2,200 research outputs found
Egg shape changes at the theropodâbird transition, and a morphometric study of amniote eggs
The eggs of amniotes exhibit a remarkable variety of shapes,
from spherical to elongate and from symmetrical to asymmetrical.
We examine eggshell geometry in a diverse sample of fossil
and living amniotes using geometric morphometrics and linear
measurements. Our goal is to quantify patterns of morphospace
occupation and shape variation in the eggs of recent through to
Mesozoic birds (neornithe plus non-neornithe avialans), as well
as in eggs attributed to non-avialan theropods. In most amniotes,
eggs show signiïŹcant deviation from sphericity, but departure
from symmetry around the equatorial axis is mostly conïŹned
to theropods and birds. Mesozoic bird eggs differ signiïŹcantly
from extant bird eggs, but extinct Cenozoic bird eggs do not. This
suggests that the range of egg shapes in extant birds had already
been attained in the Cenozoic. We conclude with a discussion
of possible biological factors imparting variation to egg shapes
during their formation in the oviduct
Finite temperature theory of the trapped two dimensional Bose gas
We present a Hartree-Fock-Bogoliubov (HFB) theoretical treatment of the
two-dimensional trapped Bose gas and indicate how semiclassical approximations
to this and other formalisms have lead to confusion. We numerically obtain
results for the fully quantum mechanical HFB theory within the Popov
approximation and show that the presence of the trap stabilizes the condensate
against long wavelength fluctuations. These results are used to show where
phase fluctuations lead to the formation of a quasicondensate.Comment: 4 pages, 3 figure
The effect of multi-layer bandage on the interface pressure applied by compression bandages
Medical compression bandages are widely used in the
treatment of chronic venous disorder. In order to design effective
compression bandages, researchers have attempted to describe the
interface pressure applied by multi-layer bandages using mathematical
models. This paper reports on the work carried out to
compare and validate the mathematical models used to describe the
interface pressure applied by multi-layer bandages. Both analytical
and experimental results showed that using simple multiplication
of a number of bandage layers with the pressure applied by one
layer of bandage or ignoring the increase in the limb radius due to
former layers of bandage will result in overestimating the pressure.
Experimental results showed that the mathematical models, which
take into consideration the increase in the limb radius due to former
bandage layers, are more accurate than the one which does not
Pnc1 piggy-back import into peroxisomes relies on Gpd1 homodimerisation.
Peroxisomes are eukaryotic organelles that posttranslationally import proteins via one of two conserved peroxisomal targeting signal (PTS1 or 2) mediated pathways. Oligomeric proteins can be imported via these pathways but evidence is accumulating that at least some PTS1-containing monomers enter peroxisomes before they assemble into oligomers. Some proteins lacking a PTS are imported by piggy-backing onto PTS-containing proteins. One of these proteins is the nicotinamidase Pnc1, that is co-imported with the PTS2-containing enzyme Glycerol-3-phosphate dehydrogenase 1, Gpd1. Here we show that Pnc1 co-import requires Gpd1 to form homodimers. A mutation that interferes with Gpd1 homodimerisation does not prevent Gpd1 import but prevents Pnc1 co-import. A suppressor mutation that restores Gpd1 homodimerisation also restores Pnc1 co-import. In line with this, Pnc1 interacts with Gpd1 in vivo only when Gpd1 can form dimers. Redirection of Gpd1 from the PTS2 import pathway to the PTS1 import pathway supports Gpd1 monomer import but not Gpd1 homodimer import and Pnc1 co-import. Our results support a model whereby Gpd1 may be imported as a monomer or a dimer but only the Gpd1 dimer facilitates co-transport of Pnc1 into peroxisomes
Kinetic Theory of Collective Excitations and Damping in Bose-Einstein Condensed Gases
We calculate the frequencies and damping rates of the low-lying collective
modes of a Bose-Einstein condensed gas at nonzero temperature. We use a complex
nonlinear Schr\"odinger equation to determine the dynamics of the condensate
atoms, and couple it to a Boltzmann equation for the noncondensate atoms. In
this manner we take into account both collisions between
noncondensate-noncondensate and condensate-noncondensate atoms. We solve the
linear response of these equations, using a time-dependent gaussian trial
function for the condensate wave function and a truncated power expansion for
the deviation function of the thermal cloud. As a result, our calculation turns
out to be characterized by two dimensionless parameters proportional to the
noncondensate-noncondensate and condensate-noncondensate mean collision times.
