387 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
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
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
Finite Line Relief Well System Design for Dams and Levees
An Analytical Method is Commonly Used to Design Relief Well Systems by Assuming the Well Line Extends an Infinite Distance Parallel to the Dam or Levee. This Assumption May Be Met in Some Cases, But When a Well Line is of Finite Length, This Can Severely Underestimate Excess Heads. Although These Consequences Have Been Historically Recognized, a Practical Graph-Based Analytical Approach for Finite Well Line Design Has Not Been Developed. Finite Well Lines Exist and Continue to Be Installed at Many Locations, So This Study Developed a Practical Design Method for Such Systems. Analytical Solutions and Numerical Models Were Used to Improve Understanding of the Performance of Partial and Full Penetration Finite Well Systems. Performance Was Found to Be Dependent on Well System Geometry, the Ratios of Effective Seepage Entry and Exit Distances to Well Spacing, and the Number of Wells. Model Results Were Used to Develop New Uplift Factors that More Accurately Define Excess Heads Along and Landward of Finite Well Systems that Fully or Partially Penetrate the Aquifer
Bose-Einstein condensation in antiferromagnets close to the saturation field
At zero temperature and strong applied magnetic fields the ground sate of an
anisotropic antiferromagnet is a saturated paramagnet with fully aligned spins.
We study the quantum phase transition as the field is reduced below an upper
critical and the system enters a XY-antiferromagnetic phase. Using a
bond operator representation we consider a model spin-1 Heisenberg
antiferromagnetic with single-ion anisotropy in hyper-cubic lattices under
strong magnetic fields. We show that the transition at can be
interpreted as a Bose-Einstein condensation (BEC) of magnons. The theoretical
results are used to analyze our magnetization versus field data in the organic
compound - (DTN) at very low temperatures. This is the
ideal BEC system to study this transition since is sufficiently low to
be reached with static magnetic fields (as opposed to pulsed fields). The
scaling of the magnetization as a function of field and temperature close to
shows excellent agreement with the theoretical predictions. It allows
to obtain the quantum critical exponents and confirm the BEC nature of the
transition at .Comment: 4 pages, 1 figure. Accepted for publication in PRB
Controlled switching of intrinsic localized modes in a 1-D antiferromagnet
Nearly steady-state locked intrinsic localized modes (ILMs) in the quasi-1d
antiferromagnet (C2H5NH3)2CuCl4 are detected via four-wave mixing emission or
the uniform mode absorption. Exploiting the long-time stability of these locked
ILMs, repeatable nonlinear switching is observed by varying the sample
temperature, and localized modes with various amplitudes are created by
modulation of the microwave driver power. This steady-state ILM locking
technique could be used to produce energy localization in other atomic
lattices.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett. v.2 :
clarifications of text and figures in response to comment
Exact results for the reactivity of a single-file system
We derive analytical expressions for the reactivity of a Single-File System
with fast diffusion and adsorption and desorption at one end. If the conversion
reaction is fast, then the reactivity depends only very weakly on the system
size, and the conversion is about 100%. If the reaction is slow, then the
reactivity becomes proportional to the system size, the loading, and the
reaction rate constant. If the system size increases the reactivity goes to the
geometric mean of the reaction rate constant and the rate of adsorption and
desorption. For large systems the number of nonconverted particles decreases
exponentially with distance from the adsorption/desorption end.Comment: 4 pages, 2 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.open131
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