437 research outputs found
Statistical properties of charged interfaces
We consider the equilibrium statistical properties of interfaces submitted to
competing interactions; a long-range repulsive Coulomb interaction inherent to
the charged interface and a short-range, anisotropic, attractive one due to
either elasticity or confinement. We focus on one-dimensional interfaces such
as strings. Model systems considered for applications are mainly aggregates of
solitons in polyacetylene and other charge density wave systems, domain lines
in uniaxial ferroelectrics and the stripe phase of oxides. At zero temperature,
we find a shape instability which lead, via phase transitions, to tilted
phases. Depending on the regime, elastic or confinement, the order of the
zero-temperature transition changes. Thermal fluctuations lead to a pure
Coulomb roughening of the string, in addition to the usual one, and to the
presence of angular kinks. We suggest that such instabilities might explain the
tilting of stripes in cuprate oxides. The 3D problem of the charged wall is
also analyzed. The latter experiences instabilities towards various tilted
phases separated by a tricritical point in the elastic regime. In the
confinement regime, the increase of dimensionality favors either the melting of
the wall into a Wigner crystal of its constituent charges or a strongly
inclined wall which might have been observed in nickelate oxides.Comment: 17 pages, 11 figure
Equation of state of a strongly magnetized hydrogen plasma
The influence of a constant uniform magnetic field on the thermodynamic
properties of a partially ionized hydrogen plasma is studied. Using the method
of Green' s function various interaction contributions to the thermodynamic
functions are calculated. The equation of state of a quantum magnetized plasma
is presented within the framework of a low density expansion up to the order
e^4 n^2 and, additionally, including ladder type contributions via the bound
states in the case of strong magnetic fields (2.35*10^{5} T << B << 2.35*10^{9}
T). We show that for high densities (n=10^{27-30} m^{-3}) and temperatures
T=10^5 - 10^6 K typical for the surface of neutron stars nonideality effects
as, e.g., Debye screening must be taken into account.Comment: 12 pages, 2 Postscript figures. uses revtex, to appear in Phys. Rev.
Correlations in two-component log-gas systems
A systematic study of the properties of particle and charge correlation
functions in the two-dimensional Coulomb gas confined to a one-dimensional
domain is undertaken. Two versions of this system are considered: one in which
the positive and negative charges are constrained to alternate in sign along
the line, and the other where there is no charge ordering constraint. Both
systems undergo a zero-density Kosterlitz-Thouless type transition as the
dimensionless coupling is varied through . In
the charge ordered system we use a perturbation technique to establish an
decay of the two-body correlations in the high temperature limit.
For , the low-fugacity expansion of the asymptotic
charge-charge correlation can be resummed to all orders in the fugacity. The
resummation leads to the Kosterlitz renormalization equations.Comment: 39 pages, 5 figures not included, Latex, to appear J. Stat. Phys.
