近日，美国哈佛大学的Philip Kim等人合作并取得一项新进展。他们对扭曲三层石墨烯超导体的超流刚度进行了研究。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。

该研究团队报道了对魔角扭曲三层石墨烯（TTG）中超流刚度ρs的测量结果，揭示了其非传统的节点-能隙超导性。研究人员利用射频反射测量技术，测量了与微波谐振器耦合的超导TTG的动感应响应，发现ρs在低温下呈现线性温度依赖性，并且在电流偏置依赖性中表现出非线性迈斯纳效应，这两者均表明超导序参量中存在节点结构。

此外，掺杂依赖性显示出零温ρs与超导转变温度Tc之间存在线性相关性，这与铜氧化物超导体中的植村关系相似，暗示了相干性受限的超导性。这项研究结果为TTG中的节点超导性提供了有力证据，并对这类基于石墨烯的超导体的机制提出了严格限制。

据悉，超导体的宏观量子性质的稳健性可以通过超流刚度ρs来表征，该参量描述了改变宏观量子波函数相位所需的能量。在非传统超导体（如铜氧化物超导体）中，由于动量空间中的无间隙点（节点）处的准粒子激发，ρs的低温行为与传统超导体显著不同。近期发现的魔角扭曲石墨烯家族，除了展现出超导态外，还揭示了与自发对称性破缺相关的强关联电子态，这引发了人们对ρs的深入研究，以期揭示其超导性质可能具有的非传统特性。

附：英文原文

Title: Superfluid stiffness of twisted trilayer graphene superconductors

Author: Banerjee, Abhishek, Hao, Zeyu, Kreidel, Mary, Ledwith, Patrick, Phinney, Isabelle, Park, Jeong Min, Zimmerman, Andrew, Wesson, Marie E., Watanabe, Kenji, Taniguchi, Takashi, Westervelt, Robert M., Yacoby, Amir, Jarillo-Herrero, Pablo, Volkov, Pavel A., Vishwanath, Ashvin, Fong, Kin Chung, Kim, Philip

Issue&Volume: 2025-02-05

Abstract: The robustness of the macroscopic quantum nature of a superconductor can be characterized by the superfluid stiffness, ρs, a quantity that describes the energy required to vary the phase of the macroscopic quantum wavefunction. In unconventional superconductors, such as cuprates, the low-temperature behaviour of ρs markedly differs from that of conventional superconductors owing to quasiparticle excitations from gapless points (nodes) in momentum space. Intensive research on the recently discovered magic-angle twisted graphene family has revealed, in addition to superconducting states, strongly correlated electronic states associated with spontaneously broken symmetries, inviting the study of ρs to uncover the potentially unconventional nature of its superconductivity. Here we report the measurement of ρs in magic-angle twisted trilayer graphene (TTG), revealing unconventional nodal-gap superconductivity. Utilizing radio-frequency reflectometry techniques to measure the kinetic inductive response of superconducting TTG coupled to a microwave resonator, we find a linear temperature dependence of ρs at low temperatures and nonlinear Meissner effects in the current-bias dependence, both indicating nodal structures in the superconducting order parameter. Furthermore, the doping dependence shows a linear correlation between the zero-temperature ρs and the superconducting transition temperature Tc, reminiscent of Uemura’s relation in cuprates, suggesting phase-coherence-limited superconductivity. Our results provide strong evidence for nodal superconductivity in TTG and put strong constraints on the mechanisms of these graphene-based superconductors.

DOI: 10.1038/s41586-024-08444-3

Source: https://www.nature.com/articles/s41586-024-08444-3

来源：科学网  小柯机器人