To access superconductivity via the electric field effect in a clean, two-dimensional device is a central goal of nanoelectronics. Recently, superconductivity has been realized in graphene moiré heterostructures^1,2,3,4; however, many of these structures are not mechanically stable, and experiments show signatures of strong disorder. Here we report the observation of superconductivity—manifesting as low or vanishing resistivity at sub-kelvin temperatures—in crystalline rhombohedral trilayer graphene^5,6, a structurally metastable carbon allotrope. Superconductivity occurs in two distinct gate-tuned regions (SC1 and SC2), and is deep in the clean limit defined by the ratio of mean free path and superconducting coherence length. Mapping of the normal state Fermi surfaces by quantum oscillations reveals that both superconductors emerge from an annular Fermi sea, and are proximal to an isospin-symmetry-breaking transition where the Fermi surface degeneracy changes⁷. SC1 emerges from a paramagnetic normal state, whereas SC2 emerges from a spin-polarized, valley-unpolarized half-metal¹⁷ and violates the Pauli limit for in-plane magnetic fields by at least one order of magnitude^8,9. We discuss our results in view of several mechanisms, including conventional phonon-mediated pairing^10,11, pairing due to fluctuations of the proximal isospin order¹², and intrinsic instabilities of the annular Fermi liquid^13,14. Our observation of superconductivity in a clean and structurally simple two-dimensional metal provides a model system to test competing theoretical models of superconductivity without the complication of modelling disorder, while enabling new classes of field-effect controlled electronic devices based on correlated electron phenomena and ballistic electron transport.

在干净的二维设备中通过电场效应获得超导性是纳米电子学的核心目标。^{最近，石墨烯云纹异质结构1,2,3,4}实现了超导性；然而，许多这些结构在机械上不稳定，并且实验显示出强烈无序的特征。在这里，我们报告了在晶体菱面体三层石墨烯中观察到的超导性——在亚开尔文温度下表现为低电阻率或电阻率消失^5,6，一种结构亚稳态的碳同素异形体。超导性发生在两个不同的栅极调谐区域（SC1 和 SC2），并且处于由平均自由程与超导相干长度之比定义的清洁极限内。通过量子振荡对正常状态费米面的映射表明，这两种超导体都从环形费米海中出现，并且接近费米面简并发生变化的同位旋对称性破缺转变⁷。SC1 从顺磁常态出现，而 SC2 从自旋极化、谷非极化半金属¹⁷出现，并且违反面内磁场的泡利极限至少一个数量级^8,9. 我们根据几种机制讨论了我们的结果，包括传统的声子介导的配对^10,11、由于近端同位旋阶数波动引起的配对¹²以及环形费米液体的固有不稳定性^13,14。我们对清洁且结构简单的二维金属中的超导性的观察提供了一个模型系统来测试超导性的竞争理论模型，而不会出现建模障碍的复杂性，同时使基于相关电子现象和弹道的新型场效应控制电子设备成为可能电子传输。