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B. Alles

Publications and source records attributed to B. Alles.

At least 19 recordsLinked to original sources
arXiv2023

Role of positional disorder in fully textured ensembles of Ising-like dipoles

We study by numerical simulation the magnetic order in ensembles of randomly packed magnetic spherical particles which, induced by their uniaxial anisotropy in the strong coupling limit, behave as Ising dipoles. We explore the role of the frozen disorder in the positions of the particles assuming a common fixed direction for the easy axes of all spheres. We look at two types of spatially disordered configurations. In the first one we consider isotropic positional distributions which can be obtained from the liquid state of the hard sphere fluid. We derive the phase diagram in the $T$-$\Phi$ plane where $T$ is the temperature and $\Phi$ the volume fraction. This diagram exhibits long-range ferromagnetic order at low $T$ for volume fractions above the threshold $\Phi_{c} = 0.157$ predicted by mean-field calculations. For $\Phi \lesssim \Phi_{c}$ a spin-glass phase forms with the same marginal behavior found for other strongly disordered dipolar systems. The second type of spatial configurations we study are anisotropic distributions that can be obtained by freezing a dipolar hard sphere liquid in its polarized state at low temperature. This structural anisotropy enhances the ferromagnetic order present in isotropic hard sphere configurations.

cond-mat.stat-mech↗
arXiv2020

Magnetic ordering of random dense packings of freely rotating dipoles

We study random dense packings of Heisenberg dipoles by numerical simulation. The dipoles are at the centers of identical spheres that occupy fixed random positions in space and fill a fraction $\Phi$ of the spatial volume. The parameter $\Phi$ ranges from rather low values, typical of amorphous ensembles, to the maximum $\Phi$=0.64 that occurs in the random-close-packed limit. We assume that the dipoles can freely rotate and have no local anisotropies. As well as the usual thermodynamical variables, the physics of such systems depends on $\Phi$. Concretely, we explore the magnetic ordering of these systems in order to depict the phase diagram in the temperature-$\Phi$ plane. For $\Phi \gtrsim0.49$ we find quasi-long-range ferromagnetic order coexisting with strong long-range spin-glass order. For $\Phi \lesssim0.49$ the ferromagnetic order disappears giving way to a spin-glass phase similar to the ones found for Ising dipolar systems with strong frozen disorder.

cond-mat.stat-mech↗
arXiv2019

Phase diagram for ensembles of random close packed Ising-like dipoles as a function of texturation

We study random close packed systems of magnetic spheres by Monte Carlo simulations in order to estimate their phase diagram. The uniaxial anisotropy of the spheres makes each of them behave as a single Ising dipole along a fixed easy axis. We explore the phase diagram in terms of the temperature and the degree of alignment (or texturation) among the easy axes of all spheres. This degree of alignment ranges from the textured case (all easy axes pointing along a common direction) to the non-textured case (randomly distributed easy axes). In the former case we find long-range ferromagnetic order at low temperature but, as the degree of alignment is diminished below a certain threshold, the ferromagnetic phase gives way to a spin-glass phase. This spin-glass phase is similar to the one previously found in other dipolar systems with strong frozen disorder. The transition between ferromagnetism and spin-glass passes through a narrow intermediate phase with quasi-long-range ferromagnetic order.

cond-mat.stat-mech↗
arXiv2018

Finite density 2d O(3) sigma model: dualization and numerical simulations

The action of the 2d O(3) non-linear sigma model on the lattice in a bath of particles, when expressed in terms of standard O(3) degrees of freedom, is complex. A reformulation of the model in terms of new variables that makes the action real is presented. This reshaping enables us to utilize Monte Carlo simulations based on usual importance sampling. Several observables, including the correlation function and the mass gap, are measured.

hep-lat↗
arXiv2016

Nature of the spin-glass phase in dense packings of Ising dipoles with random anisotropy axes

Using tempered Monte Carlo simulations, we study the the spin-glass phase of dense packings of Ising dipoles pointing along random axes. We consider systems of L^3 dipoles (a) placed on the sites of a simple cubic lattice with lattice constant $d$, (b) placed at the center of randomly closed packed spheres of diameter d that occupy a 64% of the volume. For both cases we find an equilibrium spin-glass phase below a temperature T_sg. We compute the spin-glass overlap parameter q and their associated correlation length xi_L. From the variation of xi_L with T and L we determine T_sg for both systems. In the spin-glass phase, we find (a) decreases algebraically with L, and (b) xi_L/L does not diverge as L increases. At very low temperatures we find comb-like distributions of q that are sample-dependent. We find that the fraction of samples with cross-overlap spikes higher than a certain value as well as the average width of the spikes are size independent quantities. All these results are consistent with a quasi-long-range order in the spin-glass phase, as found previously for very diluted dipolar systems.

cond-mat.stat-mech↗
arXiv2014

Behavior near $θ=π$ of the mass gap in the 2D O(3) non-linear sigma model

The validity of the Haldane's conjecture entails that the mass gap of the 2-dimensional O(3) non-linear sigma model with a $θ$-term must tend to zero as $θ$ approaches the value $π$ by following a precise law. In the present paper we extract the related critical exponents by simulating the model at imaginary $θ$.

