[论文解读] Spectroscopic Signatures of Strong Correlations and Unconventional Superconductivity in Twisted Trilayer Graphene

Hyunjin Kim, Youngjoon Choi+8 more|arXiv (Cornell University)|Sep 24, 2021

Graphene research and applications被引用 14

一句话总结

本研究利用高分辨率扫描隧道显微镜与谱学技术，揭示了魔角扭曲三层石墨烯（MATTG）中强烈的电子关联效应及非常规超导性。研究识别出从BCS到BEC超导性的门电压可调转变，其特征为具有节点的序参量，该结果由掺杂依赖的相干峰与拖拽-驼峰结构所证实，表明在镜像对称的A-tw-A堆叠构型下，系统与规范玻色子模式存在强耦合。

ABSTRACT

Magic-angle twisted trilayer graphene (MATTG) has emerged as a novel moiré material that exhibits both strong electronic correlations and unconventional superconductivity. However, spectroscopic studies of its electronic properties are lacking, and the nature of superconductivity and the corresponding order parameter in this system remain elusive. Here we perform high-resolution scanning tunneling microscopy and spectroscopy of MATTG and reveal extensive regions of atomic reconstruction that favor mirror-symmetric stacking. In these regions, we observe a cascade of symmetry-breaking electronic transitions and doping-dependent band structure deformations similar to those realized in magic-angle bilayers, as expected theoretically given the commonality of flat bands. More strikingly, in a density window spanning two to three holes per moire unit cell, spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunneling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped to a nodal superconductor, which we show theoretically is compatible with a sharp transition from a Bardeen-Cooper-Schrieffer (BCS) to a Bose-Einstein condensation (BEC) superconductor with a nodal order parameter. Within this doping window, we also detect peak-dip-hump structures suggesting that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results pave the way for further understanding of superconductivity and correlated states in graphene-based moiré structures beyond twisted bilayers, where unconventional superconductivity and nodal pairing were reported.

研究动机与目标

• 利用高分辨率谱学技术研究魔角扭曲三层石墨烯（MATTG）的电子性质。
• 确定MATTG中超导性的本质及其序参量特性，尽管其表现出强烈的电子关联效应，但其本质仍不明确。
• 探究MATTG中的电子行为是否与扭曲双层石墨烯类似，特别是在平坦能带与对称性破缺转变方面。
• 明确原子重构与堆叠顺序在稳定关联态与超导态中的作用。

提出的方法

• 对扭曲角为1.5°的MATTG器件进行高分辨率扫描隧道显微镜（STM）与谱学（STS）测量。
• 测量隧道电导率dI/dV随栅压的变化，以探测掺杂依赖的电子结构。
• 分析相干峰、拖拽-驼峰结构及能隙演化等谱学特征，以推断超导配对对称性。
• 对归一化电导率数据进行Dynes公式拟合，以区分各向同性与节点型能隙对称性。
• 利用节点型d波能隙与可变化学势，模拟BEC-BCS交叉区域的微分电导。
• 结合朗道扇形图与局域态密度（LDOS），关联朗道能级量子化与超导及关联态之间的关系。

实验结果

研究问题

• RQ1 魔角扭曲三层石墨烯中超导序参量的本质是什么？
• RQ2 超导能隙如何随静电掺杂而演化，这对配对机制有何启示？
• RQ3 观测到的谱学特征是否与从BCS到BEC超导性的转变一致？
• RQ4 峰-拖拽-驼峰结构表明玻色子模式在超导中的作用如何？
• RQ5 原子重构如何影响MATTG的堆叠构型与电子关联性？

主要发现

• 观察到具有A-tw-A堆叠的三角形莫尔超晶格，表明原子重构倾向于形成镜像对称的排列。
• 在空穴掺杂区（每莫尔超原胞2–3个空穴），隧道电导中出现显著的相干峰与拖拽-驼峰结构，表明强耦合超导性。
• 电导随温度与磁场的变化表明，系统经历从全息能隙到节点型超导体的门电压可调转变。
• 数据与从Bardeen-Cooper-Schrieffer（BCS）到Bose-Einstein凝聚（BEC）超导性的急剧转变一致，且序参量具有节点。
• 峰-拖拽-驼峰结构表明与玻色子模式存在强耦合，提取的玻色子模式能量范围为1.26至1.44 meV。
• 朗道扇形图显示，从电荷中性点及平坦能带的整数填充处均出现朗道能级量子化，支持关联态的存在。