[论文解读] Spin-polarized Correlated Insulator and Superconductor in Twisted Double Bilayer Graphene

Xiaomeng Liu, Zeyu Hao+6 more|arXiv (Cornell University)|Mar 19, 2019

Graphene research and applications被引用 39

一句话总结

该论文展示了自旋极化相关绝缘态和自旋极化超导性在 twisted double bilayer graphene (TDBG) 中，可通过位移场和密度调控，包括在平面内磁场下对超导性的增强。结果表明在莫尔花纹平带系统中存在自旋极化配对和铁磁相关性。

ABSTRACT

Ferromagnetism and superconductivity typically compete with each other since the internal magnetic field generated in a magnet suppresses the formation of spin-singlet Cooper pairs in conventional superconductors. Only a handful of ferromagnetic superconductors are known in heavy fermion systems, where many-body electron interactions promoted by the narrow energy bands play a key role in stabilizing these emergent states. Recently, interaction-driven superconductivity and ferromagnetism have been demonstrated as separate phenomena in different density regimes of flat bands enabled by graphene moire superlattices. Combining superconductivity and magnetism in a single ground state may lead to more exotic quantum phases. Here, employing van der Waals heterostructures of twisted double bilayer graphene (TDBG), we realize a flat electron band that is tunable by perpendicular electric fields. Similar to the magic angle twisted bilayer graphene, TDBG exhibits energy gaps at the half and quarter filled flat bands, indicating the emergence of correlated insulating states. We find that the gaps of these insulating states increase with in-plane magnetic field, suggesting a ferromagnetic order. Upon doping the ferromagnetic half-filled insulator, superconductivity emerges with a critical temperature controlled by both density and electric fields. We observe that the in-plane magnetic field enhances the superconductivity in the low field regime, suggesting spin-polarized electron pairing. Spin-polarized superconducting states discovered in TDBG provide a new route to engineering interaction-driven topological superconductivity.

研究动机与目标

• 证明在 TDBG 中电场可调的平带，并在半填充和四分填充时识别到相关的绝缘态。
• 显示半填充绝缘隙随平行磁场增大而增大，表明铁磁序。
• 通过掺杂铁磁绝缘体发现超导性，并绘出其对密度和位移场的依赖关系。
• 表征超导态的自旋极化性质及其对平行磁场的响应。
• 讨论相互作用驱动的拓扑超导性在莫尔系统中的含义。

提出的方法

• 使用干转移法制备 twist 角约为 1.24°–1.33° 的 TDBG 设备。
• 用顶部和底部石墨栅控制电子密度 n 和位移场 D。
• 测量四端电阻率 ρ 随 V_TG、V_BG、T 及磁场 (B_⊥, B_∥) 的变化。
• 通过 ρ(T) 的阿伦尼乌斯激活行为识别绝缘隙 Δ_n_s/2 和 Δ_n_s/(4)。
• 通过观察半填充绝缘体周围的低ρ穹顶来确定超导区域，并从 I–V 曲线中提取 BKT 特征。
• 分析 Zeeman-field 依赖（g ≈ 2）对能隙大小的影响并推断带的自旋极化。

实验结果

研究问题

• RQ1 是否可以通过电场调谐 TDBG 来在特定填充下诱导带有相关绝缘态的孤立平带？
• RQ2 TDBG 的半填充绝缘态是否表现出自发的铁磁自旋极化，并通过对平行磁场的响应来体现？
• RQ3 通过对 TDBG 中的铁磁绝缘体掺杂是否可实现超导性，且是否自旋极化？
• RQ4 平行磁场如何影响 TDBG 的超导转变温度和临界行为（如 BKT）？

主要发现

• 在位移场作用下，TDBG 产生一个平坦、可调的导带，在半填充和四分填充时产生相关的绝缘态。
• 半填充绝缘隙 Δ_n_s/2 随平面内磁场增大而增大，与自旋极化（铁磁）序一致。
• 对半填充铁磁绝缘体的掺杂会产生超导性，且对密度和位移场呈穹顶形依赖。
• 超导性在低平面内磁场下增强，并显示 BKT 转变，表明自旋极化（可能是三重态）配对。
• 超导区域在密度 n_m 和位移场 D_m 附近最强，存在 T_BKT 和临界场 B_∥^c，提示非常规自旋极化配对行为。