1,350 research outputs found
The Influence of the Degree of Heterogeneity on the Elastic Properties of Random Sphere Packings
The macroscopic mechanical properties of colloidal particle gels strongly
depend on the local arrangement of the powder particles. Experiments have shown
that more heterogeneous microstructures exhibit up to one order of magnitude
higher elastic properties than their more homogeneous counterparts at equal
volume fraction. In this paper, packings of spherical particles are used as
model structures to computationally investigate the elastic properties of
coagulated particle gels as a function of their degree of heterogeneity. The
discrete element model comprises a linear elastic contact law, particle bonding
and damping. The simulation parameters were calibrated using a homogeneous and
a heterogeneous microstructure originating from earlier Brownian dynamics
simulations. A systematic study of the elastic properties as a function of the
degree of heterogeneity was performed using two sets of microstructures
obtained from Brownian dynamics simulation and from the void expansion method.
Both sets cover a broad and to a large extent overlapping range of degrees of
heterogeneity. The simulations have shown that the elastic properties as a
function of the degree of heterogeneity are independent of the structure
generation algorithm and that the relation between the shear modulus and the
degree of heterogeneity can be well described by a power law. This suggests the
presence of a critical degree of heterogeneity and, therefore, a phase
transition between a phase with finite and one with zero elastic properties.Comment: 8 pages, 6 figures; Granular Matter (published online: 11. February
2012
H\"older equicontinuity of the integrated density of states at weak disorder
H\"older continuity, , with
a constant independent of the disorder strength is proved for the
integrated density of states associated to a discrete random
operator consisting of a translation invariant hopping
matrix and i.i.d. single site potentials with an absolutely
continuous distribution, under a regularity assumption for the hopping term.Comment: 15 Pages, typos corrected, comments and ref. [1] added, theorems 3,4
combine
A room temperature 19-channel magnetic field mapping device for cardiac signals
We present a multichannel cardiac magnetic field imaging system built in
Fribourg from optical double-resonance Cs vapor magnetometers. It consists of
25 individual sensors designed to record magnetic field maps of the beating
human heart by simultaneous measurements on a grid of 19 points over the chest.
The system is operated as an array of second order gradiometers using
sophisticated digitally controlled feedback loops.Comment: 3 pages, 3 figures, submitted to Applied Physics Letter
Diffusion of wave packets in a Markov random potential
We consider the evolution of a tight binding wave packet propagating in a
time dependent potential. If the potential evolves according to a stationary
Markov process, we show that the square amplitude of the wave packet converges,
after diffusive rescaling, to a solution of a heat equation.Comment: 19 pages, acknowledgments added and typos correcte
Does the Presence of a Measurable Blood Alcohol Level in a Potential Organ Donor Affect the Outcome of Liver Transplantation?
The widespread application of hepatic transplantation has created a tremendous demand for donor organs. An assessment of donor parameters is thought to be important in selecting good donors; however, the criteria utilized have not been standardized. This study was performed to determine the effect of a measurable donor blood alcohol level on graft survival. Fiftyâtwo patients who underwent orthotopic liver transplantation at the University of Pittsburgh were included in the study. Twentyâfive patients received liver grafts from donors having a blood alcohol level between 0.04 and 0.4 g/I with a mean of 0.17 g/I. Twentyâseven patients received a liver graft from a donor who had no measurable blood alcohol. There were no differences between these two groups of donors regarding the time of initial hospitalization until the time of donation. Graft failure within the first 30 days was 24% for those receiving an organ from an alcoholâpositive donor as compared with 22.2% in those receiving an organ from an alcohol negative donor. The recipient mortality rate was 16% and 11%, respectively. No relationships between the donor blood alcohol level and organ performance, frequency of primary graft nonfunction, or number of episodes of acute cellular rejection were evident. Based upon these data, the presence of a measurable blood alcohol level in a donor should not mitigate against organ donation. Copyright © 1991, Wiley Blackwell. All rights reserve
Diffusive propagation of wave packets in a fluctuating periodic potential
We consider the evolution of a tight binding wave packet propagating in a
fluctuating periodic potential. If the fluctuations stem from a stationary
Markov process satisfying certain technical criteria, we show that the square
amplitude of the wave packet after diffusive rescaling converges to a
superposition of solutions of a heat equation.Comment: 13 pages (v2: added a paragraph on the history of the problem, added
some references, correct a few typos; v3 minor corrections, added keywords
and subject classes
Survival Analysis Using Auxiliary Variables Via Multiple Imputation, with Application to AIDS Clinical Trial Data
We develop an approach, based on multiple imputation, to using auxiliary variables to recover information from censored observations in survival analysis. We apply the approach to data from an AIDS clinical trial comparing ZDV and placebo, in which CD4 count is the time-dependent auxiliary variable. To facilitate imputation, a joint model is developed for the data, which includes a hierarchical change-point model for CD4 counts and a time-dependent proportional hazards model for the time to AIDS. Markov chain Monte Carlo methods are used to multiply impute event times for censored cases. The augmented data are then analyzed and the results combined using standard multiple-imputation techniques. A comparison of our multiple-imputation approach to simply analyzing the observed data indicates that multiple imputation leads to a small change in the estimated effect of ZDV and smaller estimated standard errors. A sensitivity analysis suggests that the qualitative findings are reproducible under a variety of imputation models. A simulation study indicates that improved efficiency over standard analyses and partial corrections for dependent censoring can result. An issue that arises with our approach, however, is whether the analysis of primary interest and the imputation model are compatible.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66054/1/j.0006-341X.2002.00037.x.pd
A high-sensitivity laser-pumped Mx magnetometer
Abstract.: We discuss the design and performance of a laser-pumped cesium vapor magnetometer in the Mx configuration. The device will be employed in the control and stabilization of fluctuating magnetic fields and gradients in a new experiment searching for a permanent electric dipole moment of the neutron. We have determined the intrinsic sensitivity of the device to be 15âfT in a 1âHz bandwidth, limited by technical laser noise. In the shot noise limit the magnetometer can reach a sensitivity of 10âfT in a 1âHz bandwidth. We have used the device to study the fluctuations of a stable magnetic field in a multi-layer magnetic shield for integration times in the range of 2-100 seconds. The residual fluctuations for times up to a few minutes are traced back to the instability of the power supply used to generate the fiel
New gas mixtures suitable for rare event detection using a Micromegas-TPC detector
The aim of the presented work was to develop further techniques based on a
Micromegas-TPC, in order to reach a high gas gain with good energy resolution,
and to search for gas mixtures suitable for rare event detection. This paper
focuses on xenon, which is convenient for the search of neutrinoless double
beta decay in 136 Xe. Conversely, a small admixture of xenon to CF 4 can reduce
attachment in the latter. This gas mixture would be suitable for dark matter
searches and the study of solar and reactor neutrinos. Various configurations
of the Micromegas plane were investigated and are described.Comment: 14 pages, 8 figures, article, revised version with improved figures,
text modifications, accepted for publication by JINS
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