美国加州大学Young Andrea F.研究小组在最新研究中，报道了超导Bernal双层石墨烯的向列性和轨道依赖。2025年2月10日出版的《自然—物理学》杂志发表了这项成果。

据介绍，超导性是石墨同素异形体的共同特征，在伯纳尔双层、菱形三层和各种角度失调的多层中都有观察到。尽管这些系统的电子结构存在显著差异，但在WSe2衬底上支持石墨一直被观察到在载流子密度和温度方面扩大了超导性的范围。

本文报道了在具有强邻近诱导的Ising自旋轨道耦合的Bernal双层石墨烯中，观察到的两种不同的超导态。他们的量子振荡测量表明，尽管第一超导相的正常状态与单粒子带结构一致，但第二超导相是从一个旋转对称性被破坏的向列态正常状态出现的。这两种超导体都受到面内磁场的影响，但它们都没有达到自旋锁定的Ising超导体所期望的磁场。

第一超导相的费米表面几何形状表明，由于电子波函数的层极化不完美而引起的轨道依赖限制了超导性。最后，对第二超导相的输运和热力学可压缩性测量的分析表明，在其他菱面体石墨烯同素异形体中，观察到的同位旋相边界的接近可能是巧合，它们限制了这些系统中配对机制的理论。

附：英文原文

Title: Nematicity and orbital depairing in superconducting Bernal bilayer graphene

Author: Holleis, Ludwig, Patterson, Caitlin L., Zhang, Yiran, Vituri, Yaar, Yoo, Heun Mo, Zhou, Haoxin, Taniguchi, Takashi, Watanabe, Kenji, Berg, Erez, Nadj-Perge, Stevan, Young, Andrea F.

Issue&Volume: 2025-02-10

Abstract: Superconductivity is a common feature of graphite allotropes, having been observed in Bernal bilayers, rhombohedral trilayers and a wide variety of angle-misaligned multilayers. Despite notable differences in the electronic structure of these systems, supporting the graphite on a WSe2 substrate has been consistently observed to expand the range of the superconductivity in terms of carrier density and temperature. Here we report the observation of two distinct superconducting states in Bernal bilayer graphene with strong proximity-induced Ising spin–orbit coupling. Our quantum oscillation measurements show that, although the normal state of the first superconducting phase is consistent with the single-particle band structure, the second emerges from a nematic normal state with broken rotational symmetry. Both superconductors are robust to in-plane magnetic fields, but neither reach fields expected for spin–valley-locked Ising superconductors. The Fermi surface geometry of the first superconducting phase suggests that the superconductivity is limited by orbital depairing arising from the imperfect layer polarization of the electron wavefunctions. Finally, an analysis of transport and thermodynamic compressibility measurements in the second superconducting phase shows that the proximity to isospin phase boundaries, observed in other rhombohedral graphene allotropes, is probably coincidental, thus constraining theories of the pairing mechanisms in these systems.

DOI: 10.1038/s41567-024-02776-7

Source: https://www.nature.com/articles/s41567-024-02776-7