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为深入理解扭角双层石墨烯系统中关联态之间的关系和超导配对机理以及扭转角度对超导电性的影响，本文采用大尺度无偏差的约束路径量子蒙特卡洛方法在构建的扭角双层石墨烯有效二轨道哈伯德模型中进行了系统的数值模拟。首先，从电声子耦合的层面，数值模拟结果显示晶格间近邻吸引库仑相互作用强烈地增强系统主导的手性d+id超导电子配对对称性，并且发现布里渊区Γ点附近的反铁磁序也存在同样的增强效应，这说明反铁磁序是形成手性d+id超导态的先决条件。更为重要的是此理论结果表明电声子耦合对调控超导电性具有重要的作用。其次，从双层石墨烯扭转角度的层面，我们讨论了扭转角度对超导电性的影响，数值模拟结果表明扭转角度在1.08°附近，随着扭转角度的减小系统主导的手性d+id超导电子配对对称性以及反铁磁序同样表现出增强的协同效应。最后，本文的研究结果为进一步揭示扭角双层石墨烯中的超导机理和提高系统的超导临界转变温度提供了重要的研究方向。
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关键词:
- 手性d+id超导态 /
- 约束路径量子蒙特卡洛方法 /
- 扭角双层石墨烯超导体 /
- 磁性
To gain deeper insight into the relationship between correlated states and the superconducting pairing mechanism in twisted bilayer graphene, as well as the influence of the twist angle on superconductivity, we employ a large-scale, unbiased constrained-path quantum Monte Carlo method to systematically simulate the effective two-orbital Hubbard model for twisted bilayer graphene. Initially, we investigate the modulation of superconductivity by nearest-neighbor attractive Coulomb interactions, demonstrating that electron-phonon coupling plays a significant role in the system. Our numerical results reveal that the superconducting state is dominated by chiral NN–d + id superconducting electron pairing symmetry, and that such nearest-neighbor attractive Coulomb interactions significantly enhance the effective long-range pairing correlation function of chiral NN–d + id wave. From this perspective, it is evident that electronphonon coupling positively contributes to the superconductivity of the system.
Subsequently, we explore how the twist angle affects the superconducting state. The flat-band structure conducted by hopping anisotropy reflects the different twist angles of the system. Our results show that as the twist angle deviates downward from 1.08°, the effective pairing correlation function of the chiral NN–d + id wave increases substantially. Conversely, as the twist angle exceeds 1.08°, the effective correlation function of the chiral NN–d + id wave exhibits a tendency of decline. These results suggest that further reduction in the twist angle may lead to higher superconducting transition temperatures in twisted bilayer graphene system.
Ultimately, we analyze how nearest-neighbor attractive Coulomb interactions and flat-band structures influence superconductivity from the standpoint of magnetic properties. The observed enhancement of the spin structure factor near the Γ point in the Brillouin zone indicates that enhanced antiferromagnetic correlations are essential for enhancing the superconducting transition temperature and for stabilizing chiral NN–d + id wave. Through this series of investigations, our numerical findings not only contribute to a more comprehensive understanding of strongly correlated systems such as twisted bilayer graphene, but also offer guidance for identifying twist-angle systems with potentially higher superconducting transition temperatures.-
Keywords:
- Chiral d + id superconducting state /
- Constrained Path Quantum Monte Carlo (CPQMC) method /
- Twisted Bilayer Graphene /
- Magnetism
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