Surface plasmon-polariton waves with low-phase speed in carbon nanostructures can be utilized for the generation of coherent terahertz radiation through the Čerenkov mechanism, the effect being especially pronounced in bilayer and multilayer graphene. Using the many-body formalism and the tight-binding approach, we derived the dispersion equations of the surface plasmon-polariton waves in graphene. In single-layer graphene, the phase speed is about three to five times smaller than the speed of light in a vacuum. In bilayer graphene, inter-layer electron tunneling suppresses the reduction of the phase speed. Reduction of the phase speed by as much as 300 times is possible in a graphene structure with two spatially expanded monolayers, because inter-layer tunneling is suppressed, and the interlayer distance can be used to tune the plasmon frequency and the phase speed.
We studied the Raman intensities of the radial breathing mode(RBM) in carbon nanotubes excited resonantly into the first and second optical transitions of semiconducting nanotubes and the first transition of metallic nanotubes. Several variations in the maximum Raman intensities of different (n1, n2) nanotubes are observed from which we discuss the dependence on the nanotube family ν, the chiral angle θ and the optical transition Eii. By comparison to theory we attribute variations with ν and θ to variations in the exciton-phonon coupling. Differences between different Eii are dominated by variations in the linewidth of the optical transitions.
For the correct interpretation of Raman spectra it is important to realize that the scattering process in carbon nanotubes is doubly resonant, a feature which - although known from semiconductors - is greatly enhanced by the peculiar bandstructure of graphene near the K-point. We discuss the double-resonant process and, as examples of its importance, show how the relative defect concentration in a set of boron-doped multiwalled tubes can be measured and how the determination of the diameter through the radial breathing mode (RBM) is affected by double resonance.
We present results of a Raman scattering study of Bi2Sr2CaCu2O8 single crystals with Tc equals 95 K in a wide temperature (65 - 365 K) and frequency (from 640 down to 10 cm-1) range. The features due to superstructure modulation are found in the low-frequency range. We also investigated additional lines seen in z(xx)z and z(yy)z scattering configurations in the regions of 160 - 240 cm-1 and 300 - 450 cm-1 and analyzed the temperature dependence of the latter.
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