Transport across twist angle domains in moiré graphene

Abstract

Many experiments in twisted bilayer graphene (TBG) differ from each other in terms of the details of their phase diagrams. Few controllable aspects aside, this discrepancy is largely believed to arise from the presence of a varying degree of twist angle inhomogeneity across different samples. Real-space maps indeed reveal TBG devices splitting into several large domains of different twist angles. Motivated by these observations, we study the quantum mechanical tunneling across a domain wall (DW) that separates two such regions. We show that the tunneling of the moiré particles can be understood by the formation of an effective step potential at the DW. The height of this step potential is simply a measure of the difference in twist angles. These computations lead us to identify the global transport signatures for detecting and quantifying the local twist angle variations. In particular, using Landauer-Büttiker formalism, we compute single-channel conductance (dI/dV) and Fano factor for shot noise (ratio of noise power and mean current). A reduction in the low-energy conductance and a zero-bias sub-meV gap in the conductance are observed which scale with the twist angle difference. One of the key findings of our work is that transport in presence of twist angle inhomogeneity is "noisy," though sub-Poissonian. In particular, the differential Fano factor peaks near the van Hove energies corresponding to the domains in the sample. The location and the strength of the peak are simply a measure of the degree of twist angle inhomogeneity.

High transmission in twisted bilayer graphene with angle disorder

“…It is noteworthy that there is no reduction in transmission when doubling the width of the angle disorder barrier. This suggest that the transmission through angle disorder barriers of many moiré periods could decay sub-exponentially with the barrier width, in contrast to previous predictions [72]. Another interesting aspect is the complete transmission of normal incidence carriers near Γ.…”

“…The bandwidth of the flat bands, the gaps to remote bands and the sharpness of the van-Hove peaks were found to be sensitive to angle disorder. So far, transport in the presence of angle disorder has only been studied with a two-band version of the continuum model [72] and was limited to tunneling through narrow angle domain walls. Another recent study [73] describes the transmission of Dirac particles through quasi-1D angle domains, modelled as a change in the Fermi velocity of carriers plus a gauge field that shifts Dirac points in momentum space.…”

Strain-Induced Quantum Phase Transitions in Magic-Angle Graphene

“…Thus, an important question is this: what weakens the insulators in some experimental devices and destroys them in others? Twist-angle disorder is expected to be at least partly responsible for this phenomenon [23][24][25][26]. Another possible culprit is the presence of strain in the graphene sheets.…”

Transport and spectral properties of magic angle twisted bilayer graphene junctions based on local orbital models

“…For instance, to determine the local spectral density at a domain wall [71,72] where the coupling to the substrate changes sign, we use the following expression For computing transport properties in junctions we start from the general expression for the current between two leads using Keldysh Green functions in [69,73] After some algebra we get the Landauer formula…”