Thermal oxidation of reactively sputtered amorphous W_(80)N_(20) films

The oxidation behavior of reactively sputtered amorphous tungsten nitride of composition W_(80)N_(20) was investigated in dry and wet oxidizing ambient in the temperature range of 450 °C–575 °C. A single WO_3 oxide phase is observed. The growth of the oxide follows a parabolic time dependence which is attributed to a process controlled by the diffusivity of the oxidant in the oxide. The oxidation process is thermally activated with an activation energy of 2.5 ± 0.05 eV for dry ambient and 2.35 ± 0.05 eV for wet ambient. The pre‐exponential factor of the reaction constant for dry ambient is 1.1×10^(21) Å^2/min; that for wet ambient is only about 10 times less and is equal to 1.3×10^(20) Å^2/min

Identification of a forebrain motor programming network for the learned song of zebra finches

The stereotyped delivery of sequences of vocalizations by singing zebra finches is thought to be mediated by a “central motor program.” We hypothesized that electrically stimulating, and thus perturbing, the neural components of this motor program during singing should alter the subsequent singing pattern. In contrast, perturbing the activity of other neurons in the song motor pathway that do not participate directly in generating the song temporal pattern should not affect the singing pattern. We found that unilaterally stimulating the forebrain area RA of singing birds with chronically implanted electrodes distorted ongoing syllables without changing the order or timing of ensuing syllables. However, stimulating forebrain area HVc, which projects directly to RA, altered both ongoing syllables and the ensuing song pattern. These findings indicate that syllable sequencing during singing is organized in forebrain areas above RA (including HVc) and that the resulting pattern is imposed on lower structures of the motor pathway. Furthermore, the observation that unilateral forebrain perturbation was sufficient to alter the pattern of this bilaterally organized behavior suggests that (non-auditory) feedback pathways to the forebrain exist to coordinate the two hemispheres during singing. We suggest that the study of the motor control system for birdsong has provided the most direct evidence to date for localizing the programming of a skilled motor sequence to the telencephalon

Stable Propagation of a Burst Through a One-Dimensional Homogeneous Excitatory Chain Model of Songbird Nucleus HVC

We demonstrate numerically that a brief burst consisting of two to six spikes can propagate in a stable manner through a one-dimensional homogeneous feedforward chain of non-bursting neurons with excitatory synaptic connections. Our results are obtained for two kinds of neuronal models, leaky integrate-and-fire (LIF) neurons and Hodgkin-Huxley (HH) neurons with five conductances. Over a range of parameters such as the maximum synaptic conductance, both kinds of chains are found to have multiple attractors of propagating bursts, with each attractor being distinguished by the number of spikes and total duration of the propagating burst. These results make plausible the hypothesis that sparse precisely-timed sequential bursts observed in projection neurons of nucleus HVC of a singing zebra finch are intrinsic and causally related.Comment: 13 pages, 6 figure

Absence of bound states for waveguides in 2D periodic structures

We study a Helmholtz-type spectral problem in a two-dimensional medium consisting of a fully periodic background structure and a perturbation in form of a line defect. The defect is aligned along one of the coordinate axes, periodic in that direction (with the same periodicity as the background), and bounded in the other direction. This setting models a so-called "soft-wall" waveguide problem. We show that there are no bound states, i.e., the spectrum of the operator under study contains no point spectrum.Comment: This is an updated version of our paper (in slightly different form in Journal of Mathematical Physics). An anonymous reviewer kindly made us aware that ref. 10 is not applicable in our situation. An application of the theorem in ref. 10 would have proved the absence of singular continuous spectrum also. Our result on the absence of point spectrum is not affected by thi

Kinetics of four-wave mixing for a 2D magneto-plasma in strong magnetic fields

We investigate the femtosecond kinetics of an optically excited 2D magneto-plasma at intermediate and high densities under a strong magnetic field perpendicular to the quantum well (QW). We assume an additional weak lateral confinement which lifts the degeneracy of the Landau levels partially. We calculate the femtosecond dephasing and relaxation kinetics of the laser pulse excited magneto-plasma due to bare Coulomb potential scattering, because screening is under these conditions of minor importance. In particular the time-resolved and time-integrated four-wave mixing (FWM) signals are calculated by taking into account three Landau subbands in both the valance and the conduction band assuming an electron-hole symmetry. The FWM signals exhibit quantum beats mainly with twice the cyclotron frequency. Contrary to general expectations, we find no pronounced slowing down of the dephasing with increasing magnetic field. On the contrary, one obtains a decreasing dephasing time because of the increase of the Coulomb matrix elements and the number of states in a given Landau subband. In the situation when the loss of scattering channels exceeds these increasing effects, one gets a slight increase at the dephasing time. However, details of the strongly modulated scattering kinetics depend sensitively on the detuning, the plasma density, and the spectral pulse width relative to the cyclotron frequency.Comment: 13 pages, in RevTex format, 10 figures, Phys. Rev B in pres

