近日，美国俄亥俄州立大学的Chun Ning Lau&Marc Bockrath及其研究小组与韩国首尔梨花女子大学的Eun-Ah Kim等人合作并取得一项新进展。经过不懈努力，他们揭示扭曲双层石墨烯中的支配1/3填充相关绝缘体态和轨道几何阻挫。相关研究成果已于2024年7月1日在国际知名学术期刊《自然—物理学》上发表。
该研究团队报道了在大角度扭曲双层石墨烯中观察到的支配态、分母态、分数填充态和绝缘态。这些状态在磁场中持续存在，并伴随着磁性有序现象及三倍晶胞重构的特征。这些发现与强耦合理论预测的几何受阻挫分数态对称性破缺现象相吻合。
据悉，在晶格系统中，当不是所有的相互作用都能同时得到满足时，就会发生几何阻挫。最简单的例子是三角形晶格上的反铁磁耦合自旋。受阻挫系统的特点是具有许多几乎简并的基态，从而导致非平凡相，如自旋冰和自旋液体。迄今为止，大多数研究都关注自旋的几何阻挫，而轨道的几何阻挫却很少被探索。对于扭曲双层石墨烯中的电子，在扭曲双层石墨烯体系中，当电子带填充至分母为3的分数态时，预计库仑相互作用和万尼尔轨道形状会强烈约束空间电荷序，导致几何上阻挫的基态，从而产生一类新的相关绝缘态。
附：英文原文
Title: Dominant 1/3-filling correlated insulator states and orbital geometric frustration in twisted bilayer graphene
Author: Tian, Haidong, Codecido, Emilio, Mao, Dan, Zhang, Kevin, Che, Shi, Watanabe, Kenji, Taniguchi, Takashi, Smirnov, Dmitry, Kim, Eun-Ah, Bockrath, Marc, Lau, Chun Ning
Issue&Volume: 2024-07-01
Abstract: Geometric frustration occurs in a lattice system when not all the interactions can be satisfied simultaneously. The simplest example is antiferromagnetically coupled spins on a triangular lattice. Frustrated systems are characterized by having many nearly degenerate ground states, leading to non-trivial phases, such as spin ice and spin liquids. To date, most studies have looked at geometric frustration of spins whereas orbital geometric frustration has been much less explored. For electrons in twisted bilayer graphene, when the electronic bands are filled to a fraction with denominator 3, Coulomb interactions and the Wannier orbital shapes are predicted to strongly constrain spatial charge ordering, leading to geometrically frustrated ground states that produce a new class of correlated insulating states. Here we report the observation of dominant, denominator 3, fractional-filling, insulating states in large-angle twisted bilayer graphene. These states persist in magnetic fields and display magnetic ordering signatures and tripled unit cell reconstruction. These results are in agreement with a strong-coupling theory for symmetry-breaking in geometrically frustrated fractional states.
DOI: 10.1038/s41567-024-02546-5
Source: https://www.nature.com/articles/s41567-024-02546-5