Experimental Characterization of the Ising Model in Disordered Antiferromagnets

The current status of experiments on the d=2 and d=3 random-exchange and random-field Ising models, as realized in dilute anisotropic antiferromagnets, is discussed. Two areas of current investigation are emphasized. For d=3, the large random field limit is being investigated and equilibrium critical behavior is being characterized at high magnetic concentrations.Comment: 19 pages, 7 figures, Ising Centennial Colloquium, to be published in the Brazilian Journal of Physic

The specific heat and optical birefringence of Fe(0.25)Zn(0.75)F2

The specific heat (Cm) and optical birefringence (\Delta n) for the magnetic percolation threshold system Fe(0.25)Zn(0.75)F2 are analyzed with the aid of Monte Carlo (MC) simulations. Both \Delta n and the magnetic energy (Um) are governed by a linear combination of near-neighbor spin-spin correlations, which we have determined for \Delta n using MC simulations modeled closely after the real system. Near a phase transition or when only one interaction dominates, the temperature derivative of the birefringence [{d(\Delta n)}/{dT}] is expected to be proportional Cm since all relevant correlations necessarily have the same temperature dependence. Such a proportionality does not hold for Fe(0.25)Zn(0.75)F2 at low temperatures, however, indicating that neither condition above holds. MC results for this percolation system demonstrate that the shape of the temperature derivative of correlations associated with the frustrating third-nearest-neighbor interaction differs from that of the dominant second-nearest-neighbor interaction, accurately explaining the experimentally observed behavior quantitatively.Comment: 16 pages, 5 figure

The random field critical concentration in dilute antiferromagnets

Monte Carlo techniques are used to investigate the equilibrium threshold concentration, xe, in the dilute anisotropic antiferromagnet Fe(x)Zn(1-x)F2 in an applied magnetic field, considered to be an ideal random-field Ising model system. Above xe equilibrium behavior is observed whereas below xe metastability and domain formation dominate. Monte Carlo results agree very well with experimental data obtained using this system.Comment: 9 pages, 3 figure

Experiments on the random field Ising model

New advances in experiments on the random-field Ising model, as realized in dilute antiferromagnets, have brought us much closer to a full characterization of the static and dynamic critical behavior of the unusual phase transition in three dimensions (d=3). The most important experiments that have laid the ground work for our present understanding are reviewed. Comparisons of the data with Monte Carlo simulations of the d=3 critical behavior are made. We review the current experimental understanding of the destroyed d=2 transition and the experiments exploring the d=2 metastability at low T. Connections to theories most relevant to the interpretations of all the experiments are discussed.Comment: 25 pages, 5 figures, LaTeX, to be published in World Scientific "Spin Glasses and Random Fields", ed. A. P. Youn

CalcHEP 3.4 for collider physics within and beyond the Standard Model

We present version 3.4 of the CalcHEP software package which is designed for effective evaluation and simulation of high energy physics collider processes at parton level. The main features of CalcHEP are the computation of Feynman diagrams, integration over multi-particle phase space and event simulation at parton level. The principle attractive key-points along these lines are that it has: a) an easy startup even for those who are not familiar with CalcHEP; b) a friendly and convenient graphical user interface; c) the option for a user to easily modify a model or introduce a new model by either using the graphical interface or by using an external package with the possibility of cross checking the results in different gauges; d) a batch interface which allows to perform very complicated and tedious calculations connecting production and decay modes for processes with many particles in the final state. With this features set, CalcHEP can efficiently perform calculations with a high level of automation from a theory in the form of a Lagrangian down to phenomenology in the form of cross sections, parton level event simulation and various kinematical distributions. In this paper we report on the new features of CalcHEP 3.4 which improves the power of our package to be an effective tool for the study of modern collider phenomenology.Comment: 82 pages, elsarticle LaTeX, 7 Figures. Changes from v1: 1) updated reference list and Acknowledgments; 2) 2->1 processes added to CalcHEP; 3) particles decay (i.e. Higgs boson) into virtual W/Z decays added together with comparison to results from Hdecay package; 4) added interface with Root packag

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