389 research outputs found
Magnetic Susceptibility of Collinear and Noncollinear Heisenberg Antiferromagnets
Predictions of the anisotropic magnetic susceptibility chi below the
antiferromagnetic (AFM) ordering temperatures TN of local moment Heisenberg
AFMs have been made previously using molecular field theory (MFT) but are very
limited in their applicability. Here a MFT calculation of chi(T<=TN) is
presented for a wide variety of collinear and noncollinear Heisenberg AFMs
containing identical crystallographically equivalent spins without recourse to
magnetic sublattices. The results are expressed in terms of directly measurable
experimental parameters and are fitted with no adjustable parameters to
experimental chi(T<=TN) data from the literature for several collinear and
noncollinear AFMs. The influence of spin correlations and fluctuations beyond
MFT is quantified by the deviation of the theory from the data. The origin of
the universal chi(T<=TN) observed for triangular lattice AFMs exhibiting
coplanar noncollinear 120 degree AFM ordering is clarified.Comment: 5 pages, 5 figure
Alternating commensurate-incommensurate structures in the magnetic phase diagram of CsNiF3
The magnetic phase diagram of the quasi one-dimensional spinchain system
CsNiF below the N\'eel temperature is determined. For magnetic fields
perpendicular to the spin chains incommensurate phases are predicted. From
linear spin-wave theory we obtain the instability line of the paramagnetic
phase as a function of the strength and the direction of the field. The system
undergoes a transition to a commensurate or an incommensurate phase depending
on the direction of the magnetic field. In the commensurate phase the
characterizing wave vector is locked to values describing a two-sublattice
structure, whereas in the incommensurate phase the wave vector changes
continuously between the corresponding two-sublattice wave vectors.Comment: 11 pages, LaTeX, 5 figures, sent to PRB Rapid Communicatio
Renormalization of the spin-wave spectrum in three-dimentional ferromagnets with dipolar interaction
Renormalization of the spin-wave spectrum is discussed in a cubic ferromagnet
with dipolar forces at . First 1/S-corrections are considered in
detail to the bare spectrum , where is the spin-wave stiffness,
is the angle between and the magnetization and
is the characteristic dipolar energy. In accordance with previous
results we obtain the thermal renormalization of constants and
in the expression for the bare spectrum. Besides, a number of previously
unknown features are revealed. We observe terms which depend on azimuthal angle
of the momentum . It is obtained an isotropic term proportional to
which makes the spectrum linear rather than quadratic when and . In particular a spin-wave gap proportional to
is observed. Essentially, thermal contribution from the
Hartree-Fock diagram to the isotropic correction as well as to the spin-wave
gap are proportional to the demagnetizing factor in the direction of domain
magnetization. This nontrivial behavior is attributed to the long-range nature
of the dipolar interaction. It is shown that the gap screens infrared
singularities of the first 1/S-corrections to the spin-wave stiffness and
longitudinal dynamical spin susceptibility (LDSS) obtained before. We
demonstrate that higher order 1/S-corrections to these quantities are small at
. However the analysis of the entire perturbation series is still
required to derive the spectrum and LDSS when .Comment: 11 pages, 1 figur
The Lebesgue Integral As The Almost Sure Limit Of Random Riemann Sums
A method is given for generating random intermediate points for a sequence of partitions. For the corresponding random Riemann sums it is shown that they converge almost surely to the Lebesgue integral. © 1982 American Mathematical Society
Steady-State Properties of Single-File Systems with Conversion
We have used Monte-Carlo methods and analytical techniques to investigate the
influence of the characteristic parameters, such as pipe length, diffusion,
adsorption, desorption and reaction rate constants on the steady-state
properties of Single-File Systems with a reaction. We looked at cases when all
the sites are reactive and when only some of them are reactive. Comparisons
between Mean-Field predictions and Monte-Carlo simulations for the occupancy
profiles and reactivity are made. Substantial differences between Mean-Field
and the simulations are found when rates of diffusion are high. Mean-Field
results only include Single-File behavior by changing the diffusion rate
constant, but it effectively allows passing of particles. Reactivity converges
to a limit value if more reactive sites are added: sites in the middle of the
system have little or no effect on the kinetics. Occupancy profiles show
approximately exponential behavior from the ends to the middle of the system.Comment: 15 pages, 20 figure
A Validation of the p-SLLOD Equations of Motion for Homogeneous Steady-state Flows