We find in general quite good agreement with experiment, both for the
frequencies and damping of the collective modes.Comment: 10 pages, 8 figure
Far-infrared surface-plasmon quantum-cascade lasers at 21.5 mu m and 24 mu m wavelengths
Quantum-cascade lasers operating above 20 mum (at lambda similar to 21.5 mum and lambda similar to 24 mum) wavelength are reported. Pulsed operation was obtained up to 140 K and with a peak power of a few milliwatts at cryogenic temperatures. Laser action originates from interminiband transitions in "chirped" superlattice active regions. The waveguides are based on surface-plasmon modes confined at a metal-semiconductor interface. The wavelengths were chosen in order to avoid major phonon absorption bands, which are particularly strong at energies just above the reststrahlen band. We also report on a 21.5-mum-wavelength laser based on a two-sided interface-plasmon waveguide. (C) 2001 American Institute of Physics
Structure-function relationships in the feto-placental circulation from in silico interpretation of micro-CT vascular structures
A well-functioning placenta is critical for healthy fetal development, as the placenta brings fetal blood in close contact with nutrient rich maternal blood, enabling exchange of nutrients and waste between mother and fetus. The feto-placental circulation forms a complex branching structure, providing blood to fetal capillaries, which must receive sufficient blood flow to ensure effective exchange, but at a low enough pressure to prevent damage to placental circulatory structures. The branching structure of the feto-placental circulation is known to be altered in complications such as fetal growth restriction, and the presence of regions of vascular dysfunction (such as hypovascularity or thrombosis) are proposed to elevate risk of placental pathology. Here we present a methodology to combine micro-computed tomography and computational model-based analysis of the branching structure of the feto-placental circulation in ex vivo placentae from normal term pregnancies. We analyse how vascular structure relates to function in this key organ of pregnancy; demonstrating that there is a 'resilience' to placental vascular structure-function relationships. We find that placentae with variable chorionic vascular structures, both with and without a Hyrtl's anastomosis between the umbilical arteries, and those with multiple regions of poorly vascularised tissue are able to function with a normal vascular resistance. Our models also predict that by progressively introducing local heterogeneity in placental vascular structure, large increases in feto-placental vascular resistances are induced. This suggests that localised heterogeneities in placental structure could potentially provide an indicator of increased risk of placental dysfunction
Quadrupole collective modes in trapped finite-temperature Bose-Einstein condensates
Finite temperature simulations are used to study quadrupole excitations of a
trapped Bose-Einstein condensate. We focus specifically on the m=0 mode, where
a long-standing theoretical problem has been to account for an anomalous
variation of the mode frequency with temperature. We explain this behavior in
terms of the excitation of two separate modes, corresponding to coupled motion
of the condensate and thermal cloud. The relative amplitudes of the modes
depends sensitively on the temperature and on the frequency of the harmonic
drive used to excite them. Good agreement with experiment is found for
appropriate drive frequencies.Comment: 4 pages, 3 figure
Beyond Gross-Pitaevskii Mean Field Theory
A large number of effects related to the phenomenon of Bose-Einstein
Condensation (BEC) can be understood in terms of lowest order mean field
theory, whereby the entire system is assumed to be condensed, with thermal and
quantum fluctuations completely ignored. Such a treatment leads to the
Gross-Pitaevskii Equation (GPE) used extensively throughout this book. Although
this theory works remarkably well for a broad range of experimental parameters,
a more complete treatment is required for understanding various experiments,
including experiments with solitons and vortices. Such treatments should
include the dynamical coupling of the condensate to the thermal cloud, the
effect of dimensionality, the role of quantum fluctuations, and should also
describe the critical regime, including the process of condensate formation.
The aim of this Chapter is to give a brief but insightful overview of various
recent theories, which extend beyond the GPE. To keep the discussion brief,
only the main notions and conclusions will be presented. This Chapter
generalizes the presentation of Chapter 1, by explicitly maintaining
fluctuations around the condensate order parameter. While the theoretical
arguments outlined here are generic, the emphasis is on approaches suitable for
describing single weakly-interacting atomic Bose gases in harmonic traps.
Interesting effects arising when condensates are trapped in double-well
potentials and optical lattices, as well as the cases of spinor condensates,
and atomic-molecular coupling, along with the modified or alternative theories
needed to describe them, will not be covered here.Comment: Review Article (19 Pages) - To appear in 'Emergent Nonlinear
Phenomena in Bose-Einstein Condensates: Theory and Experiment', Edited by
P.G. Kevrekidis, D.J. Frantzeskakis and R. Carretero-Gonzalez (Springer
Verlag
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