Daniel A. Stariolo

Spin reorientation transition and phase diagram of ultrathin ferromagnetic films

We show results from Monte Carlo simulations of a two dimensional Heisenberg model for ultrathin films with perpendicular anisotropy. A complete phase diagram is obtained as a function of anisotropy and temperature, spanning a wide range of behavior. We discuss our results in relation with experimental findings in different ultrathin films. We observe and characterize a line of Spin Reorientation Transitions . This transition from out of plane stripe order to in plane ferromagnetic order presents a paramagnetic gap in between in a finite region in parameter space, as reported in experiments. For large anisotropies direct transitions from a low temperature stripe phase to a paramagnetic or tetragonal phase with dominant perpendicular magnetization is observed, also in agreement with experiments. We also show the phase diagram for a system without exchange, i.e. with pure dipolar and anisotropy interactions. It shows a similar behavior to the ferromagnetic case with antiferromagnetic instead of stripe phases at low temperatures. A Spin Reorientation Transition is also found in this case.

Energy landscape of the finite-size spherical three-spin glass model

Dhagash Mehta, ∗ Daniel A. Stariolo, † and Michael Kastner 4, ‡ Department of Physics, Syracuse University, Syracuse, NY 13244, USA Instituto de F́ısica, Universidade Federal do Rio Grande do Sul and National Institute of Science and Technology for Complex Systems, CP 15051, 91501-970 Porto Alegre, RS, Brasil. Institute of Theoretical Physics, University of Stellenbosch, Stellenbosch 7600, South Africa National Institute for Theoretical Physics (NITheP), Stellenbosch 7600, South Africa

Dynamics of ferromagnetic spherical spin models with power law interactions: exact solution

We solve the Langevin dynamics of d-dimensional ferromagnetic spherical models with interactions that decay with distance as r−(d+σ). The long time dynamics of correlations and responses are studied in detail in the different dynamical regimes and the validity of fluctuation–dissipation relations (or its violation) are shown. In particular, we show that the fluctuation–dissipation ratio X(t+tw,tw) is asymptotically a function only of the waiting time tw in the aging regime and that X→0 as tw→∞. The results are valid in any finite dimension d and for 0<σ<2 where short-range behavior is recovered. We also solve the T=0 Cahn–Hilliard dynamics of this model (conserved order parameter). An analysis of the multiscaling behavior of the autocorrelation function is presented.

Stripe-tetragonal first-order phase transition in ultrathin magnetic films

We analyze the nature of the phase transition from a smectic stripe phase to a tetragonal phase predicted in analytic studies by Abanov et al. [Phys. Rev. B 51, 1023 (1995)] and observed in experiments on ultrathin magnetic films by Vaterlaus et al. [Phys. Rev. Lett. 84, 2247 (2000)]. At variance with existent numerical evidence, we show results of Monte Carlo simulations on a two-dimensional model with competing exchange and dipolar interactions showing strong evidence that the transition is a weak first-order one, in agreement with the theoretical predictions of Abanov et al. Besides the numerical evidence, we give further support to the first-order nature of the transition analyzing a continuum version of the model and showing that it belongs to a large family of systems, or universality class, in which a first-order transition driven by fluctuations is expected on quite general grounds.

Ising nematic phase in ultrathin magnetic films : a Monte Carlo study

We study the critical properties of a two--dimensional Ising model with competing ferromagnetic exchange and dipolar interactions, which models an ultra-thin magnetic film with high out--of--plane anisotropy in the monolayer limit. We present numerical evidence showing that two different scenarios appear in the model for different values of the exchange to dipolar intensities ratio, namely, a single first order stripe - tetragonal phase transition or two phase transitions at different temperatures with an intermediate Ising nematic phase between the stripe and the tetragonal ones. Our results are very similar to those predicted by Abanov et al [Phys. Rev. B 51, 1023 (1995)], but suggest a much more complex critical behavior than the predicted by those authors for both the stripe-nematic and the nematic-tetragonal phase transitions. We also show that the presence of diverging free energy barriers at the stripe-nematic transition makes possible to obtain by slow cooling a metastable supercooled nematic state down to temperatures well below the transition one.

