217 research outputs found
Role of elastin anisotropy in structural strain energy functions of arterial tissue
The vascular wall exhibits nonlinear anisotropic mechanical properties. The identification of a strain energy function (SEF) is the preferred method to describe its complex nonlinear elastic properties. Earlier constituent-based SEF models, where elastin is modeled as an isotropic material, failed in describing accurately the tissue response to inflation-extension loading. We hypothesized that these shortcomings are partly due to unaccounted anisotropic properties of elastin. We performed inflation-extension tests on common carotid of rabbits before and after enzymatic degradation of elastin and applied constituent-based SEFs, with both an isotropic and an anisotropic elastin part, on the experimental data. We used transmission electron microscopy (TEM) and serial block-face scanning electron microscopy (SBFSEM) to provide direct structural evidence of the assumed anisotropy. In intact arteries, the SEF including anisotropic elastin with one family of fibers in the circumferential direction fitted better the inflation-extension data than the isotropic SEF. This was supported by TEM and SBFSEM imaging, which showed interlamellar elastin fibers in the circumferential direction. In elastin-degraded arteries, both SEFs succeeded equally well in predicting anisotropic wall behavior. In elastase-treated arteries fitted with the anisotropic SEF for elastin, collagen engaged later than in intact arteries. We conclude that constituent-based models with an anisotropic elastin part characterize more accurately the mechanical properties of the arterial wall when compared to models with simply an isotropic elastin. Microstructural imaging based on electron microscopy techniques provided evidence for elastin anisotropy. Finally, the model suggests a later and less abrupt collagen engagement after elastase treatmen
2s exciton-polariton revealed in an external magnetic field
We demonstrate the existence of the excited state of an exciton-polariton in
a semiconductor microcavity. The strong coupling of the quantum well heavy-hole
exciton in an excited 2s state to the cavity photon is observed in non-zero
magnetic field due to surprisingly fast increase of Rabi energy of the 2s
exciton-polariton in magnetic field. This effect is explained by a strong
modification of the wave-function of the relative electron-hole motion for the
2s exciton state.Comment: 5 pages, 5 figure
Laser-driven plasma waves in capillary tubes
The excitation of plasma waves over a length of up to 8 centimeters is, for
the first time, demon- strated using laser guiding of intense laser pulses
through hydrogen filled glass capillary tubes. The plasma waves are diagnosed
by spectral analysis of the transmitted laser radiation. The dependence of the
spectral redshift, measured as a function of filling pressure, capillary tube
length and incident laser energy, is in excellent agreement with simulation
results. The longitudinal accelerating field inferred from the simulations is
in the range 1 -10 GV/m
Social huddling and physiological thermoregulation are related to melanism in the nocturnal barn owl.
Endothermic animals vary in their physiological ability to maintain a constant body temperature. Since melanin-based coloration is related to thermoregulation and energy homeostasis, we predict that dark and pale melanic individuals adopt different behaviours to regulate their body temperature. Young animals are particularly sensitive to a decrease in ambient temperature because their physiological system is not yet mature and growth may be traded-off against thermoregulation. To reduce energy loss, offspring huddle during periods of cold weather. We investigated in nestling barn owls (Tyto alba) whether body temperature, oxygen consumption and huddling were associated with melanin-based coloration. Isolated owlets displaying more black feather spots had a lower body temperature and consumed more oxygen than those with fewer black spots. This suggests that highly melanic individuals display a different thermoregulation strategy. This interpretation is also supported by the finding that, at relatively low ambient temperature, owlets displaying more black spots huddled more rapidly and more often than those displaying fewer spots. Assuming that spot number is associated with the ability to thermoregulate not only in Swiss barn owls but also in other Tytonidae, our results could explain geographic variation in the degree of melanism. Indeed, in the northern hemisphere, barn owls and allies are less spotted polewards than close to the equator, and in the northern American continent, barn owls are also less spotted in colder regions. If melanic spots themselves helped thermoregulation, we would have expected the opposite results. We therefore suggest that some melanogenic genes pleiotropically regulate thermoregulatory processes
Controlling the spectrum of x-rays generated in a laser-plasma accelerator by tailoring the laser wavefront
By tailoring the wavefront of the laser pulse used in a laser-wakefield
accelerator, we show that the properties of the x-rays produced due to the
electron beam's betatron oscillations in the plasma can be controlled. By
creating a wavefront with coma, we find that the critical energy of the
synchrotron-like x-ray spectrum can be significantly increased. The coma does
not substantially change the energy of the electron beam, but does increase its
divergence and produces an energy-dependent exit angle, indicating that changes
in the x-ray spectrum are due to an increase in the electron beam's oscillation
amplitude within the wakefield.Comment: 7 pages, 2 figures, submitted to Appl. Phys. Let
Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors
We discuss the analysis of mixed-state magnetization data of type-II
superconductors using a recently developed scaling procedure. It is based on
the fact that, if the Ginzburg-Landau parameter kappa does not depend on
temperature, the magnetic susceptibility is a universal function of H/H_c2(T),
leading to a simple relation between magnetizations at different temperatures.