Shortened version of abstract belo
Prognostic study of continuous variables (white blood cell count, peripheral blast cell count, haemoglobin level, platelet count and age) in childhood acute lymphoblastic leukaemia. Analysis of a population of 1545 children treated by the French Acute Lymphoblastic Leukaemia Group (FRALLE)
Many cutpoints have been proposed to categorize continuous variables in childhood acute lymphoblastic leukaemia (white blood cell count, peripheral blast cell count, haemoglobin level, platelet count and age), and have been used to define therapeutic subgroups. This variation in the choice of cutpoints leads to a bias called the âWill Rogers phenomenonâ. The aim of this study was to analyse variations in the relative risk of relapse or death as a function of continuous prognostic variables in childhood ALL and to discuss the choice of cutpoints. We studied a population of 1545 children with ALL enrolled in three consecutive protocols named FRALLE 83, FRALLE 87 and FRALLE 89. We estimated the risk of relapse or death associated with different values of each continuous prognostic variable by dividing the sample into quintiles of the distribution of the variables. As regards age, a category of children under 1 year of age was distinguished and the rest of the population was divided into quintiles. The floated variance method was used to calculate the confidence interval of each relative risk, including the reference category. The relation between the quantitative prognostic factors and the risk was monotonic for each variable, except for age. For the white blood cell count (WBC), the relation is log linear. The risk associated with WBC values in the upper quintile was 1.9 times higher than that in the lower quintile. The peripheral blast cell count correlated strongly with WBC (correlation coefficient: 0.99). The risk increased with the haemoglobin level, and the risk in the upper quintile was 1.3 times higher than that in the lower quintile. The risk decreased as the platelet count increased: the risk in the lower quintile was 1.2 times higher than that in the upper quintile. The risk increased gradually with increasing age above one year. The small subgroup of patients (2.5% of the population) under 1 year of age at diagnosis had a risk 2.6 times higher than the reference category of patients between 3 and 4.3 years of age. When the risk associated with a quantitative prognostic factor varies monotonously, the selection of a cutpoint is arbitrary and represents a loss of information. Despite this loss of information, such arbitrary categorization may be necessary to define therapeutic stratification. In that case, consensus cutpoints must be defined if one wants to avoid the Will Rogers phenomenon. The cutpoints proposed by the Rome workshop and the NCI are arbitrary, but may represent an acceptable convention. © 2000 Cancer Research Campaign http://www.bjcancer.co
Charge and Density Fluctuations Lock Horns : Ionic Criticality with Power-Law Forces
How do charge and density fluctuations compete in ionic fluids near
gas-liquid criticality when quantum mechanical effects play a role ? To gain
some insight, long-range
interactions (with ), that encompass van der Waals forces (when
), have been incorporated in exactly soluble, -dimensional
1:1 ionic spherical models with charges and hard-core repulsions. In
accord with previous work, when (and is not too
large), the Coulomb interactions do not alter the () critical
universality class that is characterized by density correlations at criticality
decaying as with . But screening
is now algebraic, the charge-charge correlations decaying, in general, only as
; thus faithfully mimics known
\textit{non}critical quantal effects. But in the \textit{absence} of full
() ion symmetry, density and charge fluctuations mix via a transparent
mechanism: then the screening \textit{at criticality} is \textit{weaker} by a
factor . Furthermore, the otherwise valid Stillinger-Lovett sum
rule fails \textit{at} criticality whenever (as, e.g., when
) although it remains valid if (as for or in
real Ising-type systems).Comment: 8 pages, in press in J. Phys. A, Letters to the Edito
Anomalous Effects of "Guest" Charges Immersed in Electrolyte: Exact 2D Results
We study physical situations when one or two "guest" arbitrarily-charged
particles are immersed in the bulk of a classical electrolyte modelled by a
Coulomb gas of positive/negative unit point-like charges, the whole system
being in thermal equilibrium. The models are treated as two-dimensional with
logarithmic pairwise interactions among charged constituents; the
(dimensionless) inverse temperature is considered to be smaller than 2
in order to ensure the stability of the electrolyte against the collapse of
positive-negative pairs of charges. Based on recent progress in the integrable
(1+1)-dimensional sine-Gordon theory, exact formulas are derived for the
chemical potential of one guest charge and for the asymptotic large-distance
behavior of the effective interaction between two guest charges. The exact
results imply, under certain circumstances, anomalous effects such as an
effective attraction (repulsion) between like-charged (oppositely-charged)
guest particles and the charge inversion in the electrolyte vicinity of a
