Impurity Scattering of Wave Packets on a Lattice

Quantum transport in a lattice is distinct from its counterpart in continuum media. Even a free wave packet travels differently in a lattice than in the continuum. We describe quantum scattering in a one dimensional lattice using three different formulations and illustrate characteristics of quantum transport such as resonant transmission. We demonstrate the real time propagation of a wave packet and its phase shift due to impurity configurations. Spin-flip scattering is also taken into account in a spin chain system. We show how individual spins in the chain evolve as a result of a spin-flip interaction between an incoming electron and a spin chain.Comment: submitted to Phys. Rev.

Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

We present a comparative study of ultrafast photo-conversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals is activated and temperature-independent respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C$_{60}$-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.Comment:

Nonlinear ultrafast modulation of the optical absorption of few cycle terahertz pulses in n-doped semiconductors

We use an open-aperture Z-scan technique to show how intense few-cycle terahertz pulses can experience a nonlinear bleaching of absorption in an n-doped semiconductor due to terahertz-electric-field-driven intervalley scattering of electrons in the conduction band. Coherent detection of the transmitted terahertz pulse waveform also allows the nonlinear conductivity dynamics to be followed with sub-picosecond time resolution. Both the Z-scan and time-domain results are found to be in agreement with our theoretical analysis.Comment: Submitted to Physical Review B, 16 pages, 3 figures, 1 tabl

Microscopic origin of the Drude-Smith model

The Drude-Smith model has been used extensively in fitting the THz conductivities of nanomaterials with carrier confinement on the mesoscopic scale. Here, we show that the conventional "backscattering" explanation for the suppression of low-frequency conductivities in the Drude-Smith model is not consistent with a confined Drude gas of classical noninteracting electrons and we derive a modified Drude-Smith conductivity formula based on a diffusive restoring current. We perform Monte Carlo simulations of a model system and show that the modifiedDrude-Smith model reproduces the extracted conductivitieswithout free parameters. This alternate route to the Drude-Smith model provides the popular formula with a more solid physical foundation and well-defined fit parameters

Structure and Colors of Diffuse Emission in the Spitzer Galactic First Look Survey

We investigate the density structure of the interstellar medium using new high-resolution maps of the 8 micron, 24 micron, and 70 micron surface brightness towards a molecular cloud in the Gum Nebula, made as part of the Spitzer Space Telescope Galactic First Look Survey. The maps are correlated with 100 micron images measured with IRAS. At 24 and 70 micron, the spatial power spectrum of surface brightness follows a power law with spectral index -3.5. At 24 micron, the power law behavior is remarkably consistent from the 0.2 degree size of our maps down to the 5 arcsecond spatial resolution. Thus, the structure of the 24 micron emission is self-similar even at milliparsec scales. The combined power spectrum produced from Spitzer 24 micron and IRAS 25 micron images is consistent with a change in the power law exponent from -2.6 to -3.5. The decrease may be due to the transition from a two-dimensional to three-dimensional structure. Under this hypothesis, we estimate the thickness of the emitting medium to be 0.3 pc.Comment: 13 Pages, 3 Figures, to be published in Astrophysical Journal Supplement Series (Spitzer Special Issue), volume 154. Uses aastex v5.

Velocity centroids and the structure of interstellar turbulence: I. Analytical study

We present an analytical study of the statistical properties of integrated emission and velocity centroids for a slightly compressible turbulent slab model, to retrieve the underlying statistics of three-dimensional density and velocity fluctuations. Under the assumptions that the density and velocity fields are homogeneous and isotropic, we derive the expressions of the antenna temperature for an optically thin spectral line observation, and of its successive moments with respect to the line of sight velocity component, focusing on the zeroth (intensity or integrated emission I) and first (non-normalized velocity centroid C) moments. The ratio of the latter to the former is the normalized centroid C_0, whose expression can be linearized for small density fluctuations. To describe the statistics of I, C and C_0, we derive expansions of their autocorrelation functions in powers of density fluctuations and perform a lowest-order real-space calculation of their scaling behaviour, assuming that the density and velocity fields are fractional Brownian motions. We hence confirm, within the scope of this study, the property recently found numerically by Miville-Deschenes, Levrier and Falgarone (2003, ApJ, 593, 831) that the spectral index of the normalized centroid is equal to that of the full velocity field. However, it is also argued that, in order to retrieve the velocity statistics, normalization of centroids may actually not be the best way to remove the influence of density fluctuations. In this respect, we discuss the modified velocity centroids introduced by Lazarian and Esquivel (2003, ApJL, 592, 37) as a possible alternative. In a following paper, we shall present numerical studies aimed at assessing the validity domain of these results.Comment: 23 pages, 2 figures, to be published in Astronomy & Astrophysic

Prevalence and incidence of carpal tunnel syndrome in US working populations: pooled analysis of six prospective studies

OBJECTIVES: Most studies of carpal tunnel syndrome (CTS) incidence and prevalence among workers have been limited by small sample sizes or restricted to a small subset of jobs. We established a common CTS case definition and then pooled CTS prevalence and incidence data across six prospective studies of musculoskeletal outcomes to measure CTS frequency and allow better studies of etiology. METHODS: Six research groups collected prospective data at >50 workplaces including symptoms characteristic of CTS and electrodiagnostic studies (EDS) of the median and ulnar nerves across the dominant wrist. While study designs and the timing of data collection varied across groups, we were able to create a common CTS case definition incorporating both symptoms and EDS results from data that were collected in all studies. RESULTS: At the time of enrollment, 7.8% of 4321 subjects met our case definition and were considered prevalent cases of CTS. During 8833 person-years of follow-up, an additional 204 subjects met the CTS case definition for an overall incidence rate of 2.3 CTS cases per 100 person-years. CONCLUSIONS: Both prevalent and incident CTS were common in data pooled across multiple studies and sites. The large number of incident cases in this prospective study provides adequate power for future exposure– response analyses to identify work- and non-work-related risk factors for CTS. The prospective nature allows determination of the temporal relations necessary for causal inference

Genetic analysis of peripheral nerve conduction velocity in twins

We studied variation in peripheral nerve conduction velocity (PNCV) and intelligence in a group of 16-year-old Dutch twins. It has been suggested that both brain nerve conduction velocity and PNCV are positively correlated with intelligence (Reed, 1984) and that heritable differences in NCV may explain part of the well established heritability of intelligence. The Standard Progressive Matrices test was administered to 210 twin pairs to obtain IQ scores. Median nerve PNCV was determined in a subgroup of 156 pairs. Genetic analyses showed a heritability of 0.65 for Raven IQ score and 0.77 for PNCV. However, there was no significant phenotypic correlation between IQ score and PNCV. © 1995 Plenum Publishing Corporation