13,729 research outputs found
Simulations of a classical spin system with competing superexchange and double-exchange interactions
Monte-Carlo simulations and ground-state calculations have been used to map
out the phase diagram of a system of classical spins, on a simple cubic
lattice, where nearest-neighbor pairs of spins are coupled via competing
antiferromagnetic superexchange and ferromagnetic double-exchange interactions.
For a certain range of parameters, this model is relevant for some magnetic
materials, such as doped manganites, which exhibit the remarkable colossal
magnetoresistance effect. The phase diagram includes two regions in which the
two sublattice magnetizations differ in magnitude. Spin-dynamics simulations
have been used to compute the time- and space-displaced spin-spin correlation
functions, and their Fourier transforms, which yield the dynamic structure
factor for this system. Effects of the double-exchange
interaction on the dispersion curves are shown.Comment: Latex, 3 pages, 3 figure
Improved Spin Dynamics Simulations of Magnetic Excitations
Using Suzuki-Trotter decompositions of exponential operators we describe new
algorithms for the numerical integration of the equations of motion for
classical spin systems. These techniques conserve spin length exactly and, in
special cases, also conserve the energy and maintain time reversibility. We
investigate integration schemes of up to eighth order and show that these new
algorithms can be used with much larger time steps than a well established
predictor-corrector method. These methods may lead to a substantial speedup of
spin dynamics simulations, however, the choice of which order method to use is
not always straightforward.Comment: J. Mod. Phys. C (in press
Singular current response from isolated impurities in d-wave superconductors
The current response of a d-wave superconductor containing a single impurity
is calculated and shown to be singular in the low-temperature limit, leading in
the case of strong scattering to a 1/T term in the penetration depth
similar to that induced by Andreev surface bound states. For a
small number of such impurities, we argue this low- upturn could be
observable in cuprate superconductors.Comment: 4 pages, 2 .eps figures. Minor changes to match the published versio
Diffusion in a multi-component Lattice Boltzmann Equation model
Diffusion phenomena in a multiple component lattice Boltzmann Equation (LBE)
model are discussed in detail. The mass fluxes associated with different
mechanical driving forces are obtained using a Chapman-Enskog analysis. This
model is found to have correct diffusion behavior and the multiple diffusion
coefficients are obtained analytically. The analytical results are further
confirmed by numerical simulations in a few solvable limiting cases. The LBE
model is established as a useful computational tool for the simulation of mass
transfer in fluid systems with external forces.Comment: To appear in Aug 1 issue of PR
Multi-component lattice-Boltzmann model with interparticle interaction
A previously proposed [X. Shan and H. Chen, Phys. Rev. E {\bf 47}, 1815,
(1993)] lattice Boltzmann model for simulating fluids with multiple components
and interparticle forces is described in detail. Macroscopic equations
governing the motion of each component are derived by using Chapman-Enskog
method. The mutual diffusivity in a binary mixture is calculated analytically
and confirmed by numerical simulation. The diffusivity is generally a function
of the concentrations of the two components but independent of the fluid
velocity so that the diffusion is Galilean invariant. The analytically
calculated shear kinematic viscosity of this model is also confirmed
numerically.Comment: 18 pages, compressed and uuencoded postscript fil
A Lattice Boltzmann method for simulations of liquid-vapor thermal flows
We present a novel lattice Boltzmann method that has a capability of
simulating thermodynamic multiphase flows. This approach is fully
thermodynamically consistent at the macroscopic level. Using this new method, a
liquid-vapor boiling process, including liquid-vapor formation and coalescence
together with a full coupling of temperature, is simulated for the first time.Comment: one gzipped tar file, 19 pages, 4 figure
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