Orientational order in two dimensions from competing interactions at different scales

We discuss orientational order in two dimensions in the context of systems with competing isotropic interactions at different scales. We consider an extension of the Brazovskii model for stripe phases including explicitly quartic terms with nematic symmetry in the energy. We show that leading fluctuations of the mean-field nematic solution drive the isotropic-nematic transition into the Kosterlitz-Thouless universality class; i.e., these systems have a thermodynamic phase with orientational quasi-long-range order.

Fickian crossover and length scales from two point functions in supercooled liquids

Particle motion of a Lennard-Jones supercooled liquid near the glass transition is studied by molecular dynamics simulations. We analyze the wave vector dependence of relaxation times in the incoherent self-scattering function and show that at least three different regimes can be identified and its scaling properties determined. The transition from one regime to another happens at characteristic length scales. The length scale associated with the onset of Fickian diffusion corresponds to the maximum size of heterogeneities in the system, and the characteristic time scale is several times larger than the alpha relaxation time. A second crossover length scale is observed, which corresponds to the typical time and length of heterogeneities, in agreement with results from four point functions. The different regimes can be traced back to the behavior of the van Hove distribution of displacements, which shows a characteristic exponential regime in the heterogeneous region before the crossover to Gaussian diffusion and should be observable in experiments. Our results show that it is possible to obtain characteristic length scales of heterogeneities through the computation of two point functions at different times.

Violation of the Fluctuation-Dissipation Theorem in a Two-Dimensional Ising Model with Dipolar Interactions

Facultad de Matem´atica, Astronom´ia y F´isica, Universidad Nacional de C´ordoba, Ciudad Universitaria, 5000 C´ordoba,Argentina(February 1, 2008)The violation of the Fluctuation-Dissipation Theorem (FDT) in a two-dimensional Ising modelwith both ferromagnetic exchange and antiferromagnetic dipolar interactions is established andinvestigated via Monte Carlo simulations. Through the computation of the autocorrelation C(t +t

Topological hypothesis on phase transitions: The simplest case

We critically analyze the possibility of finding signatures of a phase transition by looking exclusively at static quantities of statistical systems, like, e.g., the topology of potential energy submanifolds (PESs). This topological hypothesis has been successfully tested in a few statistical models but up to now there has been no rigorous proof of its general validity. We make a new test of it analyzing the, probably, simplest example of a nontrivial system undergoing a continuous phase transition: the completely connected version of the spherical model. Going through the topological properties of its PES it is shown that, as expected, the phase transition is correlated with a change in their topology. Nevertheless, this change, as reflected in the behavior of a particular topological invariant, the Euler characteristic, is small, at variance with the strong singularity observed in other systems. Furthermore, it is shown that in the presence of an external field, when the phase transition is destroyed, a similar topology change in the submanifolds is still observed at the maximum value of the potential energy manifold, a level which nevertheless is thermodynamically inaccessible. This suggests that static properties of the PESs are not enough in order to decide whether a phase transition will take place; some input from dynamics seems necessary.

Distance between inherent structures and the influence of saddles on approaching the mode coupling transition in a simple glass former

We analyze through molecular dynamics simulations of a Lennard-Jones (LJ) binary mixture the statistics of the distances between inherent structures sampled at temperatures above the mode coupling transition temperature T(MCT). After equilibrating at T>T(MCT) we take equilibrated configurations and randomly perturb the coordinates of a given number of particles. After that we find the nearby inherent structures (IS) of both the original and perturbed configurations and evaluate the distance between them. This distance presents an inflection point at T(li) approximately 1 with a strong decrease below this temperature which goes to a small but nonzero value on approaching T(MCT). In the low-temperature region we study the statistics of events which give zero distance, i.e., dominated by minima, and find evidence that the number of saddles decreases exponentially near T(MCT). This implies that saddles continue to exist even at T<or=T(MCT). As at T(MCT) the diffusivity goes to zero, our results imply that there are saddles associated with nondiffusive events at T