Localized low-frequency vibrational modes in glasses

This is the publisher's version, also available electronically from http://journals.aps.org/prb/abstract/10.1103/PhysRevB.44.6746.Using a larger data set and more detailed analysis, we continue our examination of the zero-temperature vibrational spectrum of a glass of soft spheres by computer simulation. As in our previous study [Phys. Rev. Lett. 66, 636 (1991)], a normal-mode analysis shows clearly the existence of (quasi-)localized modes at the low-frequency end of the vibrational spectrum. The modes are found to be localized around atoms whose neighborhood structure differs significantly from the average glass environment and is reminiscent of that associated with interstitial defects in crystals. The effective masses of these modes range upward from 10 atomic masses

Localized low-frequency vibrational modes in a simple model glass

This is the publisher's version, also available electronically from http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.66.636We examine the vibrational spectrum of a glass of soft spheres produced by quenching an equilibrated liquid (produced via constant-energy molecular-dynamics simulation) to zero temperature. Normal-mode analysis shows clearly the existence of (quasi)localized modes at the low-frequency end of the vibrational spectrum. The modes are found to be localized around atoms whose neighborhood structure differs significantly from the average glass environment. The effective masses of these modes range upwards from 10 atomic masses

Hermiticity Correlation Study

No abstract availabl

Sequential Logistic Regression: A Method to Reveal Subtlety in Self-Efficacy

This paper uses self-efficacy to predict the success of women in introductory physics. We show how sequential logistic regression demonstrates the predictive ability of self-efficacy, and reveals variations with type of physics course. Also discussed are the sources of self-efficacy that have the largest impact on predictive ability

Ozone Depletion from Nearby Supernovae

Estimates made in the 1970's indicated that a supernova occurring within tens of parsecs of Earth could have significant effects on the ozone layer. Since that time, improved tools for detailed modeling of atmospheric chemistry have been developed to calculate ozone depletion, and advances have been made in theoretical modeling of supernovae and of the resultant gamma-ray spectra. In addition, one now has better knowledge of the occurrence rate of supernovae in the galaxy, and of the spatial distribution of progenitors to core-collapse supernovae. We report here the results of two-dimensional atmospheric model calculations that take as input the spectral energy distribution of a supernova, adopting various distances from Earth and various latitude impact angles. In separate simulations we calculate the ozone depletion due to both gamma-rays and cosmic rays. We find that for the combined ozone depletion roughly to double the ``biologically active'' UV flux received at the surface of the Earth, the supernova must occur at <8 pc. Based on the latest data, the time-averaged galactic rate of core-collapse supernovae occurring within 8 pc is ~1.5/Gyr. In comparing our calculated ozone depletions with those of previous studies, we find them to be significantly less severe than found by Ruderman (1974), and consistent with Whitten et al. (1976). In summary, given the amplitude of the effect, the rate of nearby supernovae, and the ~Gyr time scale for multicellular organisms on Earth, this particular pathway for mass extinctions may be less important than previously thought.Comment: 24 pages, 4 Postscript figures, to appear in The Astrophysical Journal, 2003 March 10, vol. 58

Effects of chlorinated seawater on decapod crustaceans and Mulinia larvae

Eggs and larvae of decapod crustaceans and embryos of Mulinia laterulis were exposed to chlorinated seawater for varying periods in continuous flow systems. Mortality, developmental rate, and general behavior were recorded

Deletion of astroglial CXCL10 delays clinical onset but does not affect progressive axon loss in a murine autoimmune multiple sclerosis model.

