Landau Quantization and Fermi Velocity Renormalization in Twisted Graphene Bilayers

TL;DR

This study uses STM and STS to investigate the electronic properties of twisted bilayer graphene, revealing how Fermi velocity depends on twist angle and interlayer coupling, resolving previous experimental contradictions.

Contribution

It provides direct measurements linking Fermi velocity renormalization to twist angle and interlayer coupling, clarifying conflicting experimental results.

Findings

Fermi velocity varies with twist angle and interlayer coupling

Landau quantization matches that of massless Dirac fermions

Interlayer interaction strengths differ across samples

Abstract

Currently there is a lively discussion concerning Fermi velocity renormalization in twisted bilayers and several contradicted experimental results are reported. Here we study electronic structures of the twisted bilayers by scanning tunneling microscopy (STM) and spectroscopy (STS). The interlayer coupling strengths between the adjacent bilayers are measured according to energy separations of two pronounced low-energy van Hove singularities (VHSs) in the STS spectra. We demonstrate that there is a large range of values for the interlayer interaction in different twisted bilayers. Below the VHSs, the observed Landau quantization in the twisted bilayers is identical to that of massless Dirac fermions in graphene monolayer, which allows us to measure the Fermi velocity directly. Our result indicates that the Fermi velocity of the twisted bilayers depends remarkably on both the twisted angles and the interlayer coupling strengths. This removes the discrepancy about the Fermi velocity renormalization in the twisted bilayers and provides a consistent interpretation of all current data.