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Graphene

Graphene is the ultimately thin crystal, a purely two dimensional plane of carbon atoms arranged on a honeycomb lattice. This material has stimulated a worldwide research activity because electrons in graphene have a linear electronic dispersion and hence, behave as massless particles, similar to those encountered in high energy physics. We use optical techniques (infrared transmission/reflectivity and Raman scattering) combined with high magnetic fields to study the effects of electron-electron and of electron-phonon interactions in mono and multilayer graphene, with different stackings.

Exploring the band structure of multilayer graphene

We have probed electronic excitations between Landau levels in freestanding N-layer graphene over a broad energy range, with unprecedented spectral and spatial resolution, using micro magneto-Raman scattering spectroscopy. A characteristic evolution of electronic bands in up to five Bernal-stacked graphene layers is evidenced and shown to remarkably follow a simple theoretical approach, based on an effective bilayer model. (N > 3)-layer graphenes appear as appealing candidates in the (...)

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Landau level spectroscopy of electron-electron interactions in graphene

We present magneto-Raman scattering studies of electronic inter-Landau level excitations in quasineutral graphene samples with different strengths of Coulomb interaction. The band velocity associated with these excitations is found to depend on the dielectric environment, on the index of Landau level involved, and to vary as a function of the magnetic field. This contradicts the single-particle picture of noninteracting massless Dirac electrons but is accounted for by theory when the effect (...)

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Carrier dynamics in Landau-quantized graphene featuring strong Auger scattering

The energy spectrum of common two-dimensional electron gases consists of a harmonic (that is, equidistant) ladder of Landau levels, thus preventing the possibility of optically addressing individual transitions. In graphene, however, owing to its non-harmonic spectrum, individual levels can be addressed selectively. Here, we report a time-resolved experiment directly pumping discrete Landau levels in graphene. Energetically degenerate Landau-level transitions from n = −1 to n = 0 and (...)

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Magneto-phonon resonance

Our low-temperature magneto-Raman scattering measurements performed on graphene-like locations on the surface of bulk graphite, carries the energyite reveal a new series of magneto-phonon resonances involving both K point and Γ point phonons. These are resonances between a purely electronic excitation, an electronic excitation accompanied by one phonon, and a two-phonon excitation. In particular, we observe the resonant splitting of three crossing excitation branches. We give a (...)

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Rhombohedral (ABC stacked) thin graphite

Graphene layers are known to stack in two stable configurations, namely, ABA or ABC stacking, with drastically distinct electronic properties. Unlike the ABA stacking, little has been done to experimentally investigate the electronic properties of ABC graphene multilayers. Here, we report on the first magneto optical study of a large ABC domain in a graphene multilayer flake, with ABC sequences exceeding 17 graphene sheets. ABC-stacked multilayers can be fingerprinted with a characteristic (...)

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SU4 symmetry breaking

Refined infrared magnetotransmission experiments have been performed in magnetic fields B up to 35 T on a series of multilayer epitaxial graphene samples. Following the main optical transition involving the n=0 Landau level (LL), we observe a new absorption transition increasing in intensity with magnetic fields B≥26 T. Our analysis shows that this is a signature of the breaking of the SU(4) symmetry of the n=0 LL. Using a quantitative model, we show that the only symmetry-breaking (...)

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