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ICQM faculty member publishes an article in Physical Review Letters reporting ‘Chern Kondo Insulator in an Optical Lattice’
《物理评论快报》报道量子材料科学中心对拓扑近藤绝缘体研究的最新成果“Chern Kondo Insulator in an Optical Lattice”

Recently, a postdoc Dr. Hua Chen, Prof. Xiong-Jun Liu and Prof. X. C. Xie at International Center for Quantum Materials, Peking University published a paper titled “Chern Kondo Insulator in an Optical Lattice” in Physical Review Letters 116, 046401 (2016).

The work is motivated by the recently progress of time-reversal invariant topological Kondo insulators (TKIs), which were predicted in a few heavy-fermion materials like SmB6. The topological Kondo insulators originate from the hybridization between itinerant conduction bands and strong correlated f electrons. The proposed scenario of TKIs is consistent with the transport measurement, angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. Nevertheless, the TKIs share the same topological classification as conventional TIs, and the essential difference is that in a TKI it is the strongly correlated Kondo effect that leads to the insulating gap. Measuring such strong correlation physics (for f electrons) can directly distinguish a TKI from conventional TIs, while this might be a challenging task for condensed matter systems. This motivates us to consider Kondo insulators with nontrivial topology in cold atoms, which may allow for an exact study with full controllability.

In our work, we propose to realize and observe a strongly correlated quantum anomalous Hall phase, called Chern Kondo insulator, in an optical lattice, motivated by the recent rapidly developing new technologies for cold atoms. Compared with solid state systems, the cold atoms can offer extremely clean platforms with full controllability to study many-body physics and topological phases. Here, we consider a double-well square lattice with Raman-coupling-assisted s-p orbital hybridization to observe Chern Kondo insulating phases. Due to the strong Hubbard interaction of s orbital states, the Kondo screening is achieved when the applied Raman coupling exceeds a critical value, and then a nonzero renormalized s-p orbital hybridization drives the system into a fully gapped Chern Kondo phase.

Figure: Main panel: (a) Image map of Kondo temperature and (b) direct band gap as a function of hopping integral and gauge field. Subfigure: (a) Kondo temperature and (b) band gap (open square), Chern number (open circle) versus gauge field with tsp=0.8.

We show that the predicted CK insulator can be identified by three characteristic features, namely, the existence of a critical s-p coupling strength for the CK phase transition, the nontrivial topology in the bulk band, and the Mott behavior of the s orbital. These features distinguish the strongly correlated CK insulating phase from the single-particle QAH states. The experimental schemes are proposed for such detection.

This work is supported by the National Basic Research Program of China (Grant No. 2015CB921102) and the National Natural Science Foundation of China (Grants No. 11534001 and No. 11574008). X.-J. L. is also supported in part by the Thousand-Young-Talent Program of China.

      最近,北京大学物理学院量子材料科学中心陈华博士后,刘雄军研究员,和谢心澄教授合作在《物理评论快报》发表了标题为“Chern Kondo Insulator in an Optical Lattice” 的研究论文:Physical Review Letters 116, 046401 (2016)。

      近年来,重费米子材料SmB6被理论预言为时间反演不变的拓扑近藤绝缘体。该体系中Sm原子中4f电子轨道非常局域,因而具有强电子关联,表现为局域磁性。近藤效应由导带d轨道电子和局域的Sm原子中4f电子杂化,进而对4f的局域磁性发生有效屏蔽所导致。由于出现能带反转,d轨道电子和局域的4f电子杂化导致非平凡拓扑绝缘体。理论预言符合多种实验测量。包括输运测量,角分辨光电子谱和扫描隧道谱。尽管如此,拓扑近藤绝缘体与通常的拓扑绝缘体并不存在拓扑性质差别,它们的关键差别在于拓扑近藤绝缘体中能隙是由强关联的近藤耦合所导致,以及4f电子的强关联特征。而对这些性质的直接测量在凝聚态系统中通常十分困难。这促使陈华等人在冷原子中提出合适的方案研究具有非平凡拓扑的近藤绝缘体,并研究直接的观测手段。

      得益于冷原子领域中近年来在实验和测量技术上的快速发展,该工作提出一套利用冷原子在光晶格中实现关联量子反常霍尔效应的方案,即破坏时间反演对称的拓扑近藤绝缘体。和凝聚态体系相比,冷原子为研究多体物理和拓扑相提供了易于操纵的平台。本文引入一个两套子光晶格组成的超晶格,并利用拉曼光诱导s波和p波轨道发生杂化并同时产生有效的规范场。由于s波轨道强关联效应,理论发现只有当拉曼耦合强度大于某一临界值,s波和p波轨道才能发生有效的杂化,从而打开能隙进入近藤绝缘相,并导致关联量子反常霍尔效应。此效应与非相互作用量子反常霍尔系统具有本质区别。

  

图注:主图:(a) 近藤温度和 (b) 直接带隙关于跃迁积分和规范场的相图。子图:(a) 近藤温度和 (b) 直接带隙随着规范场的曲线,此时跃迁积分为0.8。

   该量子反常霍尔近藤绝缘体在实验观测上由三个基本特征决定:1)拓扑相变存在临界拉曼耦合强度;2)拓扑态有非零陈数刻画;3)由于强关联效应,s轨道态不存在双占据现象。针对这些特征,本文提出了相应的观测手段。

      本工作陈华为第一作者,刘雄军为通讯作者。该研究得到国家重大科学研究计划、国家自然科学基金、以及中组部“青年千人”计划等项目经费的资助。