[论文解读] Correlated Insulator and Chern Insulators in Pentalayer Rhombohedral Stacked Graphene

Tonghang Han, Zhengguang Lu+8 more|arXiv (Cornell University)|May 3, 2023

Graphene research and applications参考文献 45被引用 11

一句话总结

本文报道在零电荷密度和零位移场下的相关绝缘态，在 rhombohedral-stacked pentalayer graphene 中低磁场下还存在具有 C = -5 和 C = -3 的 Chern insulating states，以及在高场时的 isospin-polarized metals。

ABSTRACT

Rhombohedral stacked multilayer graphene is an ideal platform to search for correlated electron phenomena, due to its pair of flat bands touching at zero energy and further tunability by an electric field. Furthermore, its valley-dependent Berry phase at zero energy points to possible topological states when the pseudospin symmetry is broken by electron correlation. However, experimental explorations of these opportunities are very limited so far, due to a lack of devices with optimized layer numbers and configurations. Here we present electron transport measurements of hBN-encapsulated pentalayer graphene at down to 100 milli-Kelvin. We observed a correlated insulating state with >MOhm resistance at zero charge density and zero displacement field, where the tight-binding calculation predicts a metallic ground state. By increasing the displacement field, we observed a Chern insulator state with C = -5 and two other states with C = -3 at a low magnetic field of ~1 Tesla. At high displacement fields and charge densities, we observed isospin-polarized quarter- and half-metals. Therefore, rhombohedral-stacked pentalayer graphene is the first graphene system to exhibit two different types of Fermi-surface instabilities: driven by a pair of flat bands touching at zero energy, and by the Stoner mechanism in a single flat band. Our results demonstrate a new direction to explore intertwined electron correlation and topology phenomena in natural graphitic materials without the need of moiré superlattice engineering.

研究动机与目标

• 通过平带和可调位移场来探索相关电子现象的平台，作为 rhombohedral-stacked multilayer graphene 的动力来源。
• 通过实验证明 pentalayer rhombohedral graphene 在零密度和零位移场时存在相关绝缘态，并在有限位移场和磁场下通过 Chern insulators 展示拓扑性质。
• 显示在低场下出现的 Chern 数为 C = -5 和 C = -3，在更高的位移场和密度下识别出 isospin-polarized 的 quarter- 和 half-metal 状态。

提出的方法

• 制备具有精确层控制的 hBN 封装的 rhombohedral-stacked pentalayer graphene 器件。
• 进行低至 100 milli-Kelvin 的电子输运测量，绘制电阻对密度和位移场的关系图。
• 施加位移场和低磁场以诱导并识别拓扑（Chern）绝缘态。
• 将实验观测与紧束缚模型计算进行比较，预测在某些区域的导电金属基态。
• 表征高场区域以揭示 isospin-polarized 金属相和 Stoner 型不稳定性。

实验结果

研究问题

• RQ1 rhombohedral-stacked pentalayer graphene 是否能够在零密度和零位移场下实现相关绝缘态？
• RQ2 通过位移场调控，该系统是否能够在低磁场下实现具有有限 Chern 数的 Chern insulator？
• RQ3 平带与电子相关性在推动拓扑和磁性不稳定性方面的作用是什么？
• RQ4 高位移场和高电荷密度对 quarter- 和 half-metal 状态的出现有何影响？
• RQ5 该系统是否代表一种在不使用 moiré 超晶格工程的情况下实现相关性与拓扑耦合的平台？

主要发现

• 在零电荷密度和零位移场下观察到的相关绝缘态，其电阻> MΩ，与紧束缚模型预测的金属性基态形成对比。
• 在有限位移场下、低磁场 (~1 Tesla) 出现了 Chern 数为 C = -5 的 Chern insulator state，以及额外的 C = -3 状态。
• 在更高的位移场和电荷密度下，观察到 isospin-polarized 的 quarter- 和 half-metal 状态。
• 菱方堆叠的 pentalayer graphene 在单一平带中实现了两种不同的费米面不稳定性：平带驱动和 Stoner 型。
• 该系统展示了一条在石墨烯中无需 moiré 超晶格工程即可实现电子相关性与拓扑耦合的新路径。