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Estelle Mazaleyrat

Growth, structure and electronic properties of epitaxial graphene on rhenium, towards a two-dimensional superconducting platform

Published on 18 December 2019
Thesis presented December 18, 2019

The realization of graphene-based hybrid structures, where graphene is associated with other materials, offers a promising avenue for testing a variety of phenomena. In particular, one can induce properties in graphene by proximity effects. Here, the targeted graphene-based system consists of a quasi free-standing graphene platform with induced superconducting character and in close vicinity to magnetic impurities. According to recent theoretical articles, such a sample could exhibit unconventional Yu-Shiba-Rusinov (YSR) states.
Although the targeted graphene-based system was not fabricated yet, we have addressed, with the help of surface science tools, all three ingredients required for its realization (quasi-free standing graphene, induced superconducting character and proximity to magnetic impurities).
As previously demonstrated, graphene can be rendered superconducting by growing it directly on top of a superconducting material such as rhenium. Structural aspects related to graphene grown on Re(0001) were investigated. In particular, we showed that increasing the number of annealing cycles positively contributes to growing high-quality extended graphene domains. The structure of a surface rhenium carbide, which constitutes a usually ill-characterized object, was studied as well.
Additionnally, a defect appearing as a depression in the nanorippled structure of graphene on strongly interacting metals such as Re(0001) and Ru(0001) was investigated and ascribed to stacking faults either in graphene or in the metal substrate.
Using superconducting graphene grown on Re(0001) as a starting point for the fabrication of the targeted graphene-based system, we recovered the quasi free-standing character of graphene (lost due to its strong interaction with the rhenium substrate) via intercalation of sub-monolayer to few layers of gold atoms. A high density of defects observed in gold-intercalated graphene on Re(0001) was attributed to the intercalation process itself. Besides, we demonstrated that the rhenium-induced superconducting character in graphene was not affected by gold intercalation. At this point, two of the three requirements for realizing the targeted graphene-based system were fulfilled.
Provided that we bring magnetic impurities in close proximity to such a sample, few-nanometers extended YSR states could be observed. Preliminary results involving two original magnetic verdazyl compounds were presented, one of which was deposited on a model system, namely Cu(111). Before turning to quasi-free standing superconducting graphene as a hosting material for these magnetic compounds, further investigations on model systems are needed. Indeed, we could not resolve the precise structure of the molecular assemblies covering the Cu(111) surface yet, and the thermal stability of the compounds was discussed.

graphene, intercalation, defects, molecules, supramlecular assembly

On-line thesis.