磁体“沙漠”中的月牙泉——量子自旋液体

Spring in the desert of magnets——quantum spin liquids

当温度趋于零时,如果阻挫自旋系统不呈现长程磁序也不破缺晶格对称性,而且其低能物理性质表现出分数化的量子特征,这种状态称为量子自旋液体态。量子自旋液体随着高温超导的发现而被大量研究,它超越了朗道对称自发破缺的理论框架,并促进了拓扑序理论的发展,其分类理论是以投影对称群为代表的现代数学工具。量子自旋液体中的元激发是分数化的自旋子和演生规范场的光子或量子化磁通,有能隙的量子自旋液体可应用于拓扑量子计算。研究量子自旋液体的理论工具还包括严格可解格点模型、各种不同的数值计算方法和量子场论等。实验上,三角晶格、笼目晶格、六角晶格甚至三维格点系统中的量子自旋液体候选材料相继被发现和深入研究,探测手段包括核磁共振、中子散射、热输运等,调控方法包括高磁场、高压强等。确切的量子自旋液体材料的寻找仍在持续进行中,近些年发展尤其迅速。中国研究人员做出了很多努力,并在部分方向上逐步开始起引领作用。量子自旋液体是一个充满困难和挑战的研究领域,也是一个充满魅力和活力的领域。

A quantum spin liquid(QSL) is a magnetically disordered many-body state at absolute zero temperature without any breaking of crystal symmetries, whose low energy physics exhibits features of fractionalization. Being beyond the Landau-Gintzburg paradigm of spontaneous symmetry breaking, it has been profoundly studied since the discovery of high-temperature superconductors, and has stimulated the development of topological order theory. Modern mathematical tools, such as the projective symmetry groups, are introduced to classify QSLs. The elementary excitations in a QSL are fractionalized spinons and emergent gauge photons or flux quanta, hence a gapped QSL has potential application in topological quantum computation.Theoretical approaches also include exactly solvable models, various numerical simulations, and quantum field theory. Experimentally, possible QSL candidates on triangular, kagomé, honeycomb lattices, and even three dimensional lattices, have been reported and investigated. To obtain QSLs,certain experimental conditions have been tried, such as high magnetic fields or high pressures. To detect their physical properties, various techniques have been applied, including nuclear magnetic resonance, neutron scattering, thermo transport, etc. The searching of definite QSL materials is still ongoing, and rapid progresses have been made recently. Researchers in China have made great efforts and are gradually playing a leading role in some specific directions. This research area is full of difficulties and challenges, as well as vitality and fascination.