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arXiv 2103.13459cond-mat.mes-hall
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波纹晶格对扭曲双层石墨烯轨道磁响应和Condon不稳定性的影响

Moiré lattice effects on the orbital magnetic response of twisted bilayer graphene and Condon instability

Daniele Guerci, Pascal Simon, Christophe Mora

摘要:

我们从扭曲双层石墨烯的能带结构分析了其轨道磁化率。接近电荷中性,面外磁化率继承了石墨烯的强抗磁性响应。随着掺杂量的增加,样品的抗磁性向顺磁性转变,并且在第一能带中莫尔晶格的van Hove奇点处出现对数发散。范霍夫奇点处的增强顺磁性在相对大的角度下更强,但在第一魔角附近受到平坦谱的抑制。发散的顺磁磁化率表示轨道铁磁性的不稳定性,具有轨道面外磁化和朗道能级结构。然而,不稳定的区域被发现实际上是非常小的,由电子速度与光速的比率参数抑制。我们还讨论了面内轨道磁化率在电荷中性,我们发现一个顺磁响应和对数发散的魔角。顺磁响应与负逆流电流在两个层,不承认一个半经典的描述。

英文摘要:

We analyze the orbital magnetic susceptibility from the band structure of twisted bilayer graphene. Close to charge neutrality, the out-of-plane susceptibility inherits the strong diamagnetic response from graphene. Increasing the doping, a crossover from diamagnetism to paramagnetism is obtained and a logarithmic divergence develops at the van Hove singularity of the Moiré lattice in the first band. The enhanced paramagnetism at the van Hove singularity is stronger for relatively large angle but gets suppressed by the flat spectrum towards the vicinity of the first magic angle. A diverging paramagnetic susceptibility indicates an instability towards orbital ferromagnetism with an orbital out-of-plane magnetization and a Landau level structure. The region of instability is however found to be practically very small, parametrically suppressed by the ratio of the electron velocity to the speed of light. We also discuss the in-plane orbital susceptibility at charge neutrality where we find a paramagnetic response and a logarithmic divergence at the magic angle. The paramagnetic response is associated with negative counterflow current in the two layers and does not admit a semiclassical description.

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