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近来学术界在很多多层石墨烯衍生材料中发现了超导性。作为一种与零电阻和完美抗磁响应相关的量子现象，超导性一直是物理学中最令人困惑的现象之一。因此，在这些石墨烯衍生材料中的超导性，非常自然地引起了物理学界的广泛关注。奇怪的是，尽管实验上进行了大量的努力，但迄今为止，单层石墨烯中仍未发现超导的迹象。
在最近的一项研究中【Nano Letter 24,10451 (2024)】，上海交通大学的研究人员发现了一种基于几何的简单解释。通过真实的材料计算，他们首先识别出一个系统性的趋势：如果低能电子晶格形成蜂窝状晶格，则此材料便无法产生超导性。另一方面，通过改变电子所处环境，比如扭曲堆叠或施加外部电场，将低能电子晶格变为三角形或其他形状，则在适当的条件下通常能够实现超导性。这种超导对低能电子晶格的敏感性在现有的文献中从未被报道过，同时根据标准的超导理论，这也是非常不寻常的。
为了解释这种新奇的行为，这些研究人员提出了一个简单但普适的模型。他们假设序参量存在于电子的晶格键上，利用演生玻色液体来模拟该系统，并证明在蜂窝状的电子晶格下存在一种普适的几何阻挫，从而阻碍了超导性所必需的相干相位。与此形成鲜明对比的是，对于三角形的电子晶格，同样的演生玻色液体模型则很容易产生相干的相位结构，从而支持超导性的产生。
非常有趣的是，在加外电场下的双层和三层石墨烯中，这个模型预言了一种非常规的超导态，含有交错环绕的电荷流动，从而自发地破坏了时间反演对称性。不仅如此，即使在无超导性的单层石墨烯中，上述几何阻挫对超导的抑制机理指出了在这类系统中一种罕见的诱发超导的路径。经由破坏局域对称性的方式来缓解几何阻挫（例如对单层石墨烯施加拉力），此模型预言整个系统的超导相位相干性可以在低温恢复。这些新奇的预言将可以通过许多现有的实验来进行验证。
“通过几何阻挫抑制超导相位的相干性来抑制超导性是一个有趣的提议。这一想法受到了我们之前首次实现‘均匀量子玻色金属态’的理论工作的启发【PNAS 118, e2100545118 (2021)】。我们预计这一全新视角将在许多现代功能材料和人工系统中的超导及超流性的研究中发挥重要作用。”该项研究的项目负责人顾威教授说道。
该成果执行于上海交通大学李政道研究所并发表于《Nano Letter》，上海交通大学博士生张鑫垚为第一作者，顾威教授为通讯作者，蒋庆东副教授为共同通讯作者。
Xingyao Zhang et al, Nano Letter 24, 10451 (2024)
A. Hegg, J. Hou, and W. Ku, Proc. Natl. Acad. Sci. 118, e2100545118 (2021)
https://doi.org/10.1021/acs.nanolett.4c01390
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Geometric explanation for the lack of superconductivity in single-layer graphene-derived systems suggests time-reversal symmetry broken superconductivity in bi-layer and tri-layer graphenes
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Discoveries of superconductivity have recently been reported in many multi-layer graphene-derived materials. As a quantum phenomenon associated with zero resistivity and perfect diamagnetic response, superconductivity has remained one of the most puzzling in physics. Naturally, its presence in these graphene-derived materials attracts intensive attention in scientific community. Strangely, despite enormous amount of experimental efforts, to-date superconductivity has not been found in single-layer graphene-derived materials.
In a recent study [Nano Letter 24, 10451 (2024)], researchers of Shanghai Jiao Tong University discover a simple geometric explanation. From realistic material computation, they first identify a systematic trend that superconductivity does not emerge if the low-energy electronic lattice forms a honeycomb lattice. On the other hand, changing the lattice to triangular or other lattice, for example through twisting the layer stacking or applying external electric field, would very often enable superconductivity under the right condition. Such sensitivity to the low-energy electronic lattice has never been reported in the literature, and is highly unusual according to standard lore of superconductivity.
To explain this novel behavior, these researchers then proposed a simple yet generic scenario. By assuming that the order parameter resides on the lattice bonds, they model the system with an emergent Bose liquid and demonstrate under honeycomb electronic lattice a generic geometric frustration that inhibits the necessary phase coherence of superconductivity. In contrast, for triangular electronic lattices, the same construction of emergent Bose liquid would produce coherence phase structure and support the emergence of superconductivity.
Interestingly, for the specific cases of bi-layer and tri-layer graphenes under external electric field, this model predicts an unconventional superconductivity that break time-reversal symmetry, in association with spontaneous formation of staggered circulation of superflow. Furthermore, even for non-superconducting single-layer graphene, the geometric inhibition of superconductivity suggests an unusual route to induce superconductivity. By lifting the geometric frustration through breaking local symmetry (such as stretching the graphene sheet), one expects recovery of global phase coherence of superconductivity at low temperature. These predictions can be experimentally tested by many existing experimental techniques.
“Inhibition of superconductivity due to geometric frustration of superconducting phase coherence is an interesting proposal. It was inspired by our previous discovery of ‘homogeneous quantum Bose metal’. [PNAS 118, e2100545118 (2021)] We expect this new angle to paly important roles when investigating superconductivity and superfluidity in many modern functional materials and artificial systems.” said professor Wei Ku, the project leader of this study.
This study was performed in Tsung-Dao Lee Institute of Shanghai Jiao Tong University and published in Nano Letter by the first author Ph.D. student Xinyao Zhang, under the supervision of the corresponding author Professor Wei Ku, and the co-corresponding author Associate Professor Qingdong Jiang.
Xingyao Zhang et al, Nano Letter 24, 10451 (2024)
A. Hegg, J. Hou, and W. Ku, Proc. Natl. Acad. Sci. 118, e2100545118 (2021)
https://doi.org/10.1021/acs.nanolett.4c01390
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文稿 | 顾 威 张鑫垚
编辑 | 孟闻卓
