The past fifteen years have witnessed a dramatic evolution of condensed matter physics as a new class of materials has been explored: two-dimensional materials. These are extracted from layered crystals and thinned down to the atomic layer, revealing new electronic properties absent in the parent crystal. Graphene was the first of these two-dimensional materials to be extracted and remained the most studied as it is truly unique with a linear electronic dispersion hosting Dirac fermions, an anomalous quantum Hall effect and a valley degree of freedom to name a few. The development of graphene research and fabrication techniques enabled the creation of van der Waals heterostructures consisting of multiple two-dimensional sheets stacked together. These heterostructures can combine two different materials assembled in a chosen sequence to create stacks with designer properties. It is also possible to misalign the two lattices by a twist angle to create electronic properties unseen in nature. This thesis explores the electronic properties of van der Waals heterostructures made of graphene and hexagonal boron nitride (hBN) with different twist angles at cryogenic temperatures. The twist angles enable several properties based on the long-range superlattice period and the electronic hybridisation between two neighbouring electronic bands. The experimental work consists of two parts. First, I study the effect of the superlattice potential on electronic properties. I present long-range superlattices with periods of more than 10 nm by aligning a graphene layer with hBN. This length scale is 100 times larger than the lattice constant of the parent graphene and can be made comparable with the magnetic length at high magnetic fields. In high-quality graphene superlattices in high magnetic fields, I explore the regime of Brown-Zak fermions. I find that they have ballistic motion and measure their degeneracy. Additionally, I present the superconducting proximity effect in superlattices made by stacking two graphene sheets at small angles. This kind of heterostructures is subject to strain-induced reconstruction, resulting in large domains separated by narrow conduction channels that are topologically protected against backscattering. The resulting proximity effect is found to withstand extremely high magnetic fields and is attributed to Andreev bound states propagating in the narrow domain walls. The second part focuses on the band hybridisation occurring when two layers are stacked at a singular angle. I present twisted monolayer-bilayer graphene, a system in which the C2 and time-reversal symmetries are spontaneously broken. I explore the correlated insulators created by band flattening and study the asymmetry relative to perpendicular electric fields applied to the layer. I also probe these flat bands under strong electric fields, shifting the Fermi surface out of equilibrium and study the interplay of high drift velocities with narrow bandwidths. Finally, I explore the effect of high twist angles on strong correlations. These high twists allow screening of electron-electron interactions. I notably observe the emergence of superconductivity as a result of band flattening driven by Coulomb screening. I explore this regime's limitations when the Fermi velocity becomes comparable to the superconducting condensate velocity.
Date of Award | 31 Dec 2022 |
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Original language | English |
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Awarding Institution | - The University of Manchester
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Supervisor | Andre Geim (Supervisor) & Irina Grigorieva (Supervisor) |
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- quantum Hall effect
- proximity effect
- superconductivity
- electronic transport
- graphene
- correlated insulator
Electronic properties of graphene heterostructures below 1K
Barrier, J. (Author). 31 Dec 2022
Student thesis: Phd