hep-lat↗
arXiv2012

Relativity accommodates superluminal mean velocities

Contrary to a widespread belief, measures of velocity can yield a value larger than $c$, the instantaneous light speed in vacuum, without contradicting Einstein's relativity. Nevertheless, the effect turns out to be too small to explain the recently claimed superluminal velocity by the OPERA collaboration. Several other general relativistic effects acting on the OPERA neutrinos are also analyzed. All of them are unable to explain the OPERA result.

hep-ph↗
arXiv2012

Maximal entanglement from quantum random walks

The conditions under which entanglement becomes maximal are sought in the general one--dimensional quantum random walk with two walkers. Moreover, a one--dimensional shift operator for the two walkers is introduced and its performance in generating entanglement is analyzed as a function of several free parameters, some of them coming from the shift operator itself and some others from the coin operator. To simplify the investigation an averaged entanglement is defined.

quant-ph↗
arXiv2008

Numerical Evidence for the Haldane Conjecture

The Haldane conjecture, when applied to the Heisenberg O(3) model with a θterm in two dimensions, states that the correlation length ξdiverges when θapproaches π. To verify this conjecture we have numerically simulated the model at imaginary θand then analytically continued the results to real θ. We have obtained that the value where the model should become critical is θ=3.10(5) in agreement with the expectation.

cond-mat.stat-mech↗
arXiv2008

Mass gap in the 2D O(3) non-linear sigma model with a theta=pi term

By analytic continuation to real theta of data obtained from numerical simulation at imaginary theta we study the Haldane conjecture and show that the O(3) non-linear sigma model with a theta term in 2 dimensions becomes massless at theta=3.10(5). A modified cluster algorithm has been introduced to simulate the model with imaginary theta. Two different definitions of the topological charge on the lattice have been used; one of them needs renormalization to match the continuum operator. Our work also offers a successful test for numerical methods based on analytic continuation.

cond-mat.stat-mech↗
arXiv2007

Numerical study of the mass spectrum in the 2D O(3) sigma model with a theta term

It has been conjectured that the mass spectrum of the O(3) non-linear sigma model with a theta term in 2 dimensions may possess an excited state, which decays when theta is lowered from pi below a critical value. Since the direct numerical investigation of the model is prevented by a sign problem, we try to infer some information on the mass spectrum at real theta by studying the model at imaginary theta via analytic continuation. A modified Swendsen-Wang cluster algorithm has been introduced to simulate the model with the theta term.

hep-lat↗
arXiv2006

Analysis of systematic errors in the calculation of renormalization constants of the topological susceptibility on the lattice

A Ginsparg-Wilson based calibration of the topological charge is used to calculate the renormalization constants which appear in the field-theoretical determination of the topological susceptibility on the lattice. A systematic comparison is made with calculations based on cooling. The two methods agree within present statistical errors (3%-4%). We also discuss the independence of the multiplicative renormalization constant Z from the background topological charge used to determine it.

hep-lat↗
arXiv2006

Topology, chiral and screening transitions at finite density in two colour QCD

The behaviour of the topological susceptibility in QCD with two colours and 8 flavours of quarks is studied at nonzero temperature on the lattice across the finite density transition. It is shown that its signal drops at a (pseudo-)critical chemical potential mu_c. The Polyakov loop and the chiral condensate undergo their transitions at the same value. Pauli blocking supervenes at a value of the chemical potential larger than mu_c.

hep-lat↗
arXiv2006

Behaviour of the topological susceptibility in two colour QCD across the finite density transition

The behaviour of the topological susceptibility χin QCD with two colours and 8 flavours of quarks is studied at nonzero temperature on the lattice across the finite density transition. It is shown that the signal of χdrops abruptly at a critical chemical potential μ_c, much as it happens at the finite temperature and zero density transition. The Polyakov loop and the chiral condensate undergo their transitions at the same critical value μ_c. At a value μ_s of the chemical potential, called saturation point, which in our case satisfies μ_s > μ_c, Pauli blocking supervenes and consequently the theory becomes quenched.

hep-lat↗
arXiv2006

Topology on the Lattice

We review the method developed in Pisa to determine the topological susceptibility in lattice QCD and present a collection of new and old results obtained by the method.

hep-lat↗
arXiv2005

An upper limit to the electric dipole moment of the neutron from lattice QCD

A linear increase with the volume of the topological susceptibility can signal spontaneous breaking of parity and time inversion, due to a nonzero vacuum expectation value of the topological charge. Such a breaking would produce a nonzero electric dipole moment of the neutron, d_n. An upper limit to d_n is derived from numerical simulations at increasing volumes.

hep-lat↗
arXiv2005

Study of the systematic errors in the calculation of renormalization constants of the topological susceptibility on the lattice

We present a study of the systematic effects in the nonperturbative evaluation of the renormalization constants which appear in the field-theoretical determination of the topological susceptibility in pure Yang-Mills theory. The study is performed by computing the renormalization constants on configurations that have been calibrated by use of the Ginsparg-Wilson formalism and by cooling.

hep-lat↗