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PGI-1 Seminar: Dr. Stephen Power

Impurities and magnetic interactions in graphene

11:30 PGI-Hörsaal

School of Physics, Trinity College Dublin, Dublin 2, Ireland


Graphene is a two-dimensional carbon material that until its recent discovery was assumed not to exist in the free state. Graphene-related materials have been in the scientific limelight since then due to several key discoveries regarding their production and properties. There are numerous technological applications envisaged for them. Besides the huge potential for applicability, one key feature that makes graphene particularly popular is the simplicity with which many of its physical properties can be described, primarily due to the simple dispersion relation for its electrons. In this thesis a number of different topics relating to graphene systems, and in particular those doped with impurities, are investigated using a combination of analytical and numerical methods. We consider both graphene sheets and quasi-one-dimensional strips of graphene that are called `nanoribbons'.

The electronic properties of materials can be engineered by doping, but in the case of graphene nanoribbons the introduction of two symmetry-breaking edges introduces an additional dependence on the location of an impurity across the width of the ribbon. This dependence has been noted previously in electronic transport calculations, but in this work we extend the discussion to the binding energy of the impurity and also to the magnetic moment that is formed if the impurity is magnetic. The results of simple model calculations are found to match those of more sophisticated ab initio calculations.

Magnetically-doped graphene systems are potential candidates for application in future spintronic devices. A key step is to understand the pairwise interactions that occur between magnetic impurities embedded in graphene that are mediated by the graphene conduction electrons. In this work we examine interactions between such impurities using a Green function formalism. By developing an analytical expression for the Green function in graphene, we are able to explore the distance dependence of these interactions in a mathematically transparent fashion. We also demonstrate that ab initio calculations may yield spurious results if the effects of this interaction are neglected. The quick decay with separation of the interaction in graphene, reported by many authors, is often seen as a major obstacle for the spintronic application of these systems. However, in this work we report that a significant augmentation of the interaction is possible when the impurity moments are set to precess. An experimental setup to probe this dynamic form of the magnetic interaction in graphene is also suggested.