超導體結合可能産生奇特類型的費米子元激發(parafermions)，從而為可擴展的拓撲量子計算提供了創新的方案。
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| InAs/GaSb邊緣态螺旋拉廷格液體呈現的孤子型元激發的示意圖。 |
實驗上測得的邊緣态電導随溫度和測量偏壓的變化。 |
相關文章于2015年9月25日在《物理評論快報》上發表（Physical Review Letters 115, 136804），并在其主頁以編輯推薦的形式報道（平均每六篇PRL文章才會選出一篇作為編輯推薦文章）。北大杜瑞瑞教授為文章通訊作者，北大博士生李聽昕為第一作者。
上述研究得到國家重點基礎科學研究計劃（973計劃）、國家自然科學基金等項目經費的資助。
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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.
