[论文解读] Flat bands, strange metals, and the Kondo effect

J. G. Checkelsky, B. Andrei Bernevig+3 more|arXiv (Cornell University)|Dec 17, 2023

Iron-based superconductors research参考文献 193被引用 4

一句话总结

本文提出一个统一框架，通过Kondo物理和电子拓扑，将平坦能带体系与强关联电子体系联系起来。它表明，平坦能带——尤其是拓扑平坦能带——可被理解为涌现的Kondo现象，而Kondo物理则丰富了拓扑量子物质，揭示了奇异金属行为与非传统超导性中的共享机制。

ABSTRACT

Flat band materials such as the kagome metals or moiré superlattice systems are of intense current interest. Flat bands can result from the electron motion on numerous (special) lattices and usually exhibit topological properties. Their reduced bandwidth proportionally enhances the effect of Coulomb interaction, even when the absolute magnitude of the latter is relatively small. Seemingly unrelated to these cases is the large family of strongly correlated electron systems, which includes the heavy fermion compounds, cuprate and pnictide superconductors. In addition to itinerant electrons from large, strongly overlapping orbitals, they frequently contain electrons from more localized orbitals, which are subject to a large Coulomb interaction. The question then arises as to what commonality in the physical properties and microscopic physics, if any, exists between the two broad categories of materials? A rapidly increasing body of strikingly similar phenomena across the different platforms -- from electronic localization-delocalization transitions to strange metal behavior and unconventional superconductivity -- suggests that similar underlying principles could be at play. Indeed, it has recently been suggested that flat band physics can be understood in terms of Kondo physics. Inversely, the concept of electronic topology from lattice symmetry, which is fundamental in flat band systems, is enriching the field of strongly correlated electron systems where correlation-driven topological phases are increasingly being investigated. Here we elucidate this connection, survey the new opportunities for cross-fertilization in understanding across the platforms, and assess the prospect for new insights that may be gained into both the correlation physics and its intersection with electronic topology.

研究动机与目标

• 识别平坦能带材料与强关联电子体系之间的共同物理原理，尽管其电子起源不同。
• 研究Kondo物理在平坦能带体系中局域-非局域相变与奇异金属行为中的作用。
• 探讨由晶格对称性产生的电子拓扑如何稳定并丰富关联驱动的拓扑相。
• 评估摩尔超晶格、凯库梅金属与Kondo绝缘体之间在发现新量子相方面的交叉促进潜力。
• 检验拓扑超导与分数量子霍尔绝缘体是否可从Kondo驱动的平坦能带中涌现。

提出的方法

• 通过U/W比作为控制参数，分析平坦能带体系中电子关联（U）与能带宽度（W）的相互作用。
• 应用Kondo模型框架，描述拓扑平坦能带材料中局域电子与导带的耦合。
• 利用拓扑不变量与对称性约束（如空间群对称性）对关联体系中的新型拓扑相进行分类与预测。
• 跨平台调查实验信号——如量子振荡、电阻率标度与边缘态——以识别普遍行为。
• 利用格林函数技术与非微扰方法，同时处理强关联与拓扑效应。
• 借鉴Kondo驱动的外尔节点与拓扑平坦能带之间的类比，统一描述量子临界性与奇异金属行为。

实验结果

研究问题

• RQ1 平坦能带物理是否可被理解为Kondo物理的涌现形式，特别是在具有拓扑能带结构的体系中？
• RQ2 Kondo效应与拓扑序在强关联材料（如凯库梅金属与摩尔体系）中在多大程度上共存并相互稳定？
• RQ3 量子临界性与量子纠缠在连接奇异金属行为与拓扑超导性方面起什么作用？
• RQ4 通过强关联与拓扑，是否可在Kondo绝缘体中实现分数量子霍尔绝缘态？
• RQ5 包含拓扑与强关联的对称性增强Kondo模型如何预测新的物相？

主要发现

• 平坦能带体系，特别是具有拓扑特征的体系（如凯库梅金属与摩尔超晶格），由于能带宽度趋近于零，表现出增强的关联效应，类似于强Kondo耦合。
• 当有效能带平坦度发散时，平坦能带中出现奇异金属行为，标志着类似于Kondo量子临界性的局域-非局域相变。
• 在CoSn与魔角扭曲双层石墨烯（MATBG）等材料中的拓扑平坦能带表现出分数量子霍尔绝缘体的特征，暗示其与Kondo驱动的拓扑序之间存在深层联系。
• 在Ce3Bi4Pd3等体系中，Kondo物理稳定了外尔节点并支持拓扑半金属相，表明Kondo屏蔽可产生拓扑保护。
• 平坦能带中拓扑超导性的出现与Kondo驱动的量子临界性密切相关，电阻率标度与非费米液体行为提供了证据。
• 在Kondo破坏量子临界点处，多体量子纠缠达到最大值——奇异金属的特征——暗示其与拓扑量子相（包括自旋液体）具有共同起源。