28 research outputs found
Scaling of thermal conductivity of helium confined in pores
We have studied the thermal conductivity of confined superfluids on a
bar-like geometry. We use the planar magnet lattice model on a lattice with . We have applied open boundary conditions on the bar
sides (the confined directions of length ) and periodic along the long
direction. We have adopted a hybrid Monte Carlo algorithm to efficiently deal
with the critical slowing down and in order to solve the dynamical equations of
motion we use a discretization technique which introduces errors only
in the time step . Our results demonstrate the
validity of scaling using known values of the critical exponents and we
obtained the scaling function of the thermal resistivity. We find that our
results for the thermal resistivity scaling function are in very good agreement
with the available experimental results for pores using the tempComment: 5 two-column pages, 3 figures, Revtex
Percolation properties of the 2D Heisenberg model
We analyze the percolation properties of certain clusters defined on
configurations of the 2--dimensional Heisenberg model. We find that, given any
direction \vec{n} in O(3) space, the spins almost perpendicular to \vec{n} form
a percolating cluster. This result gives indications of how the model can avoid
a previously conjectured Kosterlitz-Thouless phase transition at finite
temperature T.Comment: 4 pages, 3 eps figures. Revised version (more clear abstract, some
new references
Classical Phase Fluctuations in High Temperature Superconductors
Phase fluctuations of the superconducting order parameter play a larger role
in the cuprates than in conventional BCS superconductors because of the low
superfluid density of a doped insulator. In this paper, we analyze an XY model
of classical phase fluctuations in the high temperature superconductors using a
low-temperature expansion and Monte Carlo simulations. In agreement with
experiment, the value of the superfluid density at temperature T=0 is a quite
robust predictor of Tc, and the evolution of the superfluid density with T,
including its T-linear behavior at low temperature, is insensitive to
microscopic details.Comment: 4 pages, 1 figur
Hidden degree of freedom and critical states in a two-dimensional electron gas in the presence of a random magnetic field
We establish the existence of a hidden degree of freedom and the critical
states of a spinless electron system in a spatially-correlated random magnetic
field with vanishing mean. Whereas the critical states are carried by the
zero-field contours of the field landscape, the hidden degree of freedom is
recognized as being associated with the formation of vortices in these special
contours. It is argued that, as opposed to the coherent backscattering
mechanism of weak localization, a new type of scattering processes in the
contours controls the underlying physics of localization in the random magnetic
field system. In addition, we investigate the role of vortices in governing the
metal-insulator transition and propose a renormalization-group diagram for the
system under study.Comment: 17 pages, 16 figures; Figs. 1, 7, 9, and 10 have been reduced in
quality for e-submissio
Critical behavior of the three-dimensional XY universality class
We improve the theoretical estimates of the critical exponents for the
three-dimensional XY universality class. We find alpha=-0.0146(8),
gamma=1.3177(5), nu=0.67155(27), eta=0.0380(4), beta=0.3485(2), and
delta=4.780(2). We observe a discrepancy with the most recent experimental
estimate of alpha; this discrepancy calls for further theoretical and
experimental investigations. Our results are obtained by combining Monte Carlo
simulations based on finite-size scaling methods, and high-temperature
expansions. Two improved models (with suppressed leading scaling corrections)
are selected by Monte Carlo computation. The critical exponents are computed
from high-temperature expansions specialized to these improved models. By the
same technique we determine the coefficients of the small-magnetization
expansion of the equation of state. This expansion is extended analytically by
means of approximate parametric representations, obtaining the equation of
state in the whole critical region. We also determine the specific-heat
amplitude ratio.Comment: 61 pages, 3 figures, RevTe
Path integral Monte Carlo simulation of the second layer of helium-4 adsorbed on graphite
We have developed a path integral Monte Carlo method for simulating helium
films and apply it to the second layer of helium adsorbed on graphite. We use
helium-helium and helium-graphite interactions that are found from potentials
which realistically describe the interatomic interactions. The Monte Carlo
sampling is over both particle positions and permutations of particle labels.
