Unveiling the Electronic Origin of Anomalous Contact Conductance in Twisted Bilayer Graphene

TL;DR

This paper offers a theoretical explanation for the non-monotonic contact conductance observed in twisted bilayer graphene, attributing it to electronic band structure changes rather than structural relaxation or stacking variations.

Contribution

It develops a comprehensive theoretical framework demonstrating that conductance peaks are due to van Hove singularities shifting with twist angle and bias, challenging previous interpretations.

Findings

Maximum conductance at ~5° is due to van Hove singularities aligning with the Fermi level.

Structural relaxation and stacking zones are not responsible for the conductance peak.

Bias voltage influences the position of conductance maxima.

Abstract

This study theoretically investigates the contact conductance in twisted bilayer graphene (TBG), providing a theoretical explanation for recent experimental observations from scanning tunneling microscopy (STM) and conductive atomic force microscopy (c-AFM). These experiments revealed a surprising non-monotonic current pattern as a function of the TBG rotation angle $\theta$, with a peak at $\theta \approx 5^\circ$, a finding that markedly departs from the well-known magic angle TBG behavior. To elucidate this phenomenon, we develop a comprehensive theoretical and computational framework. Our calculations, performed on both relaxed and rigid TBG structures, simulate contact conductance by analyzing the local density of states across a range of biases and rotational angles. Contrary to the current interpretation, our results demonstrate that the maximum conductance at $\theta \approx 5^{\rm o}$ is not caused by structural relaxation or AA stacking zone changes. Instead, we attribute this peak to the evolution of the electronic band structure, specifically the shifting of van Hove singularities (vHs) to the Fermi level as the twist angle decreases. We further show that the precise location of this conductance maximum is dependent on the applied bias voltage. This interplay between twist angle, bias, and vHs energy provides a robust explanation for the experimental findings.