Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry, we observe a resonant exciton at an unusually high energy of 6.3 eV in epitaxial graphene. Surprisingly, the resonant exciton occurs at room temperature and for a very large number of graphene layers N ≈ 75, thus suggesting a poor screening in graphene. The optical conductivity (σ 1 ) of a resonant exciton scales linearly with the number of graphene layers (up to at least 8 layers), implying the quantum character of electrons in graphene. Furthermore, a prominent excitation at 5.4 eV, which is a mixture of interband transitions from π to π * at the M point and a π plasmonic excitation, is observed. In contrast, for graphite the resonant exciton is not observable but strong interband transitions are seen instead. Supported by theoretical calculations, for N 28 the σ 1 is dominated by the resonant exciton, while for N > 28 it is a mixture between exitonic and interband transitions. The latter is characteristic for graphite, indicating a crossover in the electronic structure. Our study shows that important elementary excitations in graphene occur at high binding energies and elucidate the differences in the way electrons interact in graphene and graphite. Graphene, a one-layer-thick carbon honeycomb structure, has recently attracted a lot of attention due its exotic quantum properties.1 In addition to the well-known electronic properties, such as ballistic electron transport, 1-3 quantum Hall effect, 4 tunable band gap, 5,6 and physics driven by many-body interactions, 7 graphene also displays very interesting optical properties. For instance, due to its low-energy excitation graphene becomes highly transparent in the visible spectral range and its infrared optical conductivity can be tuned using a gate voltage.8 These particular properties of graphene potentially result in exciting optoelectronic applications.Recent theoretical studies based on the ab initio GW and Bethe-Salpeter equation (BSE) approach by Yang et al. 9 and Trevisanutto et al. 10 have predicted the existence and have highlighted the importance of resonant excitonic effects in the optical absorption of graphene. However, there is disagreement as to the origin and position of the exciton. In Ref. 9 the calculations were done up to 7 eV and the resonant exciton was predicted to occur at 4.6 eV due to electron-hole interaction in the π * and π bands at the M point. In Ref. 10, however, the calculations were done at a much higher energy of 22 eV and the resonant exciton was predicted to appear at 8.3 eV due the background single-particle continuum of dipole forbidden transition at the the point. Despite their disagreement, both have agreed that resonant excitons play important role in elementary excitations in graphene and thus its understanding is crucial.A direct way to probe a resonant exciton in graphene is to measure its complex dielectric response in wide energy range.10 Despite reports on the optical properties of graphene over the visible energy r...

We study the dynamics of the superconducting order parameter in the high-Tc cuprate Bi2Sr2CaCu2O8+delta by employing a novel time-resolved pump-probe Raman experiment. We find two different coupling mechanisms that contribute equally to the pair-breaking peak. One coupling sets in very fast at 2 ps and relaxes slowly, while the other one is delayed and sets in roughly at 5 ps and relaxes fast. A model that couples holes through phonons is able to reproduce one part of the condensate dynamics; thus, we argue that hole-spin interactions are of importance as well.