Optical Phonon Lineshapes and Transport in Metallic Carbon Nanotubes under High Bias Voltage

We calculate the current-voltage characteristic of metallic nanotubes at high bias voltage showing that a bottleneck exists for short nanotubes in contrast to large ones. We attribute this to a redistribution of lower-lying acoustic phonons caused by phonon-phonon scattering with hot optical phonons. The current-voltage characteristic and the electron and phonon distribution functions are derived analytically, and serve to obtain in a self-contained way the frequency shift and line broadening of the zone center optical phonons due to the electron-phonon coupling at high bias. We obtain a positive frequency shift from the zero bias shift and no broadening of the optical phonon mode at very high voltages, in agreement with recent experiments.Comment: 16 pages, 8 figures, minor changes, pusblished in PR

Beyond the random phase approximation in the Singwi-Sj\"olander theory of the half-filled Landau level

We study the $\nu=1/2$ Chern-Simons system and consider a self-consistent field theory of the Singwi-Sj\"olander type which goes beyond the random phase approximation (RPA). By considering the Heisenberg equation of motion for the longitudinal momentum operator, we are able to show that the zero-frequency density-density response function vanishes linearly in long wavelength limit independent of any approximation. From this analysis, we derive a consistency condition for a decoupling of the equal time density-density and density-momentum correlation functions. By using the Heisenberg equation of motion of the Wigner distribution function with a decoupling of the correlation functions which respects this consistency condition, we calculate the response functions of the $\nu=1/2$ system. In our scheme, we get a density-density response function which vanishes linearly in the Coulomb case for zero-frequency in the long wavelength limit. Furthermore, we derive the compressibility, and the Landau energy as well as the Coulomb energy. These energies are in better agreement to numerical and exact results, respectively, than the energies calculated in the RPA.Comment: 9 Revtex pages, 4 eps figures, typos correcte

Modelling two-dimensional Crystals with Defects under Stress: Superelongation of Carbon Nanotubes at high Temperatures

We calculate analytically the phase diagram of a two-dimensional square crystal and its wrapped version with defects under external homogeneous stress as a function of temperature using a simple elastic lattice model that allows for defect formation. The temperature dependence turns out to be very weak. The results are relevant for recent stress experiments on carbon nanotubes. Under increasing stress, we find a crossover regime which we identify with a cracking transition that is almost independent of temperature. Furthermore, we find an almost stress-independent melting point. In addition, we derive an enhanced ductility with relative strains before cracking between 200-400%, in agreement with carbon nanotube experiments. The specific values depend on the Poisson ratio and the angle between the external force and the crystal axes. We give arguments that the results for carbon nanotubes are not much different to the wrapped square crystal.Comment: 12 pages, 6 eps figures, section VI added discussing the modifications of our model when applied to tube

Characterization of physical properties of a coastal upwelling filament with evidence of enhanced submesoscale activity and transition from balanced to unbalanced motions in the Benguela upwelling region

We combine high-resolution in situ data (acoustic Doppler current profiler (ADCP), Scanfish, and surface drifters) and remote sensing to investigate the physical characteristics of a major filament observed in the Benguela upwelling region. The 30–50 km wide and about 400 km long filament persisted for at least 40 d. Mixed-layer depths were less than 40 m in the filament and over 60 m outside of it. Observations of the Rossby number Ro from the various platforms provide the spatial distribution of Ro for different resolutions. Remote sensing focuses on geostrophic motions of the region related to the mesoscale eddies that drive the filament formation and thereby reveals |Ro|&lt;0.1. Ship-based measurements in the surface mixed layer reveal 0.5&lt;|Ro|&lt;1, indicating the presence of unbalanced, ageostrophic motions. Time series of Ro from triplets of surface drifters trapped within the filament confirm these relatively large Ro values and show a high variability along the filament. A scale-dependent analysis of Ro, which relies on the second-order velocity structure function, was applied to the latter drifter group and to another drifter group released in the upwelling zone. The two releases explored the area nearly distinctly and simultaneously and reveal that at small scales (&lt;15 km) Ro values are twice as large in the filament in comparison to its environment with Ro&gt;1 for scales smaller than ∼500 m. This suggests that filaments are hotspots of ageostrophic dynamics, pointing to the presence of a forward energy cascade. The different dynamics indicated by our Ro analysis are confirmed by horizontal kinetic energy wavenumber spectra, which exhibit a power law k−α with α∼5/3 for wavelengths 2π/k smaller than a transition scale of 15 km, supporting significant submesoscale energy at scales smaller than the first baroclinic Rossby radius (Ro1∼30 km). The detected transition scale is smaller than those found in regions with less mesoscale eddy energy, consistent with previous studies. We found evidence for the processes which drive the energy transfer to turbulent scales. Positive Rossby numbers (1) associated with cyclonic motion inhibit the occurrence of positive Ertel potential vorticity (EPV) and stabilize the water column. However, where the baroclinic component of EPV dominates, submesoscale instability analysis suggests that mostly gravitational instabilities occur and that symmetric instabilities may be important at the filament edges.</p

Personalized cancer medicine and the future of pathology

In February 2011, a group of pathologists from different departments in Europe met in Zurich, Switzerland, to discuss opportunities and challenges for pathology in the era of personalized medicine. The major topics of the meeting were assessment of the role of pathology in personalized medicine, its future profile among other biomedical disciplines with an interest in personalized medicine as well as the evolution of companion diagnostics. The relevance of novel technologies for genome analysis in clinical practice was discussed. The participants recognize that there should be more initiatives taken by the pathology community in companion diagnostics and in the emerging field of next-generation sequencing and whole genome analysis. The common view of the participants was that the pathology community has to be mobilized for stronger engagement in the future of personalized medicine. Pathologists should be aware of the challenges and the analytical opportunities of the new technologies. Challenges of clinical trial design as well as insurance and reimbursement questions were addressed. The pathology community has the responsibility to lead medical colleagues into embracing this new area of genomic medicine. Without this effort, the discipline of pathology risks losing its key position in molecular tissue diagnostic

Leptonic and charged kaon decay modes of the ϕ\phi meson measured in heavy-ion collisions at the CERN SPS

We report a measurement of $\phi$ meson production in central Pb+Au collisions at E$_{lab}$/A=158 GeV. For the first time in heavy-ion collisions, $\phi$ mesons were reconstructed in the same experiment both in the K$^+$K$^-$ and the dilepton decay channel. Near mid-rapidity, this yields rapidity densities, corrected for production at the same rapidity value, of 2.05 +- 0.14(stat) +- 0.25(syst) and 2.04 +- 0.49(stat)+-{0.32}(syst), respectively. The shape of the measured transverse momentum spectra is also in close agreement in both decay channels. The data rule out a possible enhancement of the $\phi$ yield in the leptonic over the hadronic channel by a factor larger than 1.6 at 95% CL.Comment: 11 pages, 4 figures,submitted to Phys. Rev. Let