Department of Physics, University of California, Santa Barbara, CA, USA.
International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan.
Nature. 2021 Oct;598(7881):434-438. doi: 10.1038/s41586-021-03926-0. Epub 2021 Sep 1.
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; 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, 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 changes. SC1 emerges from a paramagnetic normal state, whereas SC2 emerges from a spin-polarized, valley-unpolarized half-metal and violates the Pauli limit for in-plane magnetic fields by at least one order of magnitude. We discuss our results in view of several mechanisms, including conventional phonon-mediated pairing, pairing due to fluctuations of the proximal isospin order, and intrinsic instabilities of the annular Fermi liquid. 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.
通过在清洁的二维器件中施加电场来实现超导是纳米电子学的一个核心目标。最近，在石墨烯莫尔超晶格中已经实现了超导；然而，许多这些结构在机械上不稳定，实验显示出强烈无序的特征。在这里，我们报告了在结晶三方层状石墨烯中观察到的超导现象——在亚开尔文温度下表现为低电阻或零电阻——这是一种结构亚稳的碳同素异形体。超导发生在两个不同的栅极调谐区域（SC1 和 SC2）中，并且在由平均自由程和超导相干长度之比定义的清洁极限中很深。通过量子振荡对正常态费米面的映射表明，两个超导相都来自于环形费米面，并且接近等自旋对称性破缺转变，在此转变中费米面简并度发生变化。SC1 来自顺磁正常态，而 SC2 来自自旋极化、谷非极化的半金属，并且对于面内磁场违反了泡利极限，至少超过一个数量级。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配对、由于近邻等自旋序的涨落引起的配对，以及环形费米液体的固有不稳定性。我们在清洁和结构简单的二维金属中观察到超导现象，为测试超导的竞争理论模型提供了一个模型系统，而无需对无序进行建模，同时为基于关联电子现象和弹道电子输运的新型场效应控制电子器件开辟了道路。
