[论文解读] Tunable Phase Boundaries and Ultra-Strong Coupling Superconductivity in Mirror Symmetric Magic-Angle Trilayer Graphene

Jeong Min Park, Yuan Cao+3 more|arXiv (Cornell University)|Dec 2, 2020

Graphene research and applications被引用 64

一句话总结

这篇论文展示了镜像对称的魔角扭曲三层石墨烯(MATTG)中的可调相界和超强耦合超导性，揭示超导性被范霍夫奇点界定，并且有BCS-BEC crossover的接近证据。

ABSTRACT

Moiré superlattices have recently emerged as a novel platform where correlated physics and superconductivity can be studied with unprecedented tunability. Although correlated effects have been observed in several other moiré systems, magic-angle twisted bilayer graphene (MATBG) remains the only one where robust superconductivity has been reproducibly measured. Here we realize a new moiré superconductor, mirror symmetric magic-angle twisted trilayer graphene (MATTG) with dramatically richer tunability in electronic structure and superconducting properties. Hall effect and quantum oscillations measurements as a function of density and electric field allow us to determine the system's tunable phase boundaries in the normal state. Zero magnetic field resistivity measurements then reveal that the existence of superconductivity is intimately connected to the broken symmetry phase emerging from two carriers per moiré unit cell. Strikingly, we find that the superconducting phase gets suppressed and bounded at the van Hove singularities (vHs) partially surrounding the broken-symmetry phase, which is difficult to reconcile with weak-coupling BCS theory. Moreover, the extensive in situ tunability of our system allows us to achieve the ultra-strong coupling regime, characterized by a Ginzburg-Landau coherence length reaching the average inter-particle distance and very large $T_\mathrm{BKT}/T_{F}$ ratios in excess of 0.1, where $T_\mathrm{BKT}$ and $T_F$ are the Berezinskii-Kosterlitz-Thouless transition and Fermi temperatures, respectively. These observations suggest that MATTG can be electrically tuned close to the two-dimensional BCS-BEC crossover. Our results establish a new generation of tunable moiré superconductors with the potential to revolutionize our fundamental understanding and the applications of strong coupling superconductivity.

研究动机与目标

• 调查 MATTG 的电子相图，随载流子密度和位移场 D 变化。
• 描述 Dirac 带和扁平带在施加 D 下的共存与混杂。
• 确定超导性如何出现、演变，并被范霍夫奇点与对称性破缺相所界定。

提出的方法

• 制备镜像对称的 MATTG 设备，扭角 θ 与 −θ，并用 hBN 屏蔽封装。
• 通过磁传导和霍尔效应测量，在 ν 与 D 的函数下绘制常态相边界。
• 通过零场电阻、V-I 特性和 BKT 分析探测超导性以提取 T_BKT。
• 使用 Ginzburg-Landau 关系从 Tc 对正场数据估算相干长度 ξ_GL。
• 分析量子振荡以提取化学势偏移、有效质量 m*，以及朗道扇结构。
• 在连续模型中的带结构计算，用以说明扁平带和Dirac带及其在位移场下的混杂。

实验结果

研究问题

• RQ1 MATTG 的电子结构如何随位移场 D 和莫尔晶格填充 ν 变化？
• RQ2 在 MATTG 中，超导性与 ν=±2 附近的对称性破缺相之间的关系是什么？
• RQ3 MATTG 的超导性是否显示出超强耦合的征兆以及接近 BCS-BEC 交叉的迹象？
• RQ4 范霍夫奇点如何影响 MATTG 的超导相界？
• RQ5 镜像对称性以及 Dirac/扁平带相互作用在实现稳健超导性中起到什么作用？

主要发现

• MATTG 的超导发生在 ν=±2 附近，并可通过 D 强烈调控，T_BKT 可达到约 2.1–2.25 K。
• 在某些参数区间，超导区域被范霍夫奇点界定，这一观察对弱耦合 BCS 预期提出挑战。
• Ginzburg-Landau 相干长度可短至约 12 nm，接近最佳点的粒子间距，显示强耦合。
• T_BKT/T_F 的比值达到或超过 0.1，甚至接近 0.125 的二维 BCS-BEC 交叉上限，暗示接近强耦合或 BCS-BEC 区域。
• 约旦式 Fraunhofer 模式和长程临界场尾迹证实 MATTG 的超导相干性强。
• 量子振荡显示 Dirac 与扁平带贡献，以及载流子类型随 ν=±2 状态演化；vHs 在大 D 时界定超导区域。