7,160 research outputs found
Understanding the ideal glass transition: Lessons from an equilibrium study of hard disks in a channel
We use an exact transfer-matrix approach to compute the equilibrium
properties of a system of hard disks of diameter confined to a
two-dimensional channel of width at constant longitudinal
applied force. At this channel width, which is sufficient for
next-nearest-neighbor disks to interact, the system is known to have a great
many jammed states. Our calculations show that the longitudinal force
(pressure) extrapolates to infinity at a well-defined packing fraction
that is less than the maximum possible , the latter
corresponding to a buckled crystal. In this quasi-one-dimensional problem there
is no question of there being any \emph{real} divergence of the pressure at
. We give arguments that this avoided phase transition is a structural
feature -- the remnant in our narrow channel system of the hexatic to crystal
transition -- but that it has the phenomenology of the (avoided) ideal glass
transition. We identify a length scale as our equivalent of the
penetration length for amorphous order: In the channel system, it reaches a
maximum value of around at , which is larger than the
penetration lengths that have been reported for three dimensional systems. It
is argued that the -relaxation time would appear on extrapolation to
diverge in a Vogel-Fulcher manner as the packing fraction approaches .Comment: 17 pages, 16 figure
Absence of hyperuniformity in amorphous hard-sphere packings of nonvanishing complexity
We relate the structure factor in a system of
jammed hard spheres of number density to its complexity per particle
by the formula . We have verified this formula for
the case of jammed disks in a narrow channel, for which it is possible to find
and analytically. Hyperuniformity, which is the
vanishing of , will therefore not occur if the
complexity is nonzero. An example is given of a jammed state of hard disks in a
narrow channel which is hyperuniform when generated by dynamical rules that
produce a non-extensive complexity.Comment: 5 pages, 3 figure
The validity of capillary blood sampling in the determination of human growth hormone concentration during exercise in men
This is an open access article - Copyright © 2004 BMJ Publishing Group LtdBACKGROUND: Studies measuring human growth hormone (hGH) in blood during exercise have mainly used venous sampling. The invasive nature of this procedure makes evaluation of hGH impossible in various exercise environments.
OBJECTIVE: To determine whether capillary sampling could offer an alternative sampling method.
METHODS: Capillary and venous blood samples were collected for determination of hGH at the end of each exercise stage during an incremental exercise test in 16 male club level competitive cyclists (mean (SD) age 30.8 (8.0) years, body mass 72.2 (7.1) kg, body fat 12.9 (3.5)%, peak oxygen consumption 4.18 (0.46) l⋅min−1). Linear regression, from a plot of venous v capillary blood hGH concentration, showed a correlation coefficient of r = 0.986 (p<0.001). When geometric means and log transformations were used, a coefficient of variation of 14.2% was demonstrated between venous and capillary flow for hGH concentration. The mean ratio limits of agreement were 0.62 (1.72)—that is, 95% of the ratios were contained between 0.36 and 1.07, with a mean of 0.62.
CONCLUSIONS: Capillary blood sampling is an acceptable alternative to venous sampling for determining hGH concentration during rest and exercise. Sample sites should not be used interchangeably: one site should be chosen and its use standardised
Transition state theory and the dynamics of hard disks
The dynamics of two and five disk systems confined in a square has been
studied using molecular dynamics simulations and compared with the predictions
of transition state theory. We determine the partition functions Z and
Z^\ddagger of transition state theory using a procedure first used by Salsburg
and Wood for the pressure. Our simulations show this procedure and transition
state theory are in excellent agreement with the simulations. A generalization
of the transition state theory to the case of a large number of disks N is made
and shown to be in full agreement with simulations of disks moving in a narrow
channel. The same procedure for hard spheres in three dimensions leads to the
Vogel-Fulcher-Tammann formula for their alpha relaxation time.Comment: 1 new author, new simulations and figures, less speculation. Now 6
pages, 6 figures, 1 animation. Animation may be viewed at
http://www.theory.physics.manchester.ac.uk/~godfrey/supplement/activated_dynamics2.htm
Lentiviral manipulation of gene expression in human adult and embryonic stem cells
Human stem cells could revolutionize the field of medicine by providing a diverse range of cell types for tissue replacement therapies and drug discovery. To achieve this goal, genetic tools need to be optimized and developed for controlling and manipulating stem cells ex vivo. Here we describe a lentiviral delivery system capable of high infection rates in human mesenchymal and embryonic stem cells. The lentiviral backbone was modified to express mono- and bi-cistronic transgenes and was also used to deliver short hairpin ribonucleic acid for specific silencing of gene expression in human stem cells. We show that lentiviral transduction can be used to alter gene expression without altering the genes' ability to differentiate in vitro. These vectors will enable rapid analysis of gene function in stem cells and permit the generation of knock-in / knock-out models of human disease in the rapidly developing field of gene therapy
Two-stage clustering in genotype-by-environment analyses with missing data
Cluster analysis has been commonly used in genotype-by-environment (G x E) analyses, but current methods are inadequate when the data matrix is incomplete. This paper proposes a new method, referred to as two-stage clustering, which relies on a partitioning of squared Euclidean distance into
two independent components, the G x E interaction and the genotype main effect. These components are used in the first and second stages of clustering respectively. Two-stage clustering forms the basis for imputing missing values in the G x E matrix so that a more complete data array is available for other GxE analyses. Imputation for a given genotype uses information from genotypes with similar interaction profiles. This imputation method is shown to improve on an existing nearest cluster method that confounds the G x E interaction and the genotype main effect
Properties of the Strange Axial Mesons in the Relativized Quark Model
We studied properties of the strange axial mesons in the relativized quark
model. We calculated the decay constant in the quark model and showed how
it can be used to extract the mixing angle
() from the weak decay . The ratio is the most sensitive
measurement and also the most reliable since the largest of the theoretical
uncertainties factor out. However the current bounds extracted from the
TPC/Two-Gamma collaboration measurements are rather weak: we typically obtain
at 68\% C.L. We also calculated the
strong OZI-allowed decays in the pseudoscalar emission model and the flux-tube
breaking model and extracted a mixing angle of . Our analysis also indicates that the heavy quark limit does not give a
good description of the strange mesons.Comment: Revised version to be published in Phys. Rev. D. Minor changes. Latex
file uses revtex version 3 and epsfig, 4 postcript figures are attached. The
full postcript version with embedded figures is available at
ftp://ftp.physics.carleton.ca/pub/theory/godfrey/ocipc9512.ps.
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