From the particle configurations and static structure factor calculations, we
find that this layer possesses, in order of increasing density, a superfluid
liquid phase, a sqrt(7) x sqrt(7) commensurate solid phase that is registered
with respect to the first layer, and an incommensurate solid phases. By
applying the Maxwell construction to the dependence of the low-temperature
total energy on the coverage, we are able to identify coexistence regions
between the phases. From these, we deduce an effectively zero-temperature phase
diagram. Our phase boundaries are in agreement with heat capacity and torsional
oscillator measurements, and demonstrate that the experimentally observed
disruption of the superfluid phase is caused by the growth of the commensurate
phase. We further observe that the superfluid phase has a transition
temperature consistent with the two-dimensional value. Promotion to the third
layer occurs for densities above 0.212 atom/A^2, in good agreement with
experiment. Finally, we calculate the specific heat for each phase and obtain
peaks at temperatures in general agreement with experiment.Comment: 14 double-column pages, 10 figures, revtex. Accepted for publication
in Phys. Rev. B. 3 figures added, some text revisions, 6 figures remove
Collagen concentration and biomechanical properties of samples from the lower uterine cervix in relation to age and parity in non-pregnant women
<p>Abstract</p> <p>Background</p> <p>During normal pregnancy the cervix has a load bearing function. The cervical tissue consists mainly of an extracellular matrix (ECM) rich in collagen; important for the biomechanical properties. The aim of the present study was to evaluate how the biomechanical strength of samples from the distal cervix is associated with collagen content in relation to age and parity. This study demonstrates a method to investigate cervical tissue from women who still have their uterus in situ.</p> <p>Methods</p> <p>Cervical punch biopsies (2 Ă 15 mm) were obtained from 57 healthy women (median age: 39 years, range: 29-49 years). Biomechanical tensile testing was performed, and collagen concentration (as % of dry defatted weight (DDW)) and content (mg of collagen per mm of specimen length) was determined. Histomorphometry was used to determine the volume densities of extracellular matrix and smooth muscle cells. Smooth muscle cells were identified by immunohistochemistry. Finally, orientation of collagen fibers was estimated. Data are given as mean +/- SD.</p> <p>Results</p> <p>The mean collagen concentration (62.2 +/- 6.6%) increased with age (0.5% per year, r = 0.45, p = 0.003) and decreased with parity (1.7% per birth, r = -0.45, p = 0.033). Maximum load was positively correlated with collagen content (mg of collagen per mm of specimen length) (r = 0.76, p < 0.001). Normalized maximum stiffness was increased with age (r = 0.32, p = 0.017), whereas no correlation was found with regard to parity. In tissue samples with a length of approximately one cm, volume density of smooth muscle cells increased gradually from 8.9% in the distal part near the epithelium, to 15.5% in the proximal part (p < 0.001).</p> <p>Conclusions</p> <p>The present study shows that cervical collagen concentration increases with age and decreases with parity in non-pregnant women. In addition, collagen stiffness increased with age, whereas no change in collagen tensile strength with respect to age and parity was found. These results show that collagen contributes to cervical tissue tensile strength and age and parity should be considered confounding factors.</p
Scaling and nonscaling finite-size effects in the Gaussian and the mean spherical model with free boundary conditions
We calculate finite-size effects of the Gaussian model in a L\times \tilde
L^{d-1} box geometry with free boundary conditions in one direction and
periodic boundary conditions in d-1 directions for 2<d<4. We also consider film
geometry (\tilde L \to \infty). Finite-size scaling is found to be valid for
d3 but logarithmic deviations from finite-size scaling are found for
the free energy and energy density at the Gaussian upper borderline dimension
d* =3. The logarithms are related to the vanishing critical exponent
1-\alpha-\nu=(d-3)/2 of the Gaussian surface energy density. The latter has a
cusp-like singularity in d>3 dimensions. We show that these properties are the
origin of nonscaling finite-size effects in the mean spherical model with free
boundary conditions in d>=3 dimensions. At bulk T_c in d=3 dimensions we find
an unexpected non-logarithmic violation of finite-size scaling for the
susceptibility \chi \sim L^3 of the mean spherical model in film geometry
whereas only a logarithmic deviation \chi\sim L^2 \ln L exists for box
geometry. The result for film geometry is explained by the existence of the
lower borderline dimension d_l = 3, as implied by the Mermin-Wagner theorem,
that coincides with the Gaussian upper borderline dimension d*=3. For 3<d<4 we
find a power-law violation of scaling \chi \sim L^{d-1} at bulk T_c for box
geometry and a nonscaling temperature dependence \chi_{surface} \sim \xi^d of
the surface susceptibility above T_c. For 2<d<3 dimensions we show the validity
of universal finite-size scaling for the susceptibility of the mean spherical
model with free boundary conditions for both box and film geometry and
calculate the corresponding universal scaling functions for T>=T_c.Comment: Submitted to Physical Review