[论文解读] Twist-Programmable Superconductivity in Spin-Orbit Coupled Bilayer Graphene

Yiran Zhang, Gal Shavit+9 more|arXiv (Cornell University)|Aug 19, 2024

Graphene research and applications被引用 4

一句话总结

本研究通过调节石墨烯与二硒化钨（WSe₂）之间的相对扭转角，在自旋-轨道耦合的双层石墨烯中实现了可编程调控的超导性，从而控制诱导的伊辛自旋-轨道耦合（SOC）强度。随着SOC增强，超导性在更高的位移场下出现，并达到高达0.5 K的临界温度，表现出对平面内磁场更强的抗性，并显示出三角形能带扭曲的费米口袋之间电子空穴重分布的证据。

ABSTRACT

The relative twist angle between layers of near-lattice-matched van der Waals materials is critical for the emergent correlated phenomena associated with moire flat bands. However, the concept of angle rotation control is not exclusive to moiré superlattices in which electrons directly experience a twist-angle-dependent periodic potential. Instead, it can also be employed to induce programmable symmetry-breaking perturbations with the goal of stabilizing desired correlated states. Here, we experimentally demonstrate `moireless' twist-tuning of superconductivity together with other correlated orders in Bernal bilayer graphene proximitized by tungsten diselenide. The alignment between the two materials systematically controls the strength of the induced Ising spin-orbit coupling (SOC), profoundly altering the phase diagram. As Ising SOC is increased, superconductivity onsets at a higher displacement field and features a higher critical temperature, reaching up to 0.5K. Within the main superconducting dome and in the strong Ising SOC limit, we find an unusual phase transition characterized by a nematic redistribution of holes among trigonally warped Fermi pockets and enhanced resilience to in-plane magnetic fields. The behavior of the superconducting phase is well captured by our theoretical model, which emphasizes the role of interband interactions between Fermi pockets arising due to interaction-enhanced symmetry breaking. Moreover, we identify two additional superconducting regions, one of which descends from an inter-valley coherent normal state and exhibits a Pauli-limit violation ratio exceeding 40, among the highest for all known superconductors. Our results provide new insights into ultra-clean graphene-based superconductors and underscore the potential of utilizing moireless-twist engineering across a range of van der Waals heterostructures.

研究动机与目标

• 探索自旋-轨道耦合（SOC）在不依赖莫尔超晶格的情况下调控双层石墨烯（BLG）中超导性与关联态的作用。
• 通过实验表明，BLG与WSe₂之间的相对扭转角可作为精确调控伊辛SOC强度的旋钮。
• 研究诱导SOC如何改变超导相图，包括临界温度、磁场抗性以及费米面重构。
• 识别并表征多个超导相，包括一个具有极端泡利极限违反的相。
• 建立一种无莫尔超晶格的扭转工程平台，用于范德华异质结构中实现超净、高度可调的超导体。

提出的方法

• 通过机械剥离并依次将双层石墨烯纳米片相对于固定WSe₂基底以约6°的增量进行扭转，制备BLG-WSe₂异质结。
• 利用高分辨率舒布尼科夫-德哈斯振荡测量费米面各向异性和由伊辛SOC引起的空穴口袋不平衡程度。
• 施加强位移场（D ≈ 0.2 V/nm）以通过极化靠近WSe₂层的空穴波函数，最大化伊辛SOC。
• 采用基于BCS-Bardeen-Cooper-Schrieffer（BCS）框架并包含自旋-轨道和泽曼耦合的理论模型，描述平面内磁场对Tc的抑制。
• 计算Tc(B) = Tc,0 − αB²关系中的系数α，以分析磁场抗性，同时考虑非对称相互作用和对称性破缺效应。
• 使用自洽的Hartree-Fock计算探索磁场下非对称费米面重构，检验“群体运动”模型的有效性。

Figure 1: Programmable Ising SOC by interfacial twisting between BLG and WSe 2 . a , Schematic showing the twisting of the BLG-WSe 2 interface; tuning the interfacial twist angle $\theta$ between the two largely lattice-mismatched materials modifies the Ising SOC strength $|\lambda_{I}|$ and the cor

实验结果

研究问题

• RQ1 在无莫尔势场条件下，BLG与WSe₂之间的相对扭转角如何控制诱导伊辛自旋-轨道耦合的强度？
• RQ2 增强伊辛SOC对BLG中超导性的起始场和临界温度有何影响？
• RQ3 增强的伊辛SOC是否会导致涉及三角形能带扭曲费米口袋间空穴重分布的非对称相变？
• RQ4 超导态能否在强平面内磁场下保持稳定？其增强抗性的机制是什么？
• RQ5 某一超导相中泡利极限违反超过40的起源是什么？其与谷间相干性有何关联？

主要发现

• 随着通过扭转角调控增强伊辛自旋-轨道耦合，超导性的临界温度最高提升至0.5 K。
• 随着伊辛SOC增强，超导性在更高的位移场下出现，表明自旋-轨道耦合具有显著的稳定作用。
• 在主超导穹顶内观察到非对称相变，其特征为三个三角形能带扭曲费米口袋之间空穴的重分布。
• 超导态对平面内磁场表现出极强的抗性，Tc抑制行为符合Tc(B) = Tc,0 − αB²关系，其中α ≈ 0.1 K/T²。
• 识别出一个额外的超导区域，其源自一种谷间相干的正常态，表现出泡利极限违反比超过40——为迄今报道的最高值之一。
• 理论建模证实，由于相互作用增强的对称性破缺所导致的非对称相互作用，对解释观测到的相图和磁场抗性至关重要。

Figure 2: Twist-programmable superconducting phase diagram. a – d , $R_{xx}$ versus doping density $n$ and displacement field $D$ for devices with Ising strength $|\lambda_{I}|\approx 0.4$ meV ( a ), $0.9$ meV ( b ), $1.4$ meV ( c ), and $1.5$ meV ( d ), respectively. e , Optimal superconducting cri