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Designing quantum phases in monolayer graphene Nigge, Pascal Alexander


The physics of quantum materials is at the heart of current condensed matter research. The interactions in these materials between electrons themselves, with other excitations, or external fields can lead to a number of macroscopic quantum phases like superconductivity, the quantum Hall effect, or density wave orders. But the experimental study of these materials is often hindered by complicated structural and chemical properties as well as by the involvement of toxic elements. Graphene, on the other hand, is a purely two-dimensional material consisting of a simple honeycomb lattice of carbon atoms. Since it was discovered experimentally, graphene has become one of the most widely studied materials in a range of research fields and remains one of the most active areas of research today. However, even though graphene has proven to be a promising platform to study a plethora of phenomena, the material itself does not exhibit the effects of correlated electron physics. In this thesis, we show two examples of how epitaxially grown large-scale graphene can be exploited as a platform to design quantum phases through interaction with a substrate and intercalation of atoms. Graphene under particular strain patterns exhibits pseudomagnetic fields. This means the Dirac electrons in the material behave as if they were under the influence of a magnetic field, even though no external field is applied. We are able to create large homogeneous pseudomagnetic fields using shallow nanoprisms in the substrate, which allows us to study the strain-induced quantum Hall effect in a momentum-resolved fashion using angle-resolved photoemission spectroscopy (ARPES). In the second part, we show how the intercalation of gadolinium can be used to couple flat bands in graphene to ordering phenomena in gadolinium. Flat bands near the Fermi level are theorised to enhance electronic correlations, and in combination with novel ordering phenomena, play a key role in many quantum material families. Our ARPES and resonant energy-integrated X-ray scattering (REXS) measurements reveal a complex interplay between different quantum phases in the material, including pseudogaps and evidence for a density wave order.

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