[论文解读] A new flavor of correlation and superconductivity in small twist-angle trilayer graphene

Phum Siriviboon, Jiangxiazi Lin+9 more|arXiv (Cornell University)|Dec 14, 2021

Graphene research and applications被引用 9

一句话总结

本研究揭示了小扭转角双层石墨烯（1.25°–1.38°）中强关联电子物理的新物态，其中窄能带表现出鲁棒的超导性及丰富的分数填充态，挑战了传统的味极化态层级结构。线性温度依赖电阻率的缺失与分数填充态的主导地位表明存在更长程的库仑相互作用，暗示了一种超越自旋-谷极化的莫尔物理新类型。

ABSTRACT

When layers of graphene are rotationally misaligned by the magic angle, the moiré superlattice features extremely flat bands. Due to the enhanced density of states, the Coulomb interaction induces a variety of instabilities. The most prominent occur at integer filling and are therefore commonly attributed to spontaneous polarization of the moiré unit cell's `flavor' degrees of freedom -- spin, valley, and the flat-band degeneracy. As the dominant member of the hierarchy, these correlated states are thought to crucially determine further instabilities at lower energy scales, such as superconductivity and weaker incompressible states at fractional filling. In this work, we examine the behavior of twisted trilayer graphene in a window of twist angle around $1.3^{\circ}$, well below the expected magic angle of $1.55^{\circ}$. In this small twist angle regime, we find surprisingly narrow bands, which are populated with both an abundance of correlation-driven states at fractional filling as well as robust superconductivity. The absence of linear-in-$T$ resistivity without significant reduction of the superconducting transition temperature, provides insights into the origin of both phenomena. Most remarkably, the hierarchy between integer and fractional filling is absent, indicating that flavor polarization does not play a governing role. The prominence of fractional filling in the small twist angle regime also points towards a longer-range effective Coulomb interaction. Combined, our results shed new light on outstanding questions in the field, while establishing the small twist angle regime as a new paradigm for exploring novel flavors of moiré physics.

研究动机与目标

• 研究小扭转角（约1.3°）下的扭曲双层石墨烯（tTLG）的电子性质，该角度远低于1.55°的魔角。
• 确定在缺乏传统魔角平带的情况下，强关联态与超导性是否仍能在该小扭转角区域持续存在。
• 挑战当前普遍假设，即味极化（自旋、谷、轨道）主导莫尔系统中的关联不稳定性。
• 探索长程库仑相互作用在实现分数填充态与鲁棒超导性中的作用。
• 确立小扭转角区域作为研究二维范德华异质结构中非传统关联与超导性的新范式。

提出的方法

• 制备并测量机械剥离、镜像对称的扭曲双层石墨烯（tTLG）器件，其扭转角在1.25°至1.50°之间，进行电输运测量。
• 测量纵向电阻（Rxx）与霍尔电阻（Rxy）随莫尔填充因子（νtTLG）与外加磁场的变化，以识别关联绝缘态与超导转变。
• 采用双栅背门调控化学势，实现对莫尔能带结构中载流子密度的精确控制。
• 分析电阻随温度的变化行为，以提取超导转变温度（Tc）并评估线性温度依赖电阻率的存在与否。
• 利用连续模型对不同扭转角下的能带结构进行理论建模，确认超平带的出现。
• 将实验观测特征（如霍尔载流子密度跃迁、电阻异常）与味极化及非味极化理论的预测进行对比。

实验结果

研究问题

• RQ1 在显著低于魔角1.55°的扭转角下，双层石墨烯中是否仍存在窄能带与强电子关联？
• RQ2 在缺乏线性温度依赖电阻率的条件下，为何在小扭转角下观测到鲁棒的超导性？其起源为何？
• RQ3 味自由度（自旋、谷、轨道）在该区域的关联相中起多大作用？
• RQ4 为何在小扭转角区域分数填充态显著突出，与预期的整数填充层级结构相反？
• RQ5 该区域的有效库仑相互作用如何演化？其对电子关联性质意味着什么？

主要发现

• 在1.30°–1.38°的扭转角下，观测到带宽小于魔角（1.55°）附近的带宽，表明电子关联显著增强。
• 在小至1.25°的扭转角下，仍观测到超导转变温度（Tc）超过2 K的鲁棒超导性，且在无线性温度依赖电阻率的情况下Tc未显著降低。
• 分数填充态丰富且稳定，与传统层级结构中整数填充因味极化而占主导地位的预期相悖。
• 整数填充层级不明显且超导性持续存在，表明味极化在该区域并非主导机制。
• 分数填充态的显著性与线性温度依赖电阻率的缺失共同指向更长程的有效库仑相互作用，暗示一种新的关联物理类型。
• 小扭转角区域作为研究非传统超导性与关联效应的独立平台脱颖而出，其物理机制超越了标准莫尔物理框架。