Instituto de Estructura de la Materia, CSIC, E-28006 Madrid, Spain.
Materials Science Factory, Instituto de Ciencia de Materiales de Madrid, CSIC, E-28049 Madrid, Spain.
Phys Rev Lett. 2019 Jan 18;122(2):026801. doi: 10.1103/PhysRevLett.122.026801.
We show that the recently observed superconductivity in twisted bilayer graphene (TBG) can be explained as a consequence of the Kohn-Luttinger (KL) instability which leads to an effective attraction between electrons with originally repulsive interaction. Usually, the KL instability takes place at extremely low energy scales, but in TBG, a doubling and subsequent strong coupling of the van Hove singularities (vHS) in the electronic spectrum occurs as the magic angle is approached, leading to extended saddle points in the highest valence band with almost perfect nesting between states belonging to different valleys. The highly anisotropic screening induces an effective attraction in a p-wave channel with odd parity under the exchange of the two disjoined patches of the Fermi line. We also predict the appearance of a spin-density wave instability, adjacent to the superconducting phase, and the opening of a gap in the electronic spectrum from the condensation of spins with wave vector corresponding to the nesting vector close to the vHS.
我们表明，最近在扭曲双层石墨烯（TBG）中观察到的超导性可以解释为科恩-洛特林格（KL）不稳定性的结果，这种不稳定性导致电子之间原本排斥的相互作用产生有效吸引力。通常，KL 不稳定性发生在极低的能量尺度上，但在 TBG 中，当接近魔术角时，电子能谱中的范霍夫奇点（vHS）加倍并随后强烈耦合，导致最高价带中的扩展鞍点，不同谷之间的状态之间几乎完美嵌套。高度各向异性的屏蔽在交换费米线的两个不相连的部分时，在 p 波通道中诱导出具有奇宇称的有效吸引力。我们还预测了自旋密度波不稳定性的出现，该不稳定性与超导相相邻，并且电子谱从与 vHS 附近的嵌套向量对应的波矢的自旋凝聚中打开一个间隙。
