Kohn-Luttinger Superconductivity in Twisted Bilayer Graphene

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• Twisted bilayer graphene (TBG) exhibits superconductivity, a phenomenon not fully explained by conventional theories.
• The electronic structure of TBG near the magic angle features van Hove singularities (vHS) and strong correlations.
• Kohn-Luttinger (KL) instability typically occurs at very low energy scales, involving effective attraction from repulsive interactions.

Purpose of the Study:

• To explain the observed superconductivity in TBG using the Kohn-Luttinger instability mechanism.
• To investigate the role of van Hove singularities and electronic band structure in driving superconductivity.
• To explore associated electronic instabilities, such as spin-density waves, in TBG.

Main Methods:

• Theoretical analysis of the electronic spectrum in TBG, focusing on the highest valence band near the magic angle.
• Investigation of the Kohn-Luttinger instability in the context of strong coupling and nesting of electronic states.
• Modeling of anisotropic screening effects and their influence on electron-electron interactions.

Main Results:

• The Kohn-Luttinger instability provides a viable mechanism for superconductivity in TBG, driven by enhanced electron attraction.
• Doubled and strongly coupled van Hove singularities lead to extended saddle points and near-perfect nesting, crucial for the instability.
• Anisotropic screening induces effective p-wave attraction, and a spin-density wave instability is predicted adjacent to the superconducting phase.

Conclusions:

• Superconductivity in TBG can be understood as a consequence of the Kohn-Luttinger instability, amplified by specific band structure features.
• The study highlights the importance of van Hove singularities and electronic correlations in realizing exotic quantum phenomena in TBG.
• The findings suggest potential for novel electronic phases, including superconductivity and spin-density waves, in twisted van der Waals heterostructures.