近日，美国麻省理工学院的William D. Oliver&Pablo Jarillo-Herrero及其研究团队取得一项新进展。经过不懈努力，他们实现魔角扭曲双层石墨烯超流刚度的测定。相关研究成果已于2025年2月5日在国际权威学术期刊《自然》上发表。

该研究团队利用直流传输和微波电路量子电动力学技术，通过测量动力学电感，直接测定了超导魔角扭曲双层石墨烯（MATBG）的超流刚度。研究人员发现，其超流刚度远大于传统费米液体理论的预期值，而是与理论预测以及近期实验中关于魔角下占主导地位的量子几何效应的指示相当。超流刚度随温度的变化遵循幂律，这与各向同性的巴丁-库珀-施里弗（BCS）模型相悖。相反，无论是在费米液体框架下解释，还是考虑平带超导的量子几何特性，提取出的幂律指数均表明存在各向异性的超导能隙。

此外，研究人员还观察到超流刚度对直流和微波电流均呈现出二次依赖关系，这与金兹堡-朗道理论相一致。综上所述，这项研究结果表明MATBG是一种具有各向异性能隙的非传统超导体，并强烈暗示了MATBG中量子几何、超流刚度和非传统超导性之间存在联系。此处使用的直流-微波联合测量平台适用于其他原子级薄超导体的研究。

据悉，魔角扭曲双层石墨烯（MATBG）中的超导物理是莫尔纹系统研究中的一个热点话题，它可能为人们揭示其他强关联材料（如高临界温度超导体）的配对机制提供洞见。

附：英文原文

Title: Superfluid stiffness of magic-angle twisted bilayer graphene

Author: Tanaka, Miuko, Wang, Joel -j., Dinh, Thao H., Rodan-Legrain, Daniel, Zaman, Sameia, Hays, Max, Almanakly, Aziza, Kannan, Bharath, Kim, David K., Niedzielski, Bethany M., Serniak, Kyle, Schwartz, Mollie E., Watanabe, Kenji, Taniguchi, Takashi, Orlando, Terry P., Gustavsson, Simon, Grover, Jeffrey A., Jarillo-Herrero, Pablo, Oliver, William D.

Issue&Volume: 2025-02-05

Abstract: The physics of superconductivity in magic-angle twisted bilayer graphene (MATBG) is a topic of keen interest in moiré systems research, and it may provide an insight into the pairing mechanism of other strongly correlated materials such as high-critical-temperature superconductors. Here we use d.c. transport and microwave circuit quantum electrodynamics to directly measure the superfluid stiffness of superconducting MATBG through its kinetic inductance. We find the superfluid stiffness to be much larger than expected from conventional Fermi liquid theory. Rather, it is comparable to theoretical predictions and recent experimental indications of quantum geometric effects that are dominant at the magic angle. The temperature dependence of the superfluid stiffness follows a power law, which contraindicates an isotropic Bardeen–Cooper–Schrieffer (BCS) model. Instead, the extracted power-law exponents indicate an anisotropic superconducting gap, whether interpreted in the Fermi liquid framework or by considering the quantum geometry of flat-band superconductivity. Moreover, a quadratic dependence of the superfluid stiffness on both d.c. and microwave current is observed, which is consistent with the Ginzburg–Landau theory. Taken together, our findings show that MATBG is an unconventional superconductor with an anisotropic gap and strongly suggest a connection between quantum geometry, superfluid stiffness and unconventional superconductivity in MATBG. The combined d.c.–microwave measurement platform used here is applicable to the investigation of other atomically thin superconductors.

DOI: 10.1038/s41586-024-08494-7

Source: https://www.nature.com/articles/s41586-024-08494-7