Deformations of moiré patterns in twisted bilayer graphene

Twistology could be the study of unexpected changes or developments in stories or situations, from coups d’état to the fatherhood of Darth Vader. In condensed matter physics there is something similar, although the preferred name is twistronics (from twist and electronics). It is understood as the study of how the angle (the twist) between layers of two-dimensional materials can change their electrical properties. Materials such as twisted bilayer graphene (TBG) have been shown to have vastly different electronic behaviour, ranging from non-conductive to superconductive, that depends sensitively on the angle between the layers.

In TBG a moiré pattern forms that introduces a new length scale to the material. At the “magic” twist angle of 1.1^∘, this causes a flat band to form, yielding emergent properties such as correlated insulator behavior and superconductivity. But, and this is important, the moiré structure in TBG varies spatially, influencing the local electronic properties. This results in the wide variation observed in the phase diagrams and critical temperatures of TBG.

To tackle the problem, many experimental techniques have been applied to study local moiré variations, each only resolving part of the puzzle due to practical limitations (capping layer or device substrate, surface quality, or measurement speed).

Now, a team of researchers uses ¹ aberration-corrected low energy electron microscopy (AC-LEEM), which measures an image of the reflection of a micron-sized beam of electrons at a landing energy E₀ in real space, in reciprocal space (diffraction), or combinations thereof. This allows to perform large-scale, fast, and non-destructive imaging of TBG, including device-scale moiré images and dynamics on timescales of seconds. In addition, spectroscopic measurements, yielding information on the material’s unoccupied bands, can be done by varying E₀.

Using AC-LEEM to image moiré patterns enables high-temperature imaging and has the benefit no suspended samples are required like they are for other techniques. This means that sample geometries closely resembling device geometries can be imaged, including devices with leads. Samples consisted of two twisted graphene flakes (TBG) on top of a hexagonal boron nitride flake on a silicon substrate.

At 500 ^∘C, the researchers observe thermal fluctuations of the moiré lattice, corresponding to collective atomic displacements of less than 70 pm on a timescale of seconds, but thermal annealing can be used to decrease local disorder. Importantly, no untwisting of the layers is found, even at temperatures as high as 600 ^∘C. Finally, stable topological defects – edge dislocations – are also observed in the underlying atomic lattice, the moiré structure acting as a magnifying glass. These topological defects are anticipated to exhibit unique local electronic properties.

The methods employed extend beyond TBG, to any type of twisted system. Therefore, this work introduces a way of studying deformations of moiré patterns and of connecting these to the (local) electronic properties of this exciting class of materials.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.