Commensuration torques and lubricity in double moire systems

TL;DR

This paper investigates the energetics of layer rotation and sliding in double moire graphene systems, revealing how lattice relaxations influence commensuration torques and superlubricity at small twist angles.

Contribution

It provides new insights into the energetics and superlubricity phenomena in double moire systems, emphasizing the role of lattice relaxation and twist angles.

Findings

Layer relaxations break mirror symmetry and create local minima in rotation energy.

Sliding energy barriers are comparable to rotation minima, indicating complex energetics.

Superlubricity occurs at very small twist angle deviations, suppressing sliding energies significantly.

Abstract

We study the commensuration torques and layer sliding energetics of alternating twist trilayer graphene (t3G) and twisted bilayer graphene on hexagonal boron nitride (t2G/BN) that have two superposed moire interfaces. Lattice relaxations for typical graphene twist angles of $\sim 1^{\circ}$ in t3G or t2G/BN are found to break the out-of-plane layer mirror symmetry, give rise to layer rotation energy local minima dips of the order of $\sim 10^{-1}$ meV/atom at double moire alignment angles, and have sliding energy landscape minima between top-bottom layers of comparable magnitude. Moire superlubricity is restored for twist angles as small as $\sim 0.03^\circ$ away from alignment resulting in suppression of sliding energies by several orders of magnitude of typically $\sim 10^{-4}$ meV/atom, hence indicating the precedence of rotation over sliding in the double moire commensuration process.