A validation of the p-SLLOD equations of motion for nonequilibrium molecular dynamics simulation under homogeneous steady-state flow is presented. We demonstrate that these equations generate the correct center-of-mass trajectory of the system, are completely compatible with (and derivable from) Hamiltonian dynamics, satisfy an appropriate energy balance, and require no fictitious external force to generate the required homogeneous flow. It is also shown that no rigorous derivation of the SLLOD equations exists to date
Observations of mesoscale variability in the western North Atlantic: A comparative study
As part of the POLYMODE experiment, three clusters (labeled A, B, C) of moored current meters and temperature-pressure recorders were deployed in three relatively unexplored regions in the North Atlantic Ocean to study the mesoscale variability…
Substrate binding and catalysis by the pseudouridine synthases RluA and TruB
xi, 122 leaves : ill. (some col.) ; 29 cmPseudouridine is the most common RNA modification found in all forms of life. The exact role pseudouridines play in the cell is still relatively unknown. However, its extensive incorporation in functionally important areas of the ribosome and the fitness advantage provided to cells by pseudouridines implies that its presence is important for the cell. The enzymes responsible for this modification, pseudouridine synthases, vary greatly in substrate recognition mechanisms, but all enzymes supposedly share a universally conserved catalytic mechanism. Here, I analyze the kinetic mechanisms of pseudouridylation utilized by the exemplary pseudouridine synthase RluA in order to compare it with the previously determined rate of pseudouridylation of the pseudouridine synthase TruB. My results demonstrate that RluA has the same uniformly slow catalytic step as previously determined for TruB and TruA. This constitutes the first step towards identifying the catalytic mechanism of the pseudouridine synthase family. Additionally, it was my aim to identify the major determinants for RNA binding by pseudouridine synthases. By measuring the dissociation constants (KD) for substrate and product tRNA by nitrocellulose filtration assays, I showed that both tRNA species could bind with similar affinities. These binding studies also revealed that TruB’s interaction with the isolated T-arm is the major contact site contributing to the affinity of the enzyme to RNA. Finally, a new contact between tRNA and TruB’s PUA domain was identified which was not observed in the crystal structure. In summary, my results provide new insight into the common catalytic step of pseudouridine synthases and the specific interactions contributing to substrate binding by the enzyme TruB. These results will enable future studies on the kinetic mechanism of pseudouridine synthases, in particular the kinetics of substrate and product binding and release, as well as on the chemical mechanism of pseudouridine formation
Energetic and Entropic Elasticity of Nonisothermal Flowing Polymers: Experiment, Theory, and Simulation
The thermodynamical aspects of polymeric liquids subjected to nonisothermal flow are examined from the complementary perspectives of theory, experiment, and simulation. In particular, attention is paid to the energetic effects, in addition to the entropic ones, that occur under conditions of extreme deformation. Comparisons of experimental measurements of the temperature rise generated under elongational flow at high strain rates with macroscopic finite element simulations offer clear evidence of the persistence and importance of energetic effects under severe deformation. The performance of various forms of the temperature equation are evaluated with regard to experiment, and it is concluded that the standard form of this evolution equation, arising from the concept of purely entropic elasticity, is inadequate for describing nonisothermal flow processes of polymeric liquids under high deformation. Complete temperature equations, in the sense that they possess a direct and explicit dependence on the energetics of the microstructure of the material, provide excellent agreement with experimental data
Self-Consistent Multiscale Modeling in the Presence of Inhomogeneous Fields
Molecular dynamics (MD) simulations of a Lennard–Jones fluid in an inhomogeneous external field generate steady-state profiles of density and pressure with nanoscopic heterogeneities. The continuum level of mass, momentum, and energy transport balances is capable of reproducing the MD profiles only when the equation of state for pressure as a function of density is extracted directly from the molecular level of description. We show that the density profile resulting from simulation is consistent with both a molecular-level theoretical prediction from statistical mechanics as well as the solution of the continuum-level set of differential equations describing the conservation of mass and momentum
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