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Playing graphene physics with microwaves


Graphene, the wonder material with an atomic thickness may be the promising material of the XXI century, replacing current silicon technology. Its so many fascinating properties, like the massless propagation of the electrons, are the subject of an intense research activity. On the other hand there is a growing interest for the study of “artificial graphenes”, that is, totally different and new systems which bear exciting similarities with graphene, among them : lattices of ultracold atoms (see LPS highlight 2012), photonic lattices or “molecular graphene”.

 

Recently, physicists of the Laboratoire de Physique de la Matière Condensée (LPMC) in Nice and of the Laboratoire de Physique des Solides (LPS) in Orsay have found that these fascinating properties can be simulated with a novel setup where cm-size plastic dots replace carbon atoms and microwaves replace the electrons. The recipe is simple : take a few hundred dots sandwiched between two metal plates and arrange them in a honeycomb lattice like the carbon atoms in graphene. Then let a radio wave propagate through this artificial crystal. This new setup realizes a microwave analog simulator of the quantum propagation of electrons in graphene. This allows the observation of new effects, hardly visible or difficult to explore in graphene like a new topological transition, between a « semi-metallic » phase and an insulating phase whose characteristics had been predicted in the theory team at LPS, or strange behaviors of the waves at the edges. By fabricating an appropriately strained lattice, they could observe this topological transition and show the existence of new exotic states at the edges of the system.

 

The experimental setup developed by the Nice team permits a perfect control of the lattice made of cylindrical dielectric dots. The distance between the dots fixes the intensity of the electromagnetic coupling. The configuration of the edges, well-known to play an important role in graphene but difficult to control, is quite easy to modify here. In this work, published in the Physical Review Letters, the artificial lattice has the form of a hexagon with smooth edges (« armchair edges »). The density of electromagnetic modes of this “microwaves graphene” presents the expected conic structure in the vicinity of a « Dirac point ». It is possible to measure, for each mode, the spatial repartition of the microwave field. In the isotropic configuration, there is no energy concentrated at the edges : there are no edges states. When a strained is applied along a crystallographic axis, the density of modes changes near the Dirac points, and beyond a critical frequency, a gap appears in the density of modes. The strain also induces novel exotic states whose localization along the edges increases with the strain.

 

The future studies will address the structure of the edges for more complex deformations. Moreover with an inhomogeneous strain, it is possible to simulate effects similar to those of a magnetic field… without electric charge.

 


 

Intensity of the electric field, from white (min.) to red (max.), measured at the frequency corresponding to the Dirac point.

1. Regular structure : no state corresponds to this frequency.

2. Slightly compressed structure : new states appear.

3. Strongly compressed structure : the states are localized along the edges.

 

Reference :

 

Topological transition of Dirac points in a microwave experiment,
M. Bellec, U. Kuhl, G. Montambaux and Mortessagne,
Phys. Rev. Lett. 110, 033902 (2013).

 

Contact LPS :

 

Gilles Montambaux (gilles.montambaux@u-psud.fr)

 

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