2,238 research outputs found
Finite Temperature Time-Dependent Effective Theory For The Goldstone Field In A BCS-Type Superfluid
We extend to finite temperature the time-dependent effective theory for the
Goldstone field (the phase of the pair field) which is appropriate
for a superfluid containing one species of fermions with s-wave interactions,
described by the BCS Lagrangian. We show that, when Landau damping is
neglected, the effective theory can be written as a local time-dependent
non-linear Schr\"{o}dinger Lagrangian (TDNLSL) which preserves the Galilean
invariance of the zero temperature effective theory and is identified with the
superfluid component. We then calculate the relevant Landau terms which are
non-local and which destroy the Galilean invariance. We show that the retarded
-propagator (in momentum space) can be well represented by two poles in
the lower-half frequency plane, describing damping with a predicted
temperature, frequency and momentum dependence. It is argued that the real
parts of the Landau terms can be approximately interpreted as contributing to
the normal fluid component.Comment: 25 pages, 5 figures, references added, Introduction rewritte
Infrared divergence in QED at finite temperature
We consider various ways of treating the infrared divergence which appears in
the dynamically generated fermion mass, when the transverse part of the photon
propagator in N flavour at finite temperature is included in the
Matsubara formalism. This divergence is likely to be an artefact of taking into
account only the leading order term in the expansion when we
calculate the photon propagator and is handled here phenomenologically by means
of an infrared cutoff. Inserting both the longitudinal and the transverse part
of the photon propagator in the Schwinger-Dyson equation we find the dependence
of the dynamically generated fermion mass on the temperature and the cutoff
parameters. It turns out that consistency with certain statistical physics
arguments imposes conditions on the cutoff parameters. For parameters in the
allowed range of values we find that the ratio is approximately 6, consistently with previous calculations which
neglected the transverse photon contribution.Comment: 37 pages, 12 figures, typos corrected, references added, Introduction
rewritte
Which way up? Recognition of homologous DNA segments in parallel and antiparallel alignment
Homologous gene shuffling between DNA promotes genetic diversity and is an
important pathway for DNA repair. For this to occur, homologous genes need to
find and recognize each other. However, despite its central role in homologous
recombination, the mechanism of homology recognition is still an unsolved
puzzle. While specific proteins are known to play a role at later stages of
recombination, an initial coarse grained recognition step has been proposed.
This relies on the sequence dependence of the DNA structural parameters, such
as twist and rise, mediated by intermolecular interactions, in particular
electrostatic ones. In this proposed mechanism, sequences having the same base
pair text, or are homologous, have lower interaction energy than those
sequences with uncorrelated base pair texts; the difference termed the
recognition energy. Here, we probe how the recognition energy changes when one
DNA fragment slides past another, and consider, for the first time, homologous
sequences in antiparallel alignment. This dependence on sliding was termed the
recognition well. We find that there is recognition well for anti-parallel,
homologous DNA tracts, but only a very shallow one, so that their interaction
will differ little from the interaction between two nonhomologous tracts. This
fact may be utilized in single molecule experiments specially targeted to test
the theory. As well as this, we test previous theoretical approximations in
calculating the recognition well for parallel molecules against MC simulations,
and consider more rigorously the optimization of the orientations of the
fragments about their long axes. The more rigorous treatment affects the
recognition energy a little, when the molecules are considered rigid. However
when torsional flexibility of the DNA molecules is introduced, we find
excellent agreement between analytical approximation and simulation.Comment: Paper with supplemental material attached. 41 pages in all, 4 figures
in main text, 3 figures in supplmental. To be submitted to Journa
Renormalization group and 1/N expansion for 3-dimensional Ginzburg-Landau-Wilson models
A renormalization-group scheme is developed for the 3-dimensional
O()-symmetric Ginzburg-Landau-Wilson model, which is consistent with the
use of a 1/N expansion as a systematic method of approximation. It is motivated
by an application to the critical properties of superconductors, reported in a
separate paper. Within this scheme, the infrared stable fixed point controlling
critical behaviour appears at , where is the inverse of
the quartic coupling constant, and an efficient renormalization procedure
consists in the minimal subtraction of ultraviolet divergences at . This
scheme is implemented at next-to-leading order, and the standard results for
critical exponents calculated by other means are recovered. An apparently novel
result of this non-perturbative method of approximation is that corrections to
scaling (or confluent singularities) do not, as in perturbative analyses,
appear as simple power series in the variable . At least in
three dimensions, the power series are modified by powers of .Comment: 20 pages; 5 figure
Intra-Individual Behavioural Variability:A Trait under Genetic Control
When individuals are measured more than once in the same context they do not behave in exactly the same way each time. The degree of predictability differs between individuals, with some individuals showing low levels of variation around their behavioural mean while others show high levels of variation. This intra-individual variability in behaviour has received much less attention than between-individual variability in behaviour, and very little is known about the underlying mechanisms that affect this potentially large but understudied component of behavioural variation. In this study, we combine standardized behavioural tests in a chicken intercross to estimate intra-individual behavioural variability with a large-scale genomics analysis to identify genes affecting intra-individual behavioural variability in an avian population. We used a variety of different anxiety-related behavioural phenotypes for this purpose. Our study shows that intra-individual variability in behaviour has a direct genetic basis that is largely unique compared to the genetic architecture for the standard behavioural measures they are based on (at least in the detected quantitative trait locus). We identify six suggestive candidate genes that may underpin differences in intra-individual behavioural variability, with several of these candidates having previously been linked to behaviour and mental health. These findings demonstrate that intra-individual variability in behaviour appears to be a heritable trait in and of itself on which evolution can act
An Exploratory Investigation of Some Statistical Summaries of Oximeter Oxygen Saturation Data from Preterm Babies
Aim. To explore the potential usefulness of the mean, standard deviation (SD), and coefficient of variation (CV = SD/mean) of oximeter oxygen saturations in the clinical care of preterm babies. Methods. This was an exploratory investigation involving 31 preterm babies at 36 weeks postmenstrual age. All babies were healthy, but two were considered to be clinically unstable and required greater attention. Each baby's oxygen saturations were recorded using an oximeter and summarized by the mean, SD, and CV. The potential usefulness of each measure was assessed by its ability to distinguish the two unstable babies from the others. This was achieved using box plots and hierarchical clustering together with the Calinski-Harabasz (CH) index to quantify clustering performance (higher CH index indicates stronger clustering outcome). Results. The box plots flagged both unstable babies as outliers and none of the other babies. Successful clustering of the stable and unstable babies was achieved using the CV (CH = 72.8) and SD (CH = 63.3) but not with the mean. Conclusion. Taking the box plots and clustering results together, it seems plausible that variability might be more effective than mean level for detecting instability in oxygen saturation in preterm babies and that the combination of variability and level through the CV might be even better
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