Small Angle and Non-Monotonic Behavior of the Thermal Conductivity in Twisted Bilayer Graphene

TL;DR

This study investigates how thermal conductivity in twisted bilayer graphene varies at small twist angles, revealing a non-monotonic trend linked to mechanical and structural changes as the angle approaches zero.

Contribution

It provides the first large-scale molecular dynamics analysis of thermal conductivity in TBG at small angles, uncovering non-monotonic behavior and underlying physical mechanisms.

Findings

Thermal conductivity decreases by 50% with increasing lattice constant.

Conductivity recovers to 90% of aligned value near 1.89°.

Shear elastic constant and wrinkling intensity correlate with conductivity changes.

Abstract

Nothing is known about the thermal conductivity in twisted bilayer graphene (TBG) at small twist angles, and how it approaches its aligned value as the twist angle approaches 0 degrees. To provide insight into these questions, we perform large scale non-equilibrium molecular dynamics calculations on commensurate TBG structures with angles down to 1.87 degrees. The results show a smooth, non-monotonic behavior of the thermal conductivity with respect to the commensurate lattice constant. As the commensurate lattice constant increases, the thermal conductivity initially decreases by 50%, and then it returns to 90% of its aligned value as the angle is reduced to 1.89 degrees. These same qualitative trends are followed by the trends in the shear elastic constant, the wrinkling intensity, and the out-of-plane ZA2 phonon frequency. The picture that emerges of the physical mechanism governing the thermal conductivity is that misorientation reduces the shear elastic constant; the reduced shear elastic constant enables greater wrinkling; and the greater wrinkling reduces the thermal conductivity. The small-angle behavior of the thermal conductivity raises the question of how do response functions approach their aligned values as the twist angle approaches 0 degrees. Is the approach gradual, discontinuous, or a combination of the two?