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Electrical Switch to the Resonant Magneto-Phonon Effect in Graphene

Atomic vibrations of a graphene crystal, phonons, can hybridize with electronic excitations. This hybridization is particularly pronounced when a magnetic field is applied perpendicular to the plane of the two dimensional crystal with an intensity tuned so that the energy of electronic excitations coincides with that of the phonon. This effect is called the magneto-phonon effect and manifests itself through pronounced variations of the phonon energy each time an electronic excitation is tuned to the phonon energy. It can be observed through light scattering experiments (Raman scattering) performed at low temperatures and in high magnetic fields.

At the resonance, the energy shift of the phonon modes with respect to their energy without magnetic field is determined by the electron-phonon coupling constant, by the intensity of the magnetic field, and by the effective strength of the electronic excitation. It is this latter parameter that a joint collaboration between scientists at the LNCMI, at the Institut Néel and at the LPMMC managed to tune continuously, allowing probing the two distinct regimes for which the electronic excitation exists and efficiently hybridizes with the phonon (strong coupling), or, for which the electronic excitation is not allowed and no hybridization occurs. This external tuning of the interacting regime of the electron-phonon system is made possible by monitoring the number of carriers in the sample, and by completely filling or emptying the states involved in the electronic excitation. The result has been obtained using CVD-grown graphene, which is today the most promising production method for an industrial use of graphene.
 

For more information: http://pubs.acs.org/doi/abs/10.1021/nl404588g

 

Fig1: False color map of the scattered intensity at the phonon energy, measured at B=26T for different values of the filling factor (carrier concentration) and showing the hybrid electron-phonon modes, characteristic of the strong coupling, on both sides of the non-interacting phonon (a). Results of the calculation of the effect (b