403 research outputs found
DMRG studies of the effect of constraint release on the viscosity of polymer melts
The scaling of the viscosity of polymer melts is investigated with regard to
the molecular weight. We present a generalization of the Rubinstein-Duke model,
which takes constraint releases into account and calculate the effects on the
viscosity by the use of the Density Matrix Renormalization Group (DMRG)
algorithm. Using input from Rouse theory the rates for the constraint release
are determined in a self consistent way. We conclude that shape fluctuations of
the tube caused by constraint release are not a likely candidate for improving
Doi's crossover theory for the scaling of the polymer viscosity.Comment: 6 pages, 8 figure
Quasars and their host galaxies
This review attempts to describe developments in the fields of quasar and
quasar host galaxies in the past five. In this time period, the Sloan and 2dF
quasar surveys have added several tens of thousands of quasars, with Sloan
quasars being found to z>6. Obscured, or partially obscured quasars have begun
to be found in significant numbers. Black hole mass estimates for quasars, and
our confidence in them, have improved significantly, allowing a start on
relating quasar properties such as radio jet power to fundamental parameters of
the quasar such as black hole mass and accretion rate. Quasar host galaxy
studies have allowed us to find and characterize the host galaxies of quasars
to z>2. Despite these developments, many questions remain unresolved, in
particular the origin of the close relationship between black hole mass and
galaxy bulge mass/velocity dispersion seen in local galaxies.Comment: Review article, to appear in Astrophysics Update
Destruction of diagonal and off-diagonal long range order by disorder in two-dimensional hard core boson systems
We use quantum Monte Carlo simulations to study the effect of disorder, in
the form of a disordered chemical potential, on the phase diagram of the hard
core bosonic Hubbard model in two dimensions. We find numerical evidence that
in two dimensions, no matter how weak the disorder, it will always destroy the
long range density wave order (checkerboard solid) present at half filling and
strong nearest neighbor repulsion and replace it with a bose glass phase. We
study the properties of this glassy phase including the superfluid density,
energy gaps and the full Green's function. We also study the possibility of
other localized phases at weak nearest neighbor repulsion, i.e. Anderson
localization. We find that such a phase does not truly exist: The disorder must
exceed a threshold before the bosons (at weak nn repulsion) are localized. The
phase diagram for hard core bosons with disorder cannot be obtained easily from
the soft core phase diagram discussed in the literature.Comment: 7 pages, 10 eps figures include
SU(3) Predictions for Weak Decays of Doubly Heavy Baryons -- including SU(3) breaking terms
We find expressions for the weak decay amplitudes of baryons containing two b
quarks (or one b and one c quark -- many relationship are the same) in terms of
unknown reduced matrix elements. This project was originally motivated by the
request of the FNAL Run II b Physics Workshop organizers for a guide to
experimentalists in their search for as yet unobserved hadrons. We include an
analysis of linear SU(3) breaking terms in addition to relationships generated
by unbroken SU(3) symmetry, and relate these to expressions in terms of the
complete set of possible reduced matrix elements.Comment: 49 page
General boundary conditions for the envelope function in multiband k.p model
We have derived general boundary conditions (BC) for the multiband envelope
functions (which do not contain spurious solutions) in semiconductor
heterostructures with abrupt heterointerfaces. These BC require the
conservation of the probability flux density normal to the interface and
guarantee that the multiband Hamiltonian be self--adjoint. The BC are energy
independent and are characteristic properties of the interface. Calculations
have been performed of the effect of the general BC on the electron energy
levels in a potential well with infinite potential barriers using a coupled two
band model. The connection with other approaches to determining BC for the
envelope function and to the spurious solution problem in the multiband k.p
model are discussed.Comment: 15 pages, 2 figures; to be published in Phys. Rev. B 65, March 15
issue 200
Whole Blood Profiling of T-cell-Derived microRNA Allows the Development of Prognostic models in Inflammatory Bowel Disease
Background: MicroRNAs [miRNAs] are cell-specific small non-coding RNAs that can regulate gene expression and have been implicated in inflammatory bowel disease [IBD] pathogenesis. Here we define the cell-specific miRNA profiles and investigate its biomarker potential in IBD.
