[论文解读] Flux-dependent localisation in a disordered flat-band lattice

Fangzhao Alex An, Η. Meier+1 more|arXiv (Cornell University)|May 25, 2017

Cold Atom Physics and Bose-Einstein Condensates被引用 5

一句话总结

本研究首次利用超冷原子在合成动量空间晶格中实现了对无序平带晶格的量子模拟，通过工程化设计具有多距离跃迁和合成规范通量的之字形结构，揭示了通量依赖的局域化现象及近似平带四次能带中的局域化边界，建立了简并拓扑、无序与受挫量子系统中涌现多体现象之间的直接联系。

ABSTRACT

Physical systems with high ground state degeneracy, such as electrons in large magnetic fields [1, 2] and geometrically frustrated spins [3], provide a rich playground for exploring emergent many-body phenomena. Quantum simulations with cold atoms offer new prospects for exploring complex phases arising from frustration and interactions [4-7] through the direct engineering of these ingredients in a well-controlled environment [8, 9]. Advances in band structure engineering, through the use of sophisticated lattice potentials made from interfering lasers, have allowed for explorations of kagome [10] and Lieb [11] lattice structures that support high-degeneracy excited energy bands. The use of internal states as synthetic dimensions [12] offers even greater flexibility in creating nontrivial band structures [13-17]. Here, using synthetic lattices based on laser-coupled atomic momentum states, we perform the first exploration of high-degeneracy ground bands in a cold atom system. By combining nearest- and next-nearest-neighbour tunnellings, we form an effective zigzag lattice that naturally supports kinetic frustration and nearly-flat quartic energy bands. In a quartic band structure with time-reversal symmetry (TRS) broken by a synthetic magnetic flux, the quench dynamics of our atoms reveal hallmark signatures of spin-momentum locking. Under preserved TRS, we demonstrate the extreme sensitivity of a nearly-flat ground band to added disorder. Furthermore, we show that the ground state localisation properties are strongly modified by an added flux, relating to a flux-dependent mobility edge due to multi-range tunnelling. Our work constitutes the first quantum simulation study on the interplay of topology and disorder [18], and opens the door to studying emergent phenomena driven by frustrated kinetics and atomic interactions

研究动机与目标

• 利用超冷原子研究拓扑、无序与平带在量子系统中的相互作用。
• 通过动量空间中的合成晶格工程化实现具有动能受挫的高简并基态能带。
• 研究合成规范通量如何改变无序条件下近似平带中的局域化行为。
• 展示具有多距离跃迁的系统中通量依赖的局域化边界的实证存在。
• 提供一个受控平台，用于研究由受挫动力学与相互作用引发的涌现多体现象。

提出的方法

• 利用激光耦合原子动量态，在系统中构建具有有效最近邻与次近邻跃迁的合成晶格。
• 工程化设计一种支持高简并度近似平带四次能带的等效之字形晶格结构。
• 引入合成规范通量以打破时间反演对称性，并探测拓扑效应。
• 应用淬火动力学以观测在通量存在下自旋-动量锁定的特征信号。
• 引入无序以研究其对基态局域化的影响，并通过可调通量调控局域化边界。
• 利用时间分辨测量追踪原子密度动力学并提取局域化特性。

实验结果

研究问题

• RQ1 在无序条件下，合成规范通量如何影响近似平带中的局域化行为？
• RQ2 多距离跃迁的存在是否会在平带系统中导致通量依赖的局域化边界？
• RQ3 在时间反演对称性被破坏的平带中，自旋-动量锁定的哪些特征信号会显现？
• RQ4 无序如何影响高简并度四次能带的基态？
• RQ5 合成晶格平台能否在受控的量子模拟中实现拓扑、受挫与无序之间的相互作用？

主要发现

• 淬火动力学清晰揭示了在时间反演对称性被破坏的区域中，合成规范通量下自旋-动量锁定的显著特征。
• 当时间反演对称性被保留时，基态对无序表现出极端敏感性，表明在平带中发生安德森局域化。
• 引入合成通量后，诱导出通量依赖的局域化边界，从而改变局域化-非局域化相变行为。
• 多距离跃迁使可调谐的局域化边界得以出现，通过合成通量实现对局域化的拓扑调控。
• 该系统实现了高简并度的近似平带四次能带，为强关联与涌现量子物态提供了条件。
• 本工作首次建立了受控量子模拟中研究拓扑、无序与受挫相互作用的实验平台。