1,057 research outputs found
Simulations of a single membrane between two walls using a Monte Carlo method
Quantitative theory of interbilayer interactions is essential to interpret
x-ray scattering data and to elucidate these interactions for biologically
relevant systems. For this purpose Monte Carlo simulations have been performed
to obtain pressure P and positional fluctuations sigma. A new method, called
Fourier Monte-Carlo (FMC), that is based on a Fourier representation of the
displacement field, is developed and its superiority over the standard method
is demonstrated. The FMC method is applied to simulating a single membrane
between two hard walls, which models a stack of lipid bilayer membranes with
non-harmonic interactions. Finite size scaling is demonstrated and used to
obtain accurate values for P and sigma in the limit of a large continuous
membrane. The results are compared with perturbation theory approximations, and
numerical differences are found in the non-harmonic case. Therefore, the FMC
method, rather than the approximations, should be used for establishing the
connection between model potentials and observable quantities, as well as for
pure modeling purposes.Comment: 10 pages, 10 figure
Critical Behaviour of Structure Factors at a Quantum Phase Transition
We review the theoretical behaviour of the total and one-particle structure
factors at a quantum phase transition for temperature T=0. The predictions are
compared with exact or numerical results for the transverse Ising model, the
alternating Heisenberg chain, and the bilayer Heisenberg model. At the critical
wavevector, the results are generally in accord with theoretical expectations.
Away from the critical wavevector, however, different models display quite
different behaviours for the one-particle residues and structure factors.Comment: 17 pp, 10 figure
Soft X-ray resonant scattering study of single-crystal LaSrMnO
Soft X-ray resonant scattering studies at the Mn - and
the La - edges of single-crystal LaSrMnO are
reported. At low temperatures, below K, energy scans
with a fixed momentum transfer at the \emph{A}-type antiferromagnetic (0 0 1)
reflection around the Mn -edges with incident linear
and polarizations show strong resonant enhancements. The
splitting of the energy spectra around the Mn -edges may
indicate the presence of a mixed valence state, e.g., Mn/Mn. The
relative intensities of the resonance and the clear shoulder-feature as well as
the strong incident and polarization dependences strongly
indicate its complex electronic origin. Unexpected enhancement of the charge
Bragg (0 0 2) reflection at the La -edges with
polarization has been observed up to 300 K, with an anomaly appearing around
the orbital-ordering transition temperature, K,
suggesting a strong coupling (competition) between them.Comment: Accepted by European Physical Journal
On critical behavior of phase transitions in certain antiferromagnets with complicated ordering
Within the four-loop \ve expansion, we study the critical behavior of
certain antiferromagnets with complicated ordering. We show that an anisotropic
stable fixed point governs the phase transitions with new critical exponents.
This is supported by the estimate of critical dimensionality
obtained from six loops via the exact relation established
for the real and complex hypercubic models.Comment: Published versio
Vaporization and Layering of Alkanols at the Oil/Water Interface
This study of adsorption of normal alkanols at the oil/water interface with
x-ray reflectivity and tensiometry demonstrates that the liquid to gas
monolayer phase transition at the hexane/water interface is thermodynamically
favorable only for long-chain alkanols. As the alkanol chain length is
decreased, the change in excess interfacial entropy per area decreases to zero.
