2,209 research outputs found
Series Expansions for Excited States of Quantum Lattice Models
We show that by means of connected-graph expansions one can effectively
generate exact high-order series expansions which are informative of low-lying
excited states for quantum many-body systems defined on a lattice. In
particular, the Fourier series coefficients of elementary excitation spectra
are directly obtained. The numerical calculations involved are straightforward
extensions of those which have already been used to calculate series expansions
for ground-state correlations and susceptibilities in a wide variety of
models. As a test, we have reproduced the known elementary excitation spectrum
of the transverse-field Ising chain in its disordered phase.Comment: 9 pages, no figures, Revtex 3.0 The revised version corrects the
incorrect (and unnecessary) statement in the original that H and H^eff are
related by a unitary transformation; in fact they are related by via a
similarity transformation. This has no implications for the calculations of
spectra, but is important for matrix element
On the finite-size behavior of systems with asymptotically large critical shift
Exact results of the finite-size behavior of the susceptibility in
three-dimensional mean spherical model films under Dirichlet-Dirichlet,
Dirichlet-Neumann and Neumann-Neumann boundary conditions are presented. The
corresponding scaling functions are explicitly derived and their asymptotics
close to, above and below the bulk critical temperature are obtained. The
results can be incorporated in the framework of the finite-size scaling theory
where the exponent characterizing the shift of the finite-size
critical temperature with respect to is smaller than , with
being the critical exponent of the bulk correlation length.Comment: 24 pages, late
Disease progression in Plasmodium knowlesi malaria is linked to variation in invasion gene family members.
Emerging pathogens undermine initiatives to control the global health impact of infectious diseases. Zoonotic malaria is no exception. Plasmodium knowlesi, a malaria parasite of Southeast Asian macaques, has entered the human population. P. knowlesi, like Plasmodium falciparum, can reach high parasitaemia in human infections, and the World Health Organization guidelines for severe malaria list hyperparasitaemia among the measures of severe malaria in both infections. Not all patients with P. knowlesi infections develop hyperparasitaemia, and it is important to determine why. Between isolate variability in erythrocyte invasion, efficiency seems key. Here we investigate the idea that particular alleles of two P. knowlesi erythrocyte invasion genes, P. knowlesi normocyte binding protein Pknbpxa and Pknbpxb, influence parasitaemia and human disease progression. Pknbpxa and Pknbpxb reference DNA sequences were generated from five geographically and temporally distinct P. knowlesi patient isolates. Polymorphic regions of each gene (approximately 800 bp) were identified by haplotyping 147 patient isolates at each locus. Parasitaemia in the study cohort was associated with markers of disease severity including liver and renal dysfunction, haemoglobin, platelets and lactate, (r = ≥ 0.34, p = <0.0001 for all). Seventy-five and 51 Pknbpxa and Pknbpxb haplotypes were resolved in 138 (94%) and 134 (92%) patient isolates respectively. The haplotypes formed twelve Pknbpxa and two Pknbpxb allelic groups. Patients infected with parasites with particular Pknbpxa and Pknbpxb alleles within the groups had significantly higher parasitaemia and other markers of disease severity. Our study strongly suggests that P. knowlesi invasion gene variants contribute to parasite virulence. We focused on two invasion genes, and we anticipate that additional virulent loci will be identified in pathogen genome-wide studies. The multiple sustained entries of this diverse pathogen into the human population must give cause for concern to malaria elimination strategists in the Southeast Asian region
Field theoretical analysis of adsorption of polymer chains at surfaces: Critical exponents and Scaling
The process of adsorption on a planar repulsive, "marginal" and attractive
wall of long-flexible polymer chains with excluded volume interactions is
investigated. The performed scaling analysis is based on formal analogy between
the polymer adsorption problem and the equivalent problem of critical phenomena
in the semi-infinite n-vector model (in the limit ) with a
planar boundary. The whole set of surface critical exponents characterizing the
process of adsorption of long-flexible polymer chains at the surface is
obtained. The polymer linear dimensions parallel and perpendicular to the
surface and the corresponding partition functions as well as the behavior of
monomer density profiles and the fraction of adsorbed monomers at the surface
and in the interior are studied on the basis of renormalization group field
theoretical approach directly in d=3 dimensions up to two-loop order for the
semi-infinite n-vector model. The obtained field- theoretical
results at fixed dimensions d=3 are in good agreement with recent Monte Carlo
calculations. Besides, we have performed the scaling analysis of
center-adsorbed star polymer chains with arms of the same length and we
have obtained the set of critical exponents for such system at fixed d=3
dimensions up to two-loop order.Comment: 22 pages, 12 figures, 4 table
Order-Disorder Transition in a Two-Layer Quantum Antiferromagnet
We have studied the antiferromagnetic order -- disorder transition occurring
at in a 2-layer quantum Heisenberg antiferromagnet as the inter-plane
coupling is increased. Quantum Monte Carlo results for the staggered structure
factor in combination with finite-size scaling theory give the critical ratio
between the inter-plane and in-plane coupling constants.
The critical behavior is consistent with the 3D classical Heisenberg
universality class. Results for the uniform magnetic susceptibility and the
correlation length at finite temperature are compared with recent predictions
for the 2+1-dimensional nonlinear -model. The susceptibility is found
to exhibit quantum critical behavior at temperatures significantly higher than
the correlation length.Comment: 11 pages (5 postscript figures available upon request), Revtex 3.
Test of renormalization predictions for universal finite-size scaling functions
We calculate universal finite-size scaling functions for systems with an
n-component order parameter and algebraically decaying interactions. Just as
previously has been found for short-range interactions, this leads to a
singular epsilon-expansion, where epsilon is the distance to the upper critical
dimension. Subsequently, we check the results by numerical simulations of spin
models in the same universality class. Our systems offer the essential
advantage that epsilon can be varied continuously, allowing an accurate
examination of the region where epsilon is small. The numerical calculations
turn out to be in striking disagreement with the predicted singularity.Comment: 6 pages, including 3 EPS figures. To appear in Phys. Rev. E. Also
available as PDF file at
http://www.cond-mat.physik.uni-mainz.de/~luijten/erikpubs.htm
Structural, magnetic, and transport properties of thin films of the Heusler alloy Co2MnSi
Thin films of Co2MnSi have been grown on a-plane sapphire substrates from three elemental targets by do magnetron cosputtering. These films are single phase, have a strong (110) texture, and a, saturation magnetization of 4.95mu(B)/formula unit at 10 K. Films grown at the highest substrate temperature of 715 K showed the lowest resistivity (47 muOmega cm at 4.2 K) and the lowest coercivity (18 Oe). The spin polarization of the transport current was found to be of the order of 54% as determined by point contact Andreev reflection spectroscopy. A decrease in saturation magnetization with a decrease, in film thickness and different transport behavior in thinner films indicate graded disorder in these films grown on nonlattice matched substrates. (C) 2004 American Institute of Physics
Thermodynamics of a finite system of classical particles with short and long range interactions and nuclear fragmentation
We describe a finite inhomogeneous three dimensional system of classical
particles which interact through short and (or) long range interactions by
means of a simple analytic spin model. The thermodynamic properties of the
system are worked out in the framework of the grand canonical ensemble. It is
shown that the system experiences a phase transition at fixed average density
in the thermodynamic limit. The phase diagram and the caloric curve are
constructed and compared with numerical simulations. The implications of our
results concerning the caloric curve are discussed in connection with the
interpretation of corresponding experimental data.Comment: 11pages, LaTeX, 6 figures. Major change : A new section dealing with
numerical simulations in the framework of a cellular model has been adde
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