Mis-orientation controlled cross-plane thermoelectricity in twisted bilayer graphene

TL;DR

This study investigates how the twist angle in bilayer graphene influences out-of-plane thermoelectric properties, revealing a transition from phonon drag dominance at large angles to electronic effects at small angles, offering insights into electronic coherence.

Contribution

It provides the first detailed experimental analysis of twist-dependent cross-plane thermoelectricity in twisted bilayer graphene, highlighting the role of phononic and electronic hybridization.

Findings

Thermopower at large twist angles is dominated by phonon drag effects.

Electronic contribution to thermopower emerges at twist angles below two degrees.

Cross-plane thermoelectricity is highly sensitive to band dispersion and layer misorientation.

Abstract

The introduction of 'twist' or relative rotation between two atomically thin van der Waals (vdW) membranes gives rise to periodic Moire potential, leading to a substantial altercation of the band structure of the planar assembly. While most of the recent experiments primarily focus on the electronic-band hybridization by probing in-plane transport properties, here we report out-of-plane thermoelectric measurements across the van der Waals gap in twisted bilayer graphene (tBLG), which exhibits an interplay of twist-dependent inter-layer electronic and phononic hybridization. We show that at a large twist angle, the thermopower is entirely driven by a novel phonon drag effect at the sub-nanometer scale, while the electronic component of the thermopower is recovered only when the misorientation between the layers is reduced to less than two degrees. Our experiment shows that cross-plane thermoelectricity at a low angle is exceptionally sensitive to the nature of band dispersion and may provide fundamental insights into the coherence of electronic states in twisted bilayer graphene.