Although this scaling procedure does not provide absolute values of the upper
critical fieldH_c2(T), its temperature variation can be established rather
accurately. This provides an opportunity to validate theoretical models that
are usually employed for the evaluation of H_c2(T) from equilibrium
magnetization data. In the second part of the paper we apply this scaling
procedure for a discussion of the notorious first order phase transition in the
mixed state of high temperature superconductors. Our analysis, based on
experimental magnetization data available in the literature, shows that the
shift of the magnetization accross the transition may adopt either sign,
depending on the particular chosen sample. We argue that this observation is
inconsistent with the interpretation that this transition always represents the
melting transition of the vortex lattice.Comment: 18 pages, 12 figure
Direct Observation and Anisotropy of the Contribution of Gap nodes in the Low Temperature Specific Heat of YBa_2Cu_3O_7
The specific heat due to line nodes in the superconducting gap of YBa2Cu3O7
has been obscured up to now by magnetic terms of extrinsic origin, even for
high quality crystals. We report the specific heat of a new single crystal
grown in a non-corrosive BaZrO3 crucible, for which paramagnetic terms are
reduced to less than one spin-1/2 center for 20'000 Cu atoms. The contribution
of line nodes shows up directly in the difference C(B,T) - C(0,T) at fixed
temperatures (T < 5 K) as a function of the magnetic field parallel to the
c-axis (B<=14 T). These data illustrate the smooth crossover from C propotional
to T^2 at low fields to C propotional to TB^1/2 at high fields, and provide new
values for gap parameters which are quantitatively consistent with tunneling
spectroscopy and thermal conductivity in the framework of dx^2-y^2 pairing
symmetry. Data for B along the nodal and antinodal directions in the ab-plane
are also provided. The in-plane anisotropy predicted in the clean limit is not
observed.Comment: 29 pages(using Revtex style), 14 postscript figures, submitted to
Phys. Rev. B Content of the file changed after replacin
Systematics of two-component superconductivity in from microwave measurements of high quality single crystals
Systematic microwave surface impedance measurements of YBCO single crystals
grown in crucibles reveal new properties that are not directly seen
in similar measurements of other YBCO samples. Two key observations obtained
from complex conductivity are: a new normal conductivity peak at around 80K and
additional pairing below 65K. High pressure oxygenation of one of the crystals
still yields the same results ruling out any effect of macroscopic segregation
of O-deficient regions. A single complex order parameter cannot describe these
data, and the results suggest at least two superconducting components.
Comparisons with model calculations done for various decoupled two-component
scenarios (i.e. s+d, d+d) are presented. Systematics of three single crystals
show that the 80K quasiparticle peak is correlated with the normal state
inelastic scattering rate. Close to Tc, the data follow a mean-field behavior.
Overall, our results strongly suggest the presence of multiple pairing
temperature and energy scales in .Comment: 14 pages, 2-column, Revtex, 5 embedded postscript figures, uses
graphicx. Postscript version also available at
http://sagar.physics.neu.edu/preprints.htm
Mitochondrial simple sequenze repeats and 12s – rRNA gene reveal two distinct lineages of Crocidura russula (Mammalia, Sorcidae)
A short segment (135 bp) of the control region and a partial
sequence (394 bp) of the 12S-rRNA gene in the mitochondrial
DNA of Crocidura russula were analyzed in order to test a
previous hypothesis regarding the presence of a gene flow
disruption in northern Africa. This breakpoint would have
separated northeast-African C. russula populations from
the European (plus the northwest-African) populations. The
analysis was carried out on specimens from Tunisia (C. r. cf
agilis), Sardinia (C. r. ichnusae), and Pantelleria (C. r.
cossyrensis), and on C. r. russula from Spain and Belgium.
Two C. russula lineages were identified; they both shared R2
tandem repeated motifs of the same length (12 bp), but not the
same primary structure. These simple sequence repeats were
present in 12–23 copies in the right domain of the control
region. Within the northeast-African populations, a polymorphism
of repeat variants, not yet found in Europe, was recorded.
A neighbor-join tree, which was built by sequences of the conserved 12S-rRNA gene, separated the two sister groups; it
permitted us to date a divergence time of 0.5Myr. Our data
discriminated two different mitochondrial lineages in accordance
with the previous morphological and karyological data.
Ecoclimatic barriers formed during the Middle Pleistocene
broke the range of ancestral species in the Eastern Algeria
(Kabile Mountains), leading to two genetically separate and
modern lineages. The northeast-African lineage can today be
located in Tunisia, Pantelleria, and Sardinia. The northwest-
African lineage (Morocco and West Algeria), reaching Spain
by anthropogenic introduction, spread over north Europe in
modern times. The Palaearctic C. russula species is monophyletic,
but a taxonomical revision (ie, to provide a full species
rank for the northeast taxa and to put in synonymy some
insular taxa) is required
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