Multiple sclerosis (MS) is characterized by central nervous system (CNS) inflammation, demyelination, and axonal degeneration. CXCL10 (IP-10), a chemokine for CXCR3+ T cells, is known to regulate T cell differentiation and migration in the periphery, but effects of CXCL10 produced endogenously in the CNS on immune cell trafficking are unknown. We created floxed cxcl10 mice and crossed them with mice carrying an astrocyte-specific Cre transgene (mGFAPcre) to ablate astroglial CXCL10 synthesis. These mice, and littermate controls, were immunized with myelin oligodendrocyte glycoprotein peptide 35-55 (MOG peptide) to induce experimental autoimmune encephalomyelitis (EAE). In comparison to the control mice, spinal cord CXCL10 mRNA and protein were sharply diminished in the mGFAPcre/CXCL10fl/fl EAE mice, confirming that astroglia are chiefly responsible for EAE-induced CNS CXCL10 synthesis. Astroglial CXCL10 deletion did not significantly alter the overall composition of CD4+ lymphocytes and CD11b+ cells in the acutely inflamed CNS, but did diminish accumulation of CD4+ lymphocytes in the spinal cord perivascular spaces. Furthermore, IBA1+ microglia/macrophage accumulation within the lesions was not affected by CXCL10 deletion. Clinical deficits were milder and acute demyelination was substantially reduced in the astroglial CXCL10-deleted EAE mice, but long-term axon loss was equally severe in the two groups. We concluded that astroglial CXCL10 enhances spinal cord perivascular CD4+ lymphocyte accumulation and acute spinal cord demyelination in MOG peptide EAE, but does not play an important role in progressive axon loss in this MS model

On the connection between Gaussian statistics and excited-state linear response for time-dependent fluorescence

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/126/21/10.1063/1.2747237.Time-dependent fluorescence (TDF) of a chromophore in a polar or nonpolar solvent is frequently simulated using linear-response approximations. It is shown that one such linear-response-type approximation for the TDF Stokes shift derived by Carter and Hynes [J. Chem. Phys.94, 5961 (1991)] that is based on excited-statedynamics gives the same result as that obtained by assuming Gaussian statistics for the energy gap. The derivation provides insight into the much discussed relationship between linear response and Gaussian statistics. In particular, subtle but important differences between the two approximations are illuminated that suggest that the result is likely more generally applicable than suggested by the usual linearization procedure. In addition, the assumption of Gaussian statistics directly points to straightforward checks of the validity of the approximation with essentially no additional computational effort

Interplay between distribution of live cells and growth dynamics of solid tumours

Experiments show that simple diffusion of nutrients and waste molecules is not sufficient to explain the typical multilayered structure of solid tumours, where an outer rim of proliferating cells surrounds a layer of quiescent but viable cells and a central necrotic region. These experiments challenge models of tumour growth based exclusively on diffusion. Here we propose a model of tumour growth that incorporates the volume dynamics and the distribution of cells within the viable cell rim. The model is suggested by in silico experiments and is validated using in vitro data. The results correlate with in vivo data as well, and the model can be used to support experimental and clinical oncology

Spectroscopy of 19^{19}Ne for the thermonuclear 15^{15}O(α,γ\alpha,\gamma)19^{19}Ne and 18^{18}F(p,αp,\alpha)15^{15}O reaction rates

Uncertainties in the thermonuclear rates of the $^{15}$O($\alpha,\gamma$)$^{19}$Ne and $^{18}$F($p,\alpha$)$^{15}$O reactions affect model predictions of light curves from type I X-ray bursts and the amount of the observable radioisotope $^{18}$F produced in classical novae, respectively. To address these uncertainties, we have studied the nuclear structure of $^{19}$Ne over $E_{x} = 4.0 - 5.1$ MeV and $6.1 - 7.3$ MeV using the $^{19}$F($^{3}$He,t)$^{19}$Ne reaction. We find the $J^{\pi}$ values of the 4.14 and 4.20 MeV levels to be consistent with $9/2^{-}$ and $7/2^{-}$ respectively, in contrast to previous assumptions. We confirm the recently observed triplet of states around 6.4 MeV, and find evidence that the state at 6.29 MeV, just below the proton threshold, is either broad or a doublet. Our data also suggest that predicted but yet unobserved levels may exist near the 6.86 MeV state. Higher resolution experiments are urgently needed to further clarify the structure of $^{19}$Ne around the proton threshold before a reliable $^{18}$F($p,\alpha$)$^{15}$O rate for nova models can be determined.Comment: 5 pages, 3 figures, Phys. Rev. C (in press