Ferromagnetism and superconductivity typically compete with each other 1 since the internal magnetic field generated in a magnet suppresses the formation of spin-singlet Cooper pairs in conventional superconductors 2. Only a handful of ferromagnetic superconductors are known in heavy fermion systems 3–5, 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 moiré superlattices 6–9. 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 10.

中文翻译：

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扭曲双层石墨烯中的可调自旋极化相关态

铁磁性和超导性通常相互竞争 1，因为磁体中产生的内部磁场抑制了传统超导体中自旋单重态库珀对的形成 2。在重费米子系统 3-5 中，只有少数铁磁超导体是已知的，其中许多由窄能带促进的-体电子相互作用在稳定这些涌现状态中起着关键作用。最近，相互作用驱动的超导性和铁磁性已被证明是由石墨烯莫尔超晶格 6-9 实现的不同密度的平带密度范围内的独立现象。在单一基态中结合超导性和磁性可能会导致更奇特的量子相。在这里，采用扭曲双层石墨烯（TDBG）的范德华异质结构，我们实现了一个可以通过垂直电场调节的平坦电子带。与魔角扭曲双层石墨烯类似，TDBG 在半填充和四分之一填充的平带处表现出能隙，表明相关绝缘态的出现。我们发现这些绝缘状态的间隙随着面内磁场的增加而增加，表明存在铁磁顺序。在掺杂铁磁半填充绝缘体后，超导性会出现一个由密度和电场控制的临界温度。我们观察到面内磁场增强了低场状态下的超导性，表明自旋极化电子配对。在 TDBG 中发现的自旋极化超导状态为工程交互驱动的拓扑超导提供了一条新途径 10。