Moire superconductivity

TL;DR

This paper explains how moiré superlattices in graphene can exhibit strong-coupling superconductivity at very low densities due to enhanced local interactions and density of states, suggesting potential for high transition temperatures.

Contribution

It provides a qualitative explanation for low-density superconductivity in moiré graphene systems, emphasizing the role of periodic potentials in enhancing interactions and density of states.

Findings

Periodic potentials enhance local interactions in moiré systems.

Density of states is exponentially increased by modulation strength.

Moiré systems are natural candidates for strong-coupling superconductivity.

Abstract

Recently, superconductivity was discovered at very low densities in slightly misaligned graphene multilayers. Surprisingly, despite extremely low electronic density (about $10^{-4}$ electrons per unit cell), these systems realize strong-coupling superconductivity, with the transition temperature being a large fraction of the Fermi energy ($T_c\sim 0.1 \epsilon_F$). Here we propose a qualitative explanation for this remarkable phenomenon, highlighting similarities and qualitative differences with the conventional uniform high-density superconductivity. Most importantly, we find that periodic superimposed potential generically enhances local interactions relative to nonlocal (for instance, Coulomb) interactions. In addition, the density of states is enhanced as well, exponentially in modulation strength for low lying bands in some cases. Combination of these two effects makes moir\'e systems natural intermediate or strong-coupled superconductors, with potential for very high transition temperatures.