1,854 research outputs found
Equilibrium random-field Ising critical scattering in the antiferromagnet Fe(0.93)Zn(0.07)F2
It has long been believed that equilibrium random-field Ising model (RFIM)
critical scattering studies are not feasible in dilute antiferromagnets close
to and below Tc(H) because of severe non-equilibrium effects. The high magnetic
concentration Ising antiferromagnet Fe(0.93)Zn(0.07)F2, however, does provide
equilibrium behavior. We have employed scaling techniques to extract the
universal equilibrium scattering line shape, critical exponents nu = 0.87 +-
0.07 and eta = 0.20 +- 0.05, and amplitude ratios of this RFIM system.Comment: 4 pages, 1 figure, minor revision
Non Markovian persistence in the diluted Ising model at criticality
We investigate global persistence properties for the non-equilibrium critical
dynamics of the randomly diluted Ising model. The disorder averaged persistence
probability of the global magnetization is found to decay
algebraically with an exponent that we compute analytically in a
dimensional expansion in . Corrections to Markov process are
found to occur already at one loop order and is thus a novel
exponent characterizing this disordered critical point. Our result is
thoroughly compared with Monte Carlo simulations in , which also include a
measurement of the initial slip exponent. Taking carefully into account
corrections to scaling, is found to be a universal exponent,
independent of the dilution factor along the critical line at , and
in good agreement with our one loop calculation.Comment: 7 pages, 4 figure
Letting the Right Ones In: Whitelists, Jurisdictional Reputation, and the Racial Dynamics of Online Gambling Regulation.
Using a case study of a recent UK whitelist intended to regulate online gambling, I examine the affective politics of listing. I pay particular attention to the racial dynamics of black and white listing. By charting how the gambling whitelist worked and failed to work as a tool in the designation of jurisdictional reputation, I argue that the use and subsequent abandonment of the whitelist shows the centrality of racial dynamics to listing practices, particularly in relation to how the list was deployed in debates about the trustworthiness of the Kahnawá:ke territory and Antigua and Barbuda. In section 5 I examine what happened after the demise of the online gambling whitelist. Although non-listing techniques of governance look to be expanding, in the form of increased surveillance of individual gamblers, lists continue to play a key role in the UK government’s new model of gambling regulation. I suggest that this confirms the co-constitutive and mutually reinforcing nature of black and white lists as techniques of governance, and the value of exploring them and their racialized implications together
Computer simulation of the critical behavior of 3D disordered Ising model
The critical behavior of the disordered ferromagnetic Ising model is studied
numerically by the Monte Carlo method in a wide range of variation of
concentration of nonmagnetic impurity atoms. The temperature dependences of
correlation length and magnetic susceptibility are determined for samples with
various spin concentrations and various linear sizes. The finite-size scaling
technique is used for obtaining scaling functions for these quantities, which
exhibit a universal behavior in the critical region; the critical temperatures
and static critical exponents are also determined using scaling corrections. On
the basis of variation of the scaling functions and values of critical
exponents upon a change in the concentration, the conclusion is drawn
concerning the existence of two universal classes of the critical behavior of
the diluted Ising model with different characteristics for weakly and strongly
disordered systems.Comment: 14 RevTeX pages, 6 figure
Real-space renormalization group for the random-field Ising model
We present real--space renormalization group (RG) calculations of the
critical properties of the random--field Ising model on a cubic lattice in
three dimensions. We calculate the RG flows in a two--parameter truncation of
the Hamiltonian space. As predicted, the transition at finite randomness is
controlled by a zero temperature, disordered critical fixed point, and we
exhibit the universal crossover trajectory from the pure Ising critical point.