Dynamic ductile to brittle transition in a one-dimensional model of viscoplasticity

We study two closely related, nonlinear models of a viscoplastic solid. These models capture essential features of plasticity over a wide range of strain rates and applied stresses. They exhibit inelastic strain relaxation and steady flow above a well defined yield stress. In this paper, we describe a first step in exploring the implications of these models for theories of fracture and related phenomena. We consider a one dimensional problem of decohesion from a substrate of a membrane that obeys the viscoplastic constitutive equations that we have constructed. We find that, quite generally, when the yield stress becomes smaller than some threshold value, the energy required for steady decohesion becomes a non-monotonic function of the decohesion speed. As a consequence, steady state decohesion at certain speeds becomes unstable. We believe that these results are relevant to understanding the ductile to brittle transition as well as fracture stability.Comment: 10 pages, REVTeX, 12 postscript figure

Diabetes mellitus increases risk for colorectal adenomas in younger patients

AIM: To determine if diabetes mellitus (DM) is associated with increased risk of colorectal adenomas in younger subjects. METHODS: This was a retrospective cohort study of 375 patients undergoing index colonoscopy at a single tertiary care center in the United States. Three cohorts of patients matched for exam date and gender were compared: (1) ages 40-49 years with DM; (2) ages 40-49 years without DM; and (3) ages 50-59 years without DM. Data collected included demographics, co-morbidities, colonoscopy and pathology results. Adenoma detection rates (ADR) were calculated and compared. Conditional logistic regression analysis was performed to determine the association between each cohort and ADR. RESULTS: One hundred and twenty-five patients ages 40-49 with DM met study eligibility criteria. Patients in the other two cohorts were randomly selected, matching for date of exam and gender. ADR was 14.4% in those ages 40-49 years without DM, 30.4% in those ages 40-49 years with DM, and 32.0% in those ages 50-59 years without DM. Compared to those ages 40-49 years without DM, ADR was higher in those ages 40-49 years with DM (OR = 3.1; 95%CI: 1.5-6.4; P = 0.002) and those ages 50-59 years without DM (OR = 2.9; 95%CI: 1.5-5.6; P = 0.002). There was no difference between the ADR in those ages 40-49 years with DM and those ages 50-59 years without DM (P = 0.83). CONCLUSION: DM was associated with higher risk of colorectal adenomas in patients ages 40-49 years. These subjects harbored as many adenomas as those at the standard screening age of 50-59 years without DM

Quantum mechanics in multiply connected spaces

This paper analyses quantum mechanics in multiply connected spaces. It is shown that the multiple connectedness of the configuration space of a physical system can determine the quantum nature of physical observables, such as the angular momentum. In particular, quantum mechanics in compactified Kaluza-Klein spaces is examined. These compactified spaces give rise to an additional angular momentum which can adopt half-integer values and, therefore, may be identified with the intrinsic spin of a quantum particle.Comment: Latex 15 page

Cryptic Lineages and a Population Damned to Incipient Extinction? Insights into the Genetic Structure of a Mekong River Catfish

An understanding of the genetic composition of populations across management boundaries is vital to developing successful strategies for sustaining biodiversity and food resources. This is especially important in ecosystems where habitat fragmentation has altered baseline patterns of gene flow, dividing natural populations into smaller sub-populations and increasing potential loss of genetic variation through genetic drift. River systems can be highly fragmented by dams built for flow regulation and hydropower. We used reduced-representation sequencing to examine genomic patterns in an exploited catfish, Hemibagrus spilopterus, in a hotspot of biodiversity and hydropower development- the Mekong River basin. Our results revealed the presence of two highly-divergent coexisting genetic lineages which may be cryptic species. Within the lineage with the greatest sample sizes, pairwise FST values, principal components analysis, and a STRUCTURE analysis all suggest that long-distance migration is not common across the Lower Mekong Basin, even in areas where flood-pulse hydrology has limited genetic divergence. In tributaries, effective population size estimates were at least an order of magnitude lower than in the Mekong mainstream indicating these populations may be more vulnerable to perturbations such as human-induced fragmentation. Fish isolated upstream of several dams in one tributary exhibited particularly low genetic diversity, high amounts of relatedness, and a level of inbreeding (GIS = 0.51) that has been associated with inbreeding depression in other outcrossing species. Our results highlight the importance of assessing genetic structure and diversity in riverine fisheries populations across proposed dam development sites for the preservation of these critically-important resources