Observation of the Kohn anomaly near the K point of bilayer graphene

The dispersion of electrons and phonons near the K point of bilayer graphene was investigated in a resonant Raman study using different laser excitation energies in the near infrared and visible range. The electronic structure was analyzed within the tight-binding approximation, and the Slonczewski-Weiss-McClure (SWM) parameters were obtained from the analysis of the dispersive behavior of the Raman features. A softening of the phonon branches was observed near the K point, and results evidence the Kohn anomaly and the importance of considering electron-phonon and electron-electron interactions to correctly describe the phonon dispersion in graphene systems.Comment: 4 pages, 4 figure

Nature of the constant factor in the relation between radial breathing mode frequency and tube diameter for single-wall carbon nanotubes

Resonance Raman scattering is used to determine the radial breathing mode (RBM) frequency (ωRBM) dependence on tube diameter (dt) for single-wall carbon nanotubes (SWNTs). We establish experimentally the ωRBM=227.0/dt as the fundamental relation for pristine SWNTs. All the other RBM values found in the literature can be explained by an upshift in frequency due mostly to van der Waals interaction between SWNTs and environment

Flood risk management through a resilience lens

To prevent floods from becoming disasters, social vulnerability must be integrated into flood risk management. We advocate complementing conventional risk analysis by adopting a resilience lens in which the welfare of different societal groups is considered by adding recovery capacity, impacts of beyond design events, and distributional impacts

Quantum Noise and Polarization Fluctuations in Vertical Cavity Surface Emitting Lasers

We investigate the polarization fluctuations caused by quantum noise in quantum well vertical cavity surface emitting lasers (VCSELs). Langevin equations are derived on the basis of a generalized rate equation model in which the influence of competing gain-loss and frequency anisotropies is included. This reveals how the anisotropies and the quantum well confinement effects shape the correlations and the magnitude of fluctuations in ellipticity and in polarization direction. According to our results all parameters used in the rate equations may be obtained experimentally from precise time resolved measurements of the intensity and polarization fluctuations in the emitted laser light. To clarify the effects of anisotropies and of quantum well confinement on the laser process in VCSELs we therefore propose time resolved measurements of the polarization fluctuations in the laser light. In particular, such measurements allow to distinguish the effects of frequency anisotropy and of gain-loss anisotropy and would provide data on the spin relaxation rate in the quantum well structure during cw operation as well as representing a new way of experimentally determinig the linewidth enhancement factor alpha.Comment: 16 pages and 3 Figures, RevTex, to be published in Phys. Rev.

Controlled Dynamics of Interfaces in a Vibrated Granular Layer

We present experimental study of a topological excitation, {\it interface}, in a vertically vibrated layer of granular material. We show that these interfaces, separating regions of granular material oscillation with opposite phases, can be shifted and controlled by a very small amount of an additional subharmonic signal, mixed with the harmonic driving signal. The speed and the direction of interface motion depends sensitively on the phase and the amplitude of the subharmonic driving.Comment: 4 pages, 6 figures, RevTe

Optimal measurement of visual motion across spatial and temporal scales

Sensory systems use limited resources to mediate the perception of a great variety of objects and events. Here a normative framework is presented for exploring how the problem of efficient allocation of resources can be solved in visual perception. Starting with a basic property of every measurement, captured by Gabor's uncertainty relation about the location and frequency content of signals, prescriptions are developed for optimal allocation of sensors for reliable perception of visual motion. This study reveals that a large-scale characteristic of human vision (the spatiotemporal contrast sensitivity function) is similar to the optimal prescription, and it suggests that some previously puzzling phenomena of visual sensitivity, adaptation, and perceptual organization have simple principled explanations.Comment: 28 pages, 10 figures, 2 appendices; in press in Favorskaya MN and Jain LC (Eds), Computer Vision in Advanced Control Systems using Conventional and Intelligent Paradigms, Intelligent Systems Reference Library, Springer-Verlag, Berli

The Unique Origin of Colors of Armchair Carbon Nanotubes

The colors of suspended metallic colloidal particles are determined by their size-dependent plasma resonance, while those of semiconducting colloidal particles are determined by their size-dependent band gap. Here, we present a novel case for armchair carbon nanotubes, suspended in aqueous medium, for which the color depends on their size-dependent excitonic resonance, even though the individual particles are metallic. We observe distinct colors of a series of armchair-enriched nanotube suspensions, highlighting the unique coloration mechanism of these one-dimensional metals.Comment: 4 pages, 3 figure