highly-charged guest particle. The adequacy of the concept of renormalized
charge is confirmed in the whole stability region of inverse temperatures and
the related saturation phenomenon is revised.Comment: 21 pages, 1 figur
Phase Transitions in the Spin-Half J_1--J_2 Model
The coupled cluster method (CCM) is a well-known method of quantum many-body
theory, and here we present an application of the CCM to the spin-half J_1--J_2
quantum spin model with nearest- and next-nearest-neighbour interactions on the
linear chain and the square lattice. We present new results for ground-state
expectation values of such quantities as the energy and the sublattice
magnetisation. The presence of critical points in the solution of the CCM
equations, which are associated with phase transitions in the real system, is
investigated. Completely distinct from the investigation of the critical
points, we also make a link between the expansion coefficients of the
ground-state wave function in terms of an Ising basis and the CCM ket-state
correlation coefficients. We are thus able to present evidence of the
breakdown, at a given value of J_2/J_1, of the Marshall-Peierls sign rule which
is known to be satisfied at the pure Heisenberg point (J_2 = 0) on any
bipartite lattice. For the square lattice, our best estimates of the points at
which the sign rule breaks down and at which the phase transition from the
antiferromagnetic phase to the frustrated phase occurs are, respectively, given
(to two decimal places) by J_2/J_1 = 0.26 and J_2/J_1 = 0.61.Comment: 28 pages, Latex, 2 postscript figure
The statistical mechanics of the classical two-dimensional Coulomb gas is exactly solved
The model under consideration is a classical 2D Coulomb gas of pointlike
positive and negative unit charges, interacting via a logarithmic potential. In
the whole stability range of temperatures, the equilibrium statistical
mechanics of this fluid is exactly solvable via an equivalence with the
integrable 2D sine-Gordon field theory. The exact solution includes the bulk
thermodynamics, special cases of the surface thermodynamics, and the
large-distance asymptotic behavior of the two-body correlation functions.Comment: Talk presented at the SCCS02 meeting in Santa Fe, to appear in
J.Phys. A: Math. Ge
Alterations in plasma soluble vascular endothelial growth factor receptor-1 (sFlt-1) concentrations during coronary artery bypass graft surgery: relationships with post-operative complications
<p>Abstract</p> <p>Background</p> <p>Plasma concentrations of sFlt-1, the soluble form of the vascular endothelial growth factor receptor (VEGF), markedly increase during coronary artery bypass graft (CABG) surgery with extracorporeal circulation (ECC). We investigated if plasma sFlt-1 values might be related to the occurrence of surgical complications after CABG.</p> <p>Methods</p> <p>Plasma samples were collected from the radial artery catheter before vascular cannulation and after opening the chest, at the end of ECC just before clamp release, after cross release, after weaning from ECC, at the 6<sup>th </sup>and 24<sup>th </sup>post-operative hour. Thirty one patients were investigated. The presence of cardiovascular, haematological and respiratory dysfunctions was prospectively assessed. Plasma sFlt-1 levels were measured with commercially ELISA kits.</p> <p>Results</p> <p>Among the 31 investigated patients, 15 had uneventful surgery. Patients with and without complications had similar pre-operative plasma sFlt-1 levels. Lowered plasma sFlt-1 levels were observed at the end of ECC in patients with haematological (p = 0.001, ANOVA) or cardiovascular (p = 0.006) impairments, but not with respiratory ones (p = 0.053), as compared to patients with uneventful surgery.</p> <p>Conclusion</p> <p>These results identify an association between specific post-CABG complication and the lower release of sFlt-1 during ECC. sFlt-1-induced VEGF neutralisation might, thus, be beneficial to reduce the development of post-operative adverse effects after CABG.</p
Quietly sharing the load? The role of school psychologists in enabling teacher resilience
Teacher resilience is associated with positive student outcomes and plays an important role in teacher retention and well-being. School ecologies can enable the resilience of teachers, with prior research illustrating the importance of supportive colleagues, strong leadership, and positive school culture. There is limited research, however, exploring the role of school psychologists in supporting or enabling teacher resilience. Using data from experienced Australian school psychologists and teachers, this exploratory qualitative study examines the role of school psychologists in enabling teacher resilience. Findings show that school psychologists directly and indirectly support teacher resilience, although teachers perceive school psychologistsâ main role as work with individual students. Issues pertaining to variations in access and particular roles of school psychologists are discussed. Although further research is needed to clarify and promote the role of school psychologists, this study points to them potentially playing an important role in school ecologies that enable teacher resilience
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