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Graphene: the good, the bad, and the beauty

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If you have a question about this talk, please contact Dr G Moller.

Graphene is probably the most fascinating material ever discovered, but it has some drawbacks: it is not superconducting, it does not exhibit the quantum spin Hall effect, and its magnetic properties are still controversial. The interesting electronic properties of graphene, such as the presence of charge carriers that behave as if they would have no mass, are rooted on the honeycomb lattice of the carbon atoms. This insight provides a unique opportunity: by creating honeycomb lattices of materials other than carbon, the effects conferred by the atoms can be combined with those conferred by the honeycomb lattice and novel materials, with unexpected properties, may emerge. A key question in this regard is: if we build a honeycomb lattice out of semiconducting nanocrystals, is it going to behave like graphene or like the semiconducting building blocks?

In the first part of the talk, I will show that these systems, which have been experimentally synthesized last year [1], combine the best of the two materials. Honeycomb lattices of semiconducting nanocrystals exhibit a gap at zero energy, as well as Dirac cones are finite energies. In addition, a honeycomb lattice made of CdSe nanocrystals displays topological properties in the valence band [2], whereas for HgTe very large topological gaps are predicted to occur in the conduction p-bands [3]. These artificial materials thus open the possibility to engineer higher-orbital physics with Dirac electrons and to realize quantum (spin) Hall phases at room temperature [3].

In the second part of the talk, I will discuss how to describe the full dynamical electromagnetic interaction in 2D systems like graphene, where the electrons are constrained to move in the 2D plane, whereas the photons move in 3D. By using the so-called pseudo-QED approach, I will show how quantized edge states emerge in this system and give rise to the quantum Valley Hall Effect [4], thus opening the possibility to realize Valleytronics as an alternative to Spintronics and Electronics.

[1] M. P. Boneschanscher et al, Science 344, 1377 (2014). [2] E. Kalesaki, C. Delerue, C. Morais Smith, W. Beugeling, A. Allan, and D. Vanmaekelbergh, Phys. Rev. X 4 , 011010 (2014). [3] W. Beugeling, E. Kalesaki, C. Delerue, Y.-M. Niquet, D. Vanmaekelbergh, and C. Morais Smith, Nature Communications 6, 6316 (2015). [4] E. Marino, L. O. Nascimento, V. S. Alves, and C. Morais Smith, Phys. Rev. X 5 , (2015).

This talk is part of the Theory of Condensed Matter series.

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