Systems with small values of excess interfacial entropy form multi-molecular
layers at the interface instead of the monolayer formed by systems with much
larger excess interfacial entropy. Substitution of n-hexane by n-hexadecane
significantly alters the interfacial structure for a given alkanol surfactant,
but this substitution does not change fundamentally the phase transition
behavior of the monolayers. These data show that the critical alkanol carbon
number, at which the change in excess interfacial entropy per area decreases to
zero, is approximately six carbons larger than the number of carbons in the
alkane solvent molecules.Comment: 27 pages, 10 figures, to be published in J. Phys. Cond. Ma
Theory of x-ray absorption by laser-aligned symmetric-top molecules
We devise a theory of x-ray absorption by symmetric-top molecules which are
aligned by an intense optical laser. Initially, the density matrix of the
system is composed of the electronic ground state of the molecules and a
thermal ensemble of rigid-rotor eigenstates. We formulate equations of motion
of the two-color (laser plus x rays) rotational-electronic problem. The
interaction with the laser is assumed to be nonresonant; it is described by an
electric dipole polarizability tensor. X-ray absorption is approximated as a
one-photon process. It is shown that the equations can be separated such that
the interaction with the laser can be treated independently of the x rays. The
laser-only density matrix is propagated numerically. After each time step, the
x-ray absorption is calculated. We apply our theory to study adiabatic
alignment of bromine molecules (Br2). The required dynamic polarizabilities are
determined using the ab initio linear response methods coupled-cluster singles
(CCS), second-order approximate coupled-cluster singles and doubles (CC2), and
coupled-cluster singles and doubles (CCSD). For the description of x-ray
absorption on the sigma_g 1s --> sigma_u 4p resonance, a parameter-free
two-level model is used for the electronic structure of the molecules. Our
theory opens up novel perspectives for the quantum control of x-ray radiation.Comment: 14 pages, 4 figures, 1 table, RevTeX4, revise
A resource of genome-wide single-nucleotide polymorphisms generated by RAD tag sequencing in the critically endangered European eel
Reduced representation genome sequencing such as restriction-site-associated DNA (RAD) sequencing is finding increased use to identify and genotype large numbers of single-nucleotide polymorphisms (SNPs) in model and nonmodel species. We generated a unique resource of novel SNP markers for the European eel using the RAD sequencing approach that was simultaneously identified and scored in a genome-wide scan of 30 individuals. Whereas genomic resources are increasingly becoming available for this species, including the recent release of a draft genome, no genome-wide set of SNP markers was available until now. The generated SNPs were widely distributed across the eel genome, aligning to 4779 different contigs and 19 703 different scaffolds. Significant variation was identified, with an average nucleotide diversity of 0.00529 across individuals. Results varied widely across the genome, ranging from 0.00048 to 0.00737 per locus. Based on the average nucleotide diversity across all loci, long-term effective population size was estimated to range between 132 000 and 1 320 000, which is much higher than previous estimates based on microsatellite loci. The generated SNP resource consisting of 82 425 loci and 376 918 associated SNPs provides a valuable tool for future population genetics and genomics studies and allows for targeting specific genes and particularly interesting regions of the eel genome
Universal Window for Two Dimensional Critical Exponents
Two dimensional condensed matter is realised in increasingly diverse forms
that are accessible to experiment and of potential technological value. The
properties of these systems are influenced by many length scales and reflect
both generic physics and chemical detail. To unify their physical description
is therefore a complex and important challenge. Here we investigate the
distribution of experimentally estimated critical exponents, , that
characterize the evolution of the order parameter through the ordering
transition. The distribution is found to be bimodal and bounded within a window
, facts that are only in partial agreement with
the established theory of critical phenomena. In particular, the bounded nature
of the distribution is impossible to reconcile with existing theory for one of
the major universality classes of two dimensional behaviour - the XY model with
four fold crystal field - which predicts a spectrum of non-universal exponents
bounded only from below. Through a combination of numerical and renormalization
group arguments we resolve the contradiction between theory and experiment and
demonstrate how the "universal window" for critical exponents observed in
experiment arises from a competition between marginal operators.Comment: 26 pages, 5 figures and 6 tables. Uses longtable packag
Cooling-rate effects in a model of (ideal?) glass
Using Monte Carlo simulations we study cooling-rate effects in a
three-dimensional Ising model with four-spin interaction. During coarsening,
this model develops growing energy barriers which at low temperature lead to
very slow dynamics. We show that the characteristic zero-temperature length
increases very slowly with the inverse cooling rate, similarly to the behaviour
of ordinary glasses. For computationally accessible cooling rates the model
undergoes an ideal glassy transition, i.e., the glassy transition for very
small cooling rate coincides a thermodynamic singularity. We also study cooling
of this model with a certain fraction of spins fixed. Due to such heterogeneous
crystalization seeds the final state strongly depends on the cooling rate.Only
for sufficiently fast cooling rate does the system end up in a glassy state
while slow cooling inevitably leads to a crystal phase.Comment: 11 pages, 6 figure
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