[论文解读] Superconductivity and strong interactions in a tunable moiré quasiperiodic crystal

Aviram Uri, Sergio C. de la Barrera+11 more|arXiv (Cornell University)|Feb 1, 2023

Quasicrystal Structures and Properties被引用 8

一句话总结

该论文展示了在由三层石墨烯构成、具有两个不整平扭角的可调谐莫尔准周期晶体中存在超导性和强相互作用，揭示了具有周期性与准周期性电子行为的两种调控区，以及一种风味对称性破缺的相变。

ABSTRACT

Electronic states in quasiperiodic crystals generally preclude a Bloch description, rendering them simultaneously fascinating and enigmatic. Owing to their complexity and relative scarcity, quasiperiodic crystals are underexplored relative to periodic and amorphous structures. Here, we introduce a new type of highly tunable quasiperiodic crystal easily assembled from periodic components. By twisting three layers of graphene with two different twist angles, we form two moiré patterns with incommensurate moiré unit cells. In contrast to many common quasiperiodic structures that are defined on the atomic scale, the quasiperiodicity in our system is defined on moiré length scales of several nanometers. This novel "moiré quasiperiodic crystal" allows us to tune the chemical potential and thus the electronic system between a periodic-like regime at low energies and a strongly quasiperiodic regime at higher energies, the latter hosting a large density of weakly dispersing states. Interestingly, in the quasiperiodic regime we observe superconductivity near a flavor-symmetry-breaking phase transition, the latter indicative of the important role electronic interactions play in that regime. The prevalence of interacting phenomena in future systems with in situ tunability is not only useful for the study of quasiperiodic systems, but it may also provide insights into electronic ordering in related periodic moiré crystals. We anticipate that extending this new platform to engineer quasiperiodic crystals by varying the number of layers and twist angles, and by using different two-dimensional components, will lead to a new family of quantum materials to investigate the properties of strongly interacting quasiperiodic crystals.

研究动机与目标

• 激发对超出周期性莫尔晶体的准周期电子系统的探索。
• 引入一个由两不相等扭角构成的三层石墨烯构成的可调莫尔准周期晶体（MQC）。
• 表征载流子密度变化时电子结构从周期性样态向准周期性区间的演化。
• 研究 MQC 中由相互作用驱动的现象，包括风味对称性破缺和超导性。

提出的方法

• 制备一个 hBN 封装的有两种交替不等扭角的扭曲三层石墨烯器件。
• 调制载流子密度和电位移场以进入不同电子区间。
• 使用四端传输测量和磁场依赖性来鉴定朗道能级与量子振荡。
• 用重新标定的速度在低能 Dirac锥中建模并考虑场引起的势位移以解释朗道能级谱。
• 通过带有广义 Umklapp 散射的动量空间紧束缚计算谱函数，将理论与观测到的准周期特征联系起来。

实验结果

研究问题

• RQ1 三层石墨烯系统在两个互不整平扭角下是否能实现一个可调的莫尔准周期晶体（MQC）？
• RQ2 随着密度和位移场的变化，电子结构如何在周期性样态与准周期性区间之间演化？
• RQ3 MQC 中相互作用的标志，如风味对称性破缺和超导性，表现为何种特征？
• RQ4 在 MQC 中三层的 Dirac 锥如何混频并对 D-field 作出何种响应？
• RQ5 在该平台上准周期性与观测到的超导态之间存在怎样的关系？

主要发现

• 在约 T_BKT ≈ 300 mK 的温度下出现两个零电阻的超导口袋，Tc 约 0.4 K。
• 在 ν12 约为 -2 附近观察到伴随霍尔密度下降的风味对称性破缺相变。
• 证据显示在较高密度下存在两次莫尔晶胞填充 n_s,12 约为 4.7×10^12 cm^-2，n_s,23 在更高密度下，也证实了莫尔准晶体的存在。
• 在较高密度与位移场下呈现准周期区间，出现密集的避免交叉并降低朗道能级可观性。
• 在非“魔角”莫尔系统中呈现出强耦合样超导性，Tc/T_F 约 0.008，相干长度 ξ 约 70 nm（层间距离 d 约 7.5 nm，T_F 约 50 K）。
• 谱函数计算揭示从周期性 Bloch 样态到密集准周期谱的转变，且包含层分辨贡献。