Methods: In a two-stage prospective multi-centre case control study, next generation sequencing was performed on a discovery cohort of immunomagnetically separated leukocytes from 32 patients (nine Crohn''s disease [CD], 14 ulcerative colitis [UC], eight healthy controls) and differentially expressed signals were validated in whole blood in 294 patients [97 UC, 98 CD, 98 non-IBD, 1 IBDU] using quantitative PCR. Correlations were analysed with phenotype, including need for early treatment escalation as a marker of progressive disease using Cox proportional hazards.
Results: In stage 1, each leukocyte subset [CD4+ and CD8+ T-cells and CD14+ monocytes] was analysed in IBD and controls. Three specific miRNAs differentiated IBD from controls in CD4+ T-cells, including miR-1307-3p [p = 0.01], miR-3615 [p = 0.02] and miR-4792 [p = 0.01]. In the extension cohort, in stage 2, miR-1307-3p was able to predict disease progression in IBD (hazard ratio [HR] 1.98, interquartile range [IQR]: 1.20-3.27; logrank p = 1.80 × 10-3), in particular CD [HR 2.81; IQR: 1.11-3.53, p = 6.50 × 10-4]. Using blood-based multimarker miRNA models, the estimated chance of escalation in CD was 83% if two or more criteria were met and 90% for UC if three or more criteria are met.
Interpretation: We have identified and validated unique CD4+ T-cell miRNAs that are differentially regulated in IBD. These miRNAs may be able to predict treatment escalation and have the potential for clinical translation; further prospective evaluation is now indicated
Onset of Superfluidity in 4He Films Adsorbed on Disordered Substrates
We have studied 4He films adsorbed in two porous glasses, aerogel and Vycor,
using high precision torsional oscillator and DC calorimetry techniques. Our
investigation focused on the onset of superfluidity at low temperatures as the
4He coverage is increased. Torsional oscillator measurements of the 4He-aerogel
system were used to determine the superfluid density of films with transition
temperatures as low as 20 mK. Heat capacity measurements of the 4He-Vycor
system probed the excitation spectrum of both non-superfluid and superfluid
films for temperatures down to 10 mK. Both sets of measurements suggest that
the critical coverage for the onset of superfluidity corresponds to a mobility
edge in the chemical potential, so that the onset transition is the bosonic
analog of a superconductor-insulator transition. The superfluid density
measurements, however, are not in agreement with the scaling theory of an onset
transition from a gapless, Bose glass phase to a superfluid. The heat capacity
measurements show that the non-superfluid phase is better characterized as an
insulator with a gap.Comment: 15 pages (RevTex), 21 figures (postscript
Quintessence and Gravitational Waves
We investigate some aspects of quintessence models with a non-minimally
coupled scalar field and in particular we show that it can behave as a
component of matter with . We study the
properties of gravitational waves in this class of models and discuss their
energy spectrum and the cosmic microwave background anisotropies they induce.
We also show that gravitational waves are damped by the anisotropic stress of
the radiation and that their energy spectrum may help to distinguish between
inverse power law potential and supergravity motivated potential. We finish by
a discussion on the constraints arising from their density parameter
\Omega_\GW.Comment: 21 pages, 18 figures, fianl version, accepted for publication in PR
First Observation of Coherent Production in Neutrino Nucleus Interactions with 2 GeV
The MiniBooNE experiment at Fermilab has amassed the largest sample to date
of s produced in neutral current (NC) neutrino-nucleus interactions at
low energy. This paper reports a measurement of the momentum distribution of
s produced in mineral oil (CH) and the first observation of coherent
production below 2 GeV. In the forward direction, the yield of events
observed above the expectation for resonant production is attributed primarily
to coherent production off carbon, but may also include a small contribution
from diffractive production on hydrogen. Integrated over the MiniBooNE neutrino
flux, the sum of the NC coherent and diffractive modes is found to be (19.5
1.1 (stat) 2.5 (sys))% of all exclusive NC production at
MiniBooNE. These measurements are of immediate utility because they quantify an
important background to MiniBooNE's search for
oscillations.Comment: Submitted to Phys. Lett.
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