10,818 research outputs found
Comment on ``Scaling Laws for a System with Long-Range Interactions within Tsallis Statistics''
In their recent Letter [Phys. Rev. Lett. 83, 4233 (1999)], Salazar and Toral
(ST) study numerically a finite Ising chain with non-integrable interactions
decaying like 1/r^(d+sigma) where -d <= sigma <= 0 (like ST, we discuss general
dimensionality d). In particular, they explore a presumed connection between
non-integrable interactions and Tsallis's non-extensive statistics. We point
out that (i) non-integrable interactions provide no more motivation for Tsallis
statistics than do integrable interactions, i.e., Gibbs statistics remain
meaningful for the non-integrable case, and in fact provide a {\em complete and
exact treatment}; and (ii) there are undesirable features of the method ST use
to regulate the non-integrable interactions.Comment: Accepted for publication in Phys. Rev. Let
Single-Species Three-Particle Reactions in One Dimension
Renormalization group calculations for fluctuation-dominated
reaction-diffusion systems are generally in agreement with simulations and
exact solutions. However, simulations of the single-species reactions
3A->(0,A,2A) at their upper critical dimension d_c=1 have found asymptotic
densities argued to be inconsistent with renormalization group predictions. We
show that this discrepancy is resolved by inclusion of the leading corrections
to scaling, which we derive explicitly and show to be universal, a property not
shared by the A+A->(0,A) reactions. Finally, we demonstrate that two previous
Smoluchowski approaches to this problem reduce, with various corrections, to a
single theory which yields, surprisingly, the same asymptotic density as the
renormalization group.Comment: 8 pages, 5 figs, minor correction
Local Electronic and Magnetic Studies of an Artificial La2FeCrO6 Double Perovskite
Through the utilization of element-resolved polarized x-ray probes, the
electronic and magnetic state of an artificial La2FeCrO6 double perovskite were
explored. Applying unit-cell level control of thin film growth on SrTiO3 (111),
the rock salt double perovskite structure can be created for this system, which
does not have an ordered perovskite phase in the bulk. We find that the Fe and
Cr are in the proper 3+ valence state, but, contrary to previous studies, the
element-resolved magnetic studies find the moments in field are small and show
no evidence of a sizable magnetic moment in the remanent state.Comment: 3 pages, 4 figure
Scaling of Reaction Zones in the A+B->0 Diffusion-Limited Reaction
We study reaction zones in three different versions of the A+B->0 system. For
a steady state formed by opposing currents of A and B particles we derive
scaling behavior via renormalization group analysis. By use of a previously
developed analogy, these results are extended to the time-dependent case of an
initially segregated system. We also consider an initially mixed system, which
forms reaction zones for dimension d<4. In this case an extension of the
steady-state analogy gives scaling results characterized by new exponents.Comment: 4 pages, REVTeX 3.0 with epsf, 2 uuencoded postscript figures
appended, OUTP-94-33
Fast and Accurate Coarsening Simulation with an Unconditionally Stable Time Step
We present Cahn-Hilliard and Allen-Cahn numerical integration algorithms that
are unconditionally stable and so provide significantly faster
accuracy-controlled simulation. Our stability analysis is based on Eyre's
theorem and unconditional von Neumann stability analysis, both of which we
present. Numerical tests confirm the accuracy of the von Neumann approach,
which is straightforward and should be widely applicable in phase-field
modeling. We show that accuracy can be controlled with an unbounded time step
Delta-t that grows with time t as Delta-t ~ t^alpha. We develop a
classification scheme for the step exponent alpha and demonstrate that a class
of simple linear algorithms gives alpha=1/3. For this class the speed up
relative to a fixed time step grows with the linear size of the system as N/log
N, and we estimate conservatively that an 8192^2 lattice can be integrated 300
times faster than with the Euler method.Comment: 14 pages, 6 figure
Surface Enhanced Raman Spectroscopy of Organic Molecules on Magnetite (Fe_3O_4) Nanoparticles
Surface-enhanced Raman spectroscopy (SERS) of species bound to environmentally relevant oxide nanoparticles is largely limited to organic molecules structurally related to catechol that facilitate a chemical enhancement of the Raman signal. Here, we report that magnetite (Fe_3O_4) nanoparticles provide a SERS signal from oxalic acid and cysteine via an electric field enhancement. Magnetite thus likely provides an oxide substrate for SERS study of any adsorbed organic molecule. This substrate combines benefits from both metal-based and chemical SERS by providing an oxide surface for studies of environmentally and catalytically relevant detailed chemical bonding information with fewer restrictions of molecular structure or binding mechanisms. Therefore, the magnetite-based SERS demonstrated here provides a new approach to establishing the surface interactions of environmentally relevant organic ligands and mineral surfaces
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