675 research outputs found
Scattering properties of a cut-circle billiard waveguide with two conical leads
We examine a two-dimensional electron waveguide with a cut-circle cavity and
conical leads. By considering Wigner delay times and the Landauer-B\"{u}ttiker
conductance for this system, we probe the effects of the closed billiard energy
spectrum on scattering properties in the limit of weakly coupled leads. We
investigate how lead placement and cavity shape affect these conductance and
time delay spectra of the waveguide.Comment: 18 pages, 11 figures, accepted for publication in Phys. Rev. E (Jan.
2001
Integrability and action operators in quantum Hamiltonian systems
For a (classically) integrable quantum mechanical system with two degrees of
freedom, the functional dependence of the
Hamiltonian operator on the action operators is analyzed and compared with the
corresponding functional relationship in
the classical limit of that system. The former is shown to converge toward the
latter in some asymptotic regime associated with the classical limit, but the
convergence is, in general, non-uniform. The existence of the function
in the integrable regime of a parametric
quantum system explains empirical results for the dimensionality of manifolds
in parameter space on which at least two levels are degenerate. The comparative
analysis is carried out for an integrable one-parameter two-spin model.
Additional results presented for the (integrable) circular billiard model
illuminate the same conclusions from a different angle.Comment: 9 page
Ninth and Tenth Order Virial Coefficients for Hard Spheres in D Dimensions
We evaluate the virial coefficients B_k for k<=10 for hard spheres in
dimensions D=2,...,8. Virial coefficients with k even are found to be negative
when D>=5. This provides strong evidence that the leading singularity for the
virial series lies away from the positive real axis when D>=5. Further analysis
provides evidence that negative virial coefficients will be seen for some k>10
for D=4, and there is a distinct possibility that negative virial coefficients
will also eventually occur for D=3.Comment: 33 pages, 12 figure
Monte Carlo results for the hydrogen Hugoniot
We propose a theoretical Hugoniot obtained by combining results for the
equation of state (EOS) from the Direct Path Integral Monte Carlo technique
(DPIMC) and those from Reaction Ensemble Monte Carlo (REMC) simulations. The
main idea of such proposal is based on the fact that DPMIC provides
first-principle results for a wide range of densities and temperatures
including the region of partially ionized plasmas. On the other hand, for lower
temperatures where the formation of molecules becomes dominant, DPIMC
simulations become cumbersome and inefficient. For this region it is possible
to use accurate REMC simulations where bound states (molecules) are treated on
the Born-Oppenheimer level using a binding potential calculated by Kolos and
Wolniewicz. The remaining interaction is then reduced to the scattering between
neutral particles which is reliably treated classically applying effective
potentials. The resulting Hugoniot is located between the experimental values
of Knudson {\textit{et al.}} \cite{1} and Collins {\textit{et al.}} \cite{2}.Comment: 10 pges, 2 figures, 2 table
On the non-persistence of Hamiltonian identity cycles
We study the leading term of the holonomy map of a perturbed plane polynomial
Hamiltonian foliation. The non-vanishing of this term implies the
non-persistence of the corresponding Hamiltonian identity cycle. We prove that
this does happen for generic perturbations and cycles, as well for cycles which
are commutators in Hamiltonian foliations of degree two. Our approach relies on
the Chen's theory of iterated path integrals which we briefly resume.Comment: 17 page
Structural Phase Transition at High Temperatures in Solid Molecular Hydrogen and Deuterium
We study the effect of temperature up to 1000K on the structure of dense
molecular para-hydrogen and ortho-deuterium, using the path-integral Monte
Carlo method. We find a structural phase transition from orientationally
disordered hexagonal close packed (hcp) to an orthorhombic structure of Cmca
symmetry before melting. The transition is basically induced by thermal
fluctuations, but quantum fluctuations of protons (deuterons) are important in
determining the transition temperature through effectively hardening the
intermolecular interaction. We estimate the phase line between hcp and Cmca
phases as well as the melting line of the Cmca solid.Comment: 8 pages, 7 figures; accepted in Phys. Rev.
