Probing topological valley currents by angular layer alignment in van der Waals heterostructures

Project Information

TWISTRONICS

Grant agreement ID: 853282

DOI

10.3030/853282

Project closed

EC signature date 2 October 2019

Start date 1 January 2020

End date 31 December 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 499 543,00

EU contribution

€ 1 499 543,00

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 4 - TWISTRONICS (Probing topological valley currents by angular layer alignment in van der Waals heterostructures)

Reporting period: 2024-07-01 to 2025-12-31

Summary of the context and overall objectives of the project

van der Waals heterostructures are artificial materials created by stacking layer-by-layer 2D materials. The layer-by-layer stacking allow us to bypass most of the problems of MBE growth materials, such as strain created in the interface. Additionally, the layer-by-layer stacking allow us to access a new degree of freedom not possible before the rotational crystallographic orientation, this means that we can take any combination of 2D materials to create a van der Waals heterostructure and additionally we can further modify its properties by changing the rotational alignment of the layers. However, controlling the angular alignment is not a simple task and required so far the fabrication of a large number of samples. The impossibility of studing the angular effect in only one sample brings a large sample-to-sample variation and therefore a strong uncertanty while comparing different angular alignments.

The objective of TWISTRONICS is to control this new degree of freedom, so-called twist, and use it to modify the topological properties of graphene/BN heterostructures. The way we do this is by designing heterostructures where the upper most layer is free to move when we push it using an AFM tip. Our unique experimental setup allow us to measure the electronic transport properties of the van der Waals heterostructures while changing the angle and therefore giving us an angular precision better than 0.2 degrees at room temperature. Using this angular control we will investigate the topological properties of graphene/BN heterostructures, knowing the relation of crystallographic alignment and the appearence of these properties will shed new light over the mechanisms responsible for the creation of these effects.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

In the first phase of the TWISTRONICS project, we set up our low-temperature cryostat and refined our sample fabrication techniques. Additionally, we implemented new measurement techniques, including non-local electron transport. These non-local measurements are highly sensitive to electronic noise and connection schemes, which required us to adjust the architecture of our samples for proper implementation.
Using these advancements, we demonstrated that the crystallographic orientation of hexagonal boron nitride (BN) and bilayer graphene significantly impacts the formation of the valley Hall effect. Specifically, we found that two perfectly aligned positions—0° and 60°—are not equivalent for bilayer graphene, contrary to expectations for hexagonal structures.
Following this, we developed dual-gated rotatable devices, allowing us to control another parameter of the heterostructure: the displacement field. This enabled us to understand the role of alignment in the development of an intrinsic crystal field and how it is influenced by the near-alignment commensurate state.
Surprisingly, we also observed a regime of anomalous gating effects, which turned out to be related not to graphene/BN alignment but to the alignment of the two BN layers. This phenomenon, observable even at room temperature, holds strong potential for applications in neuromorphic systems.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We demonstrated that electron transport properties of van der Waals heterostructures are strongly dependent on the atomic configuration of each leayer revealing that two in principle identical moire superlattices do not have the same intrinsic properties. Now that our system is working and it is well stablished we will use our new sample architectures and low temperature system to investigate the topological properties of these heterostructures. We have started with the quantum Hall effect in the non-identical moire formed by bilayer graphene/BN.

We have measured by the first time the twist angle dependence of the crystal field in a heterostructure, which results from the relative position of different atoms in the heterostructure. Also, we have demonstrated that the anomalous agting effect, previously associated with the presence of a moire superlattice, it is in fact related to the alignment between the two graphene layers.

Another very important result is the mapping in twist angle of the formation of quasi-Brillouin zones in heterostructures made of doubly aligned BN/Graphene/BN heterostructures.

Experimental schematics