We extract scaling fields and critical exponents, and study the distribution of
barrier heights between states as a function of length scale.Comment: 12 pages, CU-MSC-757
Ground state numerical study of the three-dimensional random field Ising model
The random field Ising model in three dimensions with Gaussian random fields
is studied at zero temperature for system sizes up to 60^3. For each
realization of the normalized random fields, the strength of the random field,
Delta and a uniform external, H is adjusted to find the finite-size critical
point. The finite-size critical point is identified as the point in the H-Delta
plane where three degenerate ground states have the largest discontinuities in
the magnetization. The discontinuities in the magnetization and bond energy
between these ground states are used to calculate the magnetization and
specific heat critical exponents and both exponents are found to be near zero.Comment: 10 pages, 6 figures; new references and small changes to tex
Effective and Asymptotic Critical Exponents of Weakly Diluted Quenched Ising Model: 3d Approach Versus -Expansion
We present a field-theoretical treatment of the critical behavior of
three-dimensional weakly diluted quenched Ising model. To this end we analyse
in a replica limit n=0 5-loop renormalization group functions of the
-theory with O(n)-symmetric and cubic interactions (H.Kleinert and
V.Schulte-Frohlinde, Phys.Lett. B342, 284 (1995)). The minimal subtraction
scheme allows to develop either the -expansion series or to
proceed in the 3d approach, performing expansions in terms of renormalized
couplings. Doing so, we compare both perturbation approaches and discuss their
convergence and possible Borel summability. To study the crossover effect we
calculate the effective critical exponents providing a local measure for the
degree of singularity of different physical quantities in the critical region.
We report resummed numerical values for the effective and asymptotic critical
exponents. Obtained within the 3d approach results agree pretty well with
recent Monte Carlo simulations. -expansion does not allow
reliable estimates for d=3.Comment: 35 pages, Latex, 9 eps-figures included. The reference list is
refreshed and typos are corrected in the 2nd versio
Monte Carlo Simulation of a Random-Field Ising Antiferromagnet
Phase transitions in the three-dimensional diluted Ising antiferromagnet in
an applied magnetic field are analyzed numerically. It is found that random
magnetic field in a system with spin concentration below a certain threshold
induces a crossover from second-order phase transition to first-order
transition to a new phase characterized by a spin-glass ground state and
metastable energy states at finite temperatures.Comment: 10 pages, 11 figure
Monte Carlo study of the random-field Ising model
Using a cluster-flipping Monte Carlo algorithm combined with a generalization
of the histogram reweighting scheme of Ferrenberg and Swendsen, we have studied
the equilibrium properties of the thermal random-field Ising model on a cubic
lattice in three dimensions. We have equilibrated systems of LxLxL spins, with
values of L up to 32, and for these systems the cluster-flipping method appears
to a large extent to overcome the slow equilibration seen in single-spin-flip
methods. From the results of our simulations we have extracted values for the
critical exponents and the critical temperature and randomness of the model by
finite size scaling. For the exponents we find nu = 1.02 +/- 0.06, beta = 0.06
+/- 0.07, gamma = 1.9 +/- 0.2, and gammabar = 2.9 +/- 0.2.Comment: 12 pages, 6 figures, self-expanding uuencoded compressed PostScript
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Correlation functions in the two-dimensional random-field Ising model
Transfer-matrix methods are used to study the probability distributions of
spin-spin correlation functions in the two-dimensional random-field Ising
model, on long strips of width sites, for binary field
distributions at generic distance , temperature and field intensity
. For moderately high , and of the order of magnitude used in
most experiments, the distributions are singly-peaked, though rather
asymmetric. For low temperatures the single-peaked shape deteriorates, crossing
over towards a double- ground-state structure. A connection is obtained
between the probability distribution for correlation functions and the
underlying distribution of accumulated field fluctuations. Analytical
expressions are in good agreement with numerical results for ,
low , not too small, and near G=1. From a finite-size {\it ansatz} at
, , averaged correlation functions are predicted to
scale with , . From numerical data we estimate y=0.875 \pm
0.025WR/L=1W \sim h_0^{\kappa} f(L h_0^u)\kappa \simeq 0.45u \simeq 0.8f(x)x \to \inftyW \sim
h_0^{\kappa}d=2$.Comment: RevTeX code for 8 pages, 7 eps figures, to appear in Physical Review
E (1999
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