A longitudinal study of allergy and intestinal helminth infections in semi urban and rural areas of Flores, Indonesia (ImmunoSPIN Study)
BACKGROUND: The prevalence of asthma and atopic disease has been reported to be low in low income countries, however helminth infections are likely to be high among these communities. The question of whether helminth infections play a role in allergic diseases can best be addressed by intervention studies. None of the studies so far have been based on a large scale placebo-controlled trial. METHOD/DESIGN: This study was designed to assess how intestinal helminth infections can influence the immune response and atopic and allergic disorders in children in Indonesia. The relations between allergic outcomes and infection and lifestyle factors will be addressed. This study was set up among school-age children in semi urban and rural areas, located in Ende District of Flores Island, Indonesia. A randomized placebo-controlled anthelmintic treatment trial to elucidate the impact of helminth infections on the prevalence of skin prick test (SPT) reactivity and symptoms of allergic diseases will be performed. The children living in these semi-urban and rural areas will be assessed for SPT to allergens before and after 1 and 2 years of treatment as the primary outcome of the study; the secondary outcome is symptoms (asthma and atopic dermatitis); while the tertiary outcome is immune responses (both antibody levels to allergens and cellular immune responses). DISCUSSION: The study will provide information on the influence of helminth infections and anthelmintic treatment on immune response, atopy and allergic disorders. TRIAL REGISTRATION: Current Controlled Trials ISRCTN: ISRCTN8383081
Gut microbiota composition and development of atopic manifestations in infancy: the KOALA birth cohort study
BACKGROUND AND AIMS: Perturbations in intestinal microbiota composition due to lifestyle changes may be involved in the development of atopic diseases. We examined gut microbiota composition in early infancy and the subsequent development of atopic manifestations and sensitisation. METHODS: The faeces of 957 infants aged 1 month and participating in the KOALA Birth Cohort Study were analysed using quantitative real-time PCR. Information on atopic symptoms (eczema, wheeze) and potential confounders was acquired through repeated questionnaires. Total and specific IgE were measured in venous blood samples collected during home visits when the infant was 2 years old. During these home visits a clinical diagnosis of atopic dermatitis was made according to the UK-Working Party criteria. RESULTS: The presence of Escherichia coli was associated with a higher risk of developing eczema (OR(adj) = 1.87; 95% CI 1.15 to 3.04), this risk being increased with increasing numbers of E coli (p(for trend) = 0.016). Infants colonised with Clostridium difficile were at higher risk of developing eczema (OR(adj) = 1.40; 95% CI 1.02 to 1.91), recurrent wheeze (OR(adj) = 1.75; 95% CI 1.09 to 2.80) and allergic sensitisation (OR(adj) = 1.54; 95% CI 1.02 to 2.31). Furthermore, the presence of C difficile was also associated with a higher risk of a diagnosis of atopic dermatitis during the home visit (OR(adj) = 1.73; 95% CI 1.08 to 2.78). CONCLUSION: This study demonstrates that differences in gut microbiota composition precede the development of atopy. Since E coli was only associated with eczema and C difficile was associated with all atopic outcomes, the underlying mechanisms explaining these association may be differen
GALEX UV Color-Magnitude Relations and Evidence for Recent Star Formation in Early-type Galaxies
We have used the GALEX UV photometric data to construct a first
near-ultraviolet (NUV) color-magnitude relation (CMR) for the galaxies
pre-classified as early-type by SDSS studies. The NUV CMR is a powerful tool
for tracking the recent star formation history in early-type galaxies, owing to
its high sensitivity to the presence of young stellar populations. Our NUV CMR
for UV-weak galaxies shows a well-defined slope and thus will be useful for
interpreting the restframe NUV data of distant galaxies and studying their star
formation history. Compared to optical CMRs, the NUV CMR shows a substantially
larger scatter, which we interpret as evidence of recent star formation
activities. Roughly 15% of the recent epoch (z < 0.13) bright (M[r] < -22)
early-type galaxies show a sign of recent (< 1Gyr) star formation at the 1-2%
level (lower limit) in mass compared to the total stellar mass. This implies
that low level residual star formation was common during the last few billion
years even in bright early-type galaxies.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http://www.galex.caltech.edu/PUBLICATIONS/ after
November 22, 200
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