Periodic Reporting for period 3 - TWISTRONICS (Probing topological valley currents by angular layer alignment in van der Waals heterostructures)
Periodo di rendicontazione: 2023-01-01 al 2024-06-30
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.
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.
In the first part of TWISTRONICS we have set up our low temperature cryostat and improve our sample fabrication techniques. Additionally, we have implemented new measurement techniques, such as non-local electron transport. These non-local measurements are highly demanding in terms of electronic noise and connection schemes and for the correct implementation we had to adjust the architecture of our samples.
All this have been used to demonstrate that the crystallographic orientation of hexagonal boron nitride (BN) and bilayer graphene have a strong impact in the formation of valley Hall effect. In fact we demonstrated that two perfectly aligned positions, namely 0 and 60 degrees of alignment, are not equivalent for bilayer graphene, as it will be expected in hexagonal structures.
All this have been used to demonstrate that the crystallographic orientation of hexagonal boron nitride (BN) and bilayer graphene have a strong impact in the formation of valley Hall effect. In fact we demonstrated that two perfectly aligned positions, namely 0 and 60 degrees of alignment, are not equivalent for bilayer graphene, as it will be expected in hexagonal structures.
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 and we will continue to monolayer graphene. We are at the same time developing a new measurement scheme that will allow us to measure in the presence of a displacement field, an extra knob that will allow us to modify further the properties of bilayer graphene.