Fabio Cinti

One-dimensional Ising ferromagnet frustrated by long-range interactions at finite temperatures

We consider a one-dimensional lattice of Ising-type variables where the ferromagnetic exchange interaction J between neighboring sites is frustrated by a long-ranged anti-ferromagnetic interaction of strength g between the sites i and j, decaying as |i-j|^-alpha, with alpha>1. For alpha smaller than a certain threshold alpha_0, which is larger than 2 and depends on the ratio J/g, the ground state consists of an ordered sequence of segments with equal length and alternating magnetization. The width of the segments depends on both alpha and the ratio J/g. Our Monte Carlo study shows that the on-site magnetization vanishes at finite temperatures and finds no indication of any phase transition. Yet, the modulation present in the ground state is recovered at finite temperatures in the two-point correlation function, which oscillates in space with a characteristic spatial period: The latter depends on alpha and J/g and decreases smoothly from the ground-state value as the temperature is increased. Such an oscillation of the correlation function is exponentially damped over a characteristic spatial scale, the correlation length, which asymptotically diverges roughly as the inverse of the temperature as T=0 is approached. This suggests that the long-range interaction causes the Ising chain to fall into a universality class consistent with an underlying continuous symmetry. The e^(Delta/T)-temperature dependence of the correlation length and the uniform ferromagnetic ground state, characteristic of the g=0 discrete Ising symmetry, are recovered for alpha > alpha_0.

Structure, Bose-Einstein condensation, and superfluidity of two-dimensional confined dipolar assemblies

Low temperature properties of harmonically confined two-dimensional assemblies of dipolar bosons are systematically investigated by Monte Carlo simulations. Calculations carried out for different numbers of particles and strengths of the confining potential yield evidence of a quantum phase transition from a superfluid to a crystal-like phase, consistently with what is observed in the ho- mogeneous system. It is found that the crystal phase nucleates in the center of the trap, as the density increases. Bose-Einstein condensation vanishes at T = 0 upon entering the crystalline phase, concurrently with the disappearance of the superfluid response.

Monte Carlo simulations of rare-earth holmium ultrathin films

Motivated by recent experimental results in ultra-thin helimagnetic Holmium films, we have performed an extensive classical Monte Carlo simulation of films of different thickness, assuming a Hamiltonian with six inter-layer exchange constants. Both magnetic structure and critical properties have been analyzed. For n > 16 (n being the number of spin layers in the film) a correct bulk limit is reached, while for lower n the film properties are clearly affected by the strong competition among the helical pitch and the surface effects, which involve the majority of the spin layers: In the thickness range n = 9 ÷16 three different magnetic phases emerge, with the high-temperature, disordered, paramagnetic phase and the low-temperature, long-range ordered one separated by an intriguing intermediate-temperature block phase, where outer ordered layers coexist with some inner, disordered ones. The phase transition of these inner layers displays the signatures of a Kosterlitz-Thouless one. Finally, for n . 7 the film collapse once and for all to a quasi-collinear order. A comparison of our Monte Carlo simulation outcomes with available experimental data is also proposed, and further experimental investigations are suggested.

Interplay among helical order, surface effects, and range of interacting layers in ultrathin films

The properties of helical thin films have been thoroughly investigated by classical Monte Carlo simulations. The employed model assumes classical planar spins in a body-centered tetragonal lattice, where the helical arrangement along the film growth direction has been modeled by nearest neighbor and next-nearest neighbor competing interactions, the minimal requirement to get helical order. We obtain that, while the in-plane transition temperatures remain essentially unchanged with respect to the bulk ones, the helical/fan arrangement is stabilized at more and more low temperature when the film thickness, n, decreases; in the ordered phase, increasing the temperature, a softening of the helix pitch wave-vector is also observed. Moreover, we show also that the simulation data around both transition temperatures lead us to exclude the presence of a first order transition for all analyzed sizes. Finally, by comparing the results of the present work with those obtained for other models previously adopted in literature, we can get a deeper insight about the entwined role played by the number (range) of interlayer interactions and surface effects in non-collinear thin films.

Phases of dipolar bosons in a bilayer geometry

CITATION: Cinti, F., Wang, D. W. & Boninsegni, M. 2017. Phases of dipolar bosons in a bilayer geometry. Physical Review A, 95(2):023622, doi:10.1103/PhysRevA.95.023622.

Classical and quantum filaments in the ground state of trapped dipolar Bose gases

CITATION: Cinti, F. & Boninsegni, M. 2017. Classical and quantum filaments in the ground state of trapped dipolar Bose gases. Physical Review A, 96(1):013627, doi:10.1103/PhysRevA.96.013627.

Temperature-Induced Domain Shrinking in Ising Ferromagnets Frustrated by a Long-Range Interaction

We investigate a spin model in which a ferromagnetic short-range interaction competes with a long-range antiferromagnetic interaction decaying spatially as \(\frac{1}{r^{d+\sigma}}\), d being the dimensionality of the lattice. For σ smaller than a certain threshold \(\hat{\sigma}\) (with \(\hat{\sigma}>1\)), the long-range interaction is able to prevent global phase separation, the uniformly magnetized state favored by the exchange interaction for spin systems. The ground state then consists of a mono-dimensional modulation of the order parameter resulting in a superlattice of domains with positive and negative magnetization. We find that the period of modulation shrinks with increasing temperature T and suggest that this is a universal property of the considered model. For d = 2 and σ= 1 (dipolar interaction) Mean-Field (MF) calculations find a striking agreement with experiments performed on atomically-thin Fe/Cu(001) films. Monte Carlo (MC) results for d = 1 also support the generality of our arguments beyond the MF approach.

Magnetic phases in ultrathin helimagnetic holmium films

Detailed Monte Carlo simulations of Ho magnetic thin films are here presented. The system is described by a Heisenberg model with easy-plane single-ion anisotropy and seven coupling constants, as obtained by experimental neutron scattering measurements. The critical behavior landscape comes out extremely complex and fascinating. In fact, for the analyzed thicknesses of 8–16, it is seen that by increasing the temperature the films show, after the distorted helical configuration, a temperature range where disordered inner planes coexist with surface ones in a block ordered state. At low temperature, it is also found that the progressive reduction in the number of layers do not imply a sudden helical order suppression but rather, as a consequence of surface effects, a gradual passage to a fanlike order, accompanied by the helix distortion, is observed.