2,678 research outputs found
Phase Transition in the Random Anisotropy Model
The influence of a local anisotropy of random orientation on a ferromagnetic
phase transition is studied for two cases of anisotropy axis distribution. To
this end a model of a random anisotropy magnet is analyzed by means of the
field theoretical renormalization group approach in two loop approximation
refined by a resummation of the asymptotic series. The one-loop result of
Aharony indicating the absence of a second-order phase transition for an
isotropic distribution of random anisotropy axis at space dimension is
corroborated. For a cubic distribution the accessible stable fixed point leads
to disordered Ising-like critical exponents.Comment: 10 pages, 2 latex figures and a style file include
Critical slowing down in random anisotropy magnets
We study the purely relaxational critical dynamics with non-conserved order
parameter (model A critical dynamics) for three-dimensional magnets with
disorder in a form of the random anisotropy axis. For the random axis
anisotropic distribution, the static asymptotic critical behaviour coincides
with that of random site Ising systems. Therefore the asymptotic critical
dynamics is governed by the dynamical exponent of the random Ising model.
However, the disorder influences considerably the dynamical behaviour in the
non-asymptotic regime. We perform a field-theoretical renormalization group
analysis within the minimal subtraction scheme in two-loop approximation to
investigate asymptotic and effective critical dynamics of random anisotropy
systems. The results demonstrate the non-monotonic behaviour of the dynamical
effective critical exponent .Comment: 11 pages, 4 figures, style file include
Model C critical dynamics of random anisotropy magnets
We study the relaxational critical dynamics of the three-dimensional random
anisotropy magnets with the non-conserved n-component order parameter coupled
to a conserved scalar density. In the random anisotropy magnets the structural
disorder is present in a form of local quenched anisotropy axes of random
orientation. When the anisotropy axes are randomly distributed along the edges
of the n-dimensional hypercube, asymptotical dynamical critical properties
coincide with those of the random-site Ising model. However structural disorder
gives rise to considerable effects for non-asymptotic critical dynamics. We
investigate this phenomenon by a field-theoretical renormalization group
analysis in the two-loop order. We study critical slowing down and obtain
quantitative estimates for the effective and asymptotic critical exponents of
the order parameter and scalar density. The results predict complex scenarios
for the effective critical exponent approaching an asymptotic regime.Comment: 8 figures, style files include
Static and dynamic structure factors in three-dimensional randomly diluted Ising models
We consider the three-dimensional randomly diluted Ising model and study the
critical behavior of the static and dynamic spin-spin correlation functions
(static and dynamic structure factors) at the paramagnetic-ferromagnetic
transition in the high-temperature phase. We consider a purely relaxational
dynamics without conservation laws, the so-called model A. We present Monte
Carlo simulations and perturbative field-theoretical calculations. While the
critical behavior of the static structure factor is quite similar to that
occurring in pure Ising systems, the dynamic structure factor shows a
substantially different critical behavior. In particular, the dynamic
correlation function shows a large-time decay rate which is momentum
independent. This effect is not related to the presence of the Griffiths tail,
which is expected to be irrelevant in the critical limit, but rather to the
breaking of translational invariance, which occurs for any sample and which, at
the critical point, is not recovered even after the disorder average.Comment: 43 page
Real-Time Imaging of K atoms on Graphite: Interactions and Diffusion
Scanning tunneling microscopy (STM) at liquid helium temperature is used to
image potassium adsorbed on graphite at low coverage (~0.02 monolayer). Single
atoms appear as protrusions on STM topographs. A statistical analysis of the
position of the atoms demonstrates repulsion between adsorbates, which is
quantified by comparison with molecular dynamics simulations. This gives access
to the dipole moment of a single adsorbate, found to be 10.5 Debye. Time lapse
imaging shows that long range order is broken by thermally activated diffusion,
with a 32 meV barrier to hopping between graphite lattice sites
Critical dynamics of diluted relaxational models coupled to a conserved density (diluted model C)
We consider the influence of quenched disorder on the relaxational critical
dynamics of a system characterized by a non-conserved order parameter coupled
to the diffusive dynamics of a conserved scalar density (model C). Disorder
leads to model A critical dynamics in the asymptotics, however it is the
effective critical behavior which is often observed in experiments and in
computer simulations and this is described by the full set of dynamical
equations of diluted model C. Indeed different scenarios of effective critical
behavior are predicted.Comment: 4 pages, 5 figure
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