近日，美国斯坦福大学的David Goldhaber-Gordon及其研究小组与美国国家强磁场实验室的Oskar Vafek等人合作，并取得一项新进展。经过不懈努力，他们揭示了应变诱导开放费米表面扭曲双层石墨烯中的异常磁输运现象。相关研究成果已于2023年8月14日在国际知名学术期刊《美国科学院院刊》上发表。

　　据悉，最近，玩具型Hofstadter模型中的各向异性跳跃被用来解释，在远离魔角的扭曲双层石墨烯中测量到的丰富而令人惊讶的朗道光谱。

　　该研究团队怀疑玩具型Hofstadter模型中的各向异性可能是由单轴应变引起的。为了验证这一想法，研究人员将Bistritzer-MacDonald模型扩展到包括单轴异质应变，并详细分析了其对能带结构和磁输运的影响。研究人员发现，这种应变强烈地影响了能带结构，将三个本应简并的van Hove点转移到不同的能量上。结合玻尔兹曼磁输运计算，这再现了以前无法解释的在填充ν=±2附近的密度范围内的非饱和B2磁电阻，并预测了实验数据中未注意到的更微妙的特征。与纵向电阻率的显著特征相反，霍尔系数几乎不受单轴应变的影响。

　　在某种程度上，霍尔系数在电荷中性点的每一侧仍然显示出单符号变化，但这种符号变化不再发生在van Hove点。此外，该理论还预测，即使对于固定应变和刚性带，电输运主轴作为填充的函数也会有明显的旋转。因此，更仔细地研究相互作用诱导的向列序与扭曲双层石墨烯中的应变效应可能是有必要的。

附：英文原文

Title: Unusual magnetotransport in twisted bilayer graphene from strain-induced open Fermi surfaces

Author: Wang, Xiaoyu, Finney, Joe, Sharpe, Aaron L., Rodenbach, Linsey K., Hsueh, Connie L., Watanabe, Kenji, Taniguchi, Takashi, Kastner, M. A., Vafek, Oskar, Goldhaber-Gordon, David

Issue&Volume: 2023-8-14

Abstract: Anisotropic hopping in a toy Hofstadter model was recently invoked to explain a rich and surprising Landau spectrum measured in twisted bilayer graphene away from the magic angle. Suspecting that such anisotropy could arise from unintended uniaxial strain, we extend the Bistritzer–MacDonald model to include uniaxial heterostrain and present a detailed analysis of its impact on band structure and magnetotransport. We find that such strain strongly influences band structure, shifting the three otherwise-degenerate van Hove points to different energies. Coupled to a Boltzmann magnetotransport calculation, this reproduces previously unexplained nonsaturating B2 magnetoresistance over broad ranges of density near filling ν=±2 and predicts subtler features that had not been noticed in the experimental data. In contrast to these distinctive signatures in longitudinal resistivity, the Hall coefficient is barely influenced by strain, to the extent that it still shows a single sign change on each side of the charge neutrality point—surprisingly, this sign change no longer occurs at a van Hove point. The theory also predicts a marked rotation of the electrical transport principal axes as a function of filling even for fixed strain and for rigid bands. More careful examination of interaction-induced nematic order versus strain effects in twisted bilayer graphene could thus be in order.

DOI: 10.1073/pnas.2307151120

Source: https://www.pnas.org/doi/10.1073/pnas.2307151120