arXiv subjects

Stefano Fantoni

Publications and source records attributed to Stefano Fantoni.

14 recordsLinked to original sources

arXiv2013

Anisotropic Ginzburg-Landau and Lawrence-Doniach Models for Layered Ultracold Fermi Gases

We study the anisotropic Ginzburg-Landau and Lawrence-Doniach models describing a layered superfluid ultracold Fermi gas in optical lattices. We derive the coefficients of the anisotropic Ginzburg-Landau and the mass tensor as a function of anisotropy, filling and interaction, showing that near the unitary limit the effective anisotropy of the masses is significantly reduced. The anisotropy parameter is shown to vary in realistic setups in a wide range of values. We also derive the Lawrence-Doniach model - often used to describe the 2D-3D dimensional crossover in layered superconductors - for a layered ultracold Fermi gas, obtaining a relation between the interlayer Josephson couplings and the Ginzburg-Landau masses. Comparing to the Ginzburg-Landau description, we find that the region of validity of the Lawrence-Doniach model is near the unitary limit.

cond-mat.quant-gas↗

arXiv2012

Density-dependent nucleon-nucleon interaction from UIX three-nucleon force

A density-dependent two-nucleon potential has been derived in the formalism of correlated basis function. The effects of 3-particle interactions has been included by integrating out the degrees of freedom of the third nucleon. The potential can be easily employed in nuclear matter calculations. It yields results in agreement with those obtained from the underlying three-body potential. The use of the density dependent potential allowed us to study the effects of three-nucleon interactions in symmetric nuclear matter within the Auxiliary Field Diffusion Monte Carlo (AFDMC) computational scheme.

nucl-th↗

arXiv2011

Vortex Lines Distribution in Inhomogeneous Lattices

We study the distribution of vortex lines in a three-dimensional lattice with inhomogeneous couplings. We investigate the spatial distribution of the number of vortex lines, showing how the vortex lines are expelled from the region with higher couplings. Results for the correlation functions are presented, together with a discussion of the implications of the obtained results for the vortex distribution in ultracold trapped gases and in neutron stars.

cond-mat.stat-mech↗

arXiv2011

Recent progress on the accurate determination of the equation of state of neutron and nuclear matter

The problem of accurately determining the equation of state of nuclear and neutron matter at density near and beyond saturation is still an open challenge. In this paper we will review the most recent progress made by means of Quantum Monte Carlo calculations, which are at present the only ab-inito method capable to treat a sufficiently large number of particles to give meaningful estimates depending only on the choice of the nucleon-nucleon interaction. In particular, we will discuss the introduction of density-dependent interactions, the study of the temperature dependence of the equation of state, and the possibility of accurately studying the effect of the onset of hyperons by developing an accurate hyperon-nucleon and hyperon-nucleon-nucleon interaction.

nucl-th↗

arXiv2011

Comparative study of three-nucleon potentials in nuclear matter

A new generation of local three-body potentials providing an excellent description of the properties of light nuclei, as well as of the neutron-deuteron doublet scattering length, has been recently derived. We have performed a comparative analysis of the equations of state of both pure neutron matter and symmetric nuclear matter obtained using these models of three-nucleon forces. None of the considered potentials simultaneously explains the empirical equilibrium density and binding energy of symmetric nuclear matter. However, two of them provide reasonable values of the saturation density. The ambiguity concerning the treatment of the contact term of the chiral inspired potentials is discussed.

nucl-th↗

arXiv2011

Periodic Box FHNC calculations of neutron star crustal matter. (I)

Neutron star crustal matter, whose properties are relevant in many models aimed at explaining observed astrophysical phenomena, has so far always been studied using a mean field approach. In order to check the results obtained in this way, a sensible next step is to make use of a realistic nuclear potential. The present paper extends the periodic-box Fermi HyperNetted Chain method to include longitudinal-isospin dependence of the correlations, making feasible a study of asymmetric crustal matter. Results are presented for the symmetry energy, the low-density neutron star equation of state and the single particle neutron and proton energies.

nucl-th↗

arXiv2010

Density-dependent nucleon-nucleon interaction from three-nucleon forces

Microscopic calculations based on realistic nuclear hamiltonians, while yielding accurate results for the energies of the ground and low-lying excited states of nuclei with $A \leq 12$, fail to reproduce the empirical equilibrium properties of nuclear matter, that are known to be significantly affected by three-nucleon forces. We discuss a scheme suitable to construct a density-dependent two-nucleon potential, in which the effects of $n$-particle interactions can be included by integrating out the degrees of freedom of $(n-2)$-nucleons. Our approach, based on the formalism of correlated basis function and state-of-the-art models of the two- and three-nucleon potentials, leads to an effective interactionthat can be easily employed in nuclear matter calculations, yielding results in good agreement with those obtained from the underlying three-body potential.

nucl-th↗

arXiv2009

Effect of random on-site energies on the critical temperature of a lattice Bose gas

We study the effect of random on-site energies on the critical temperature of a non-interacting Bose gas on a lattice. In our derivation the on-site energies are distributed according a Gaussian probability distribution function having vanishing average and variance $v_o^2$. By using the replicated action obtained by averaging on the disorder, we perform a perturbative expansion for the Green functions of the disordered system. We evaluate the shift of the chemical potential induced by the disorder and we compute, for $v_o^2 << 1$, the critical temperature for condensation. We find that, for large filling, disorder slightly enhances the critical temperature for condensation.

cond-mat.stat-mech↗

arXiv2008

Monte Carlo approach to nuclei and nuclear matter

We report on the most recent applications of the Auxiliary Field Diffusion Monte Carlo (AFDMC) method. The equation of state (EOS) for pure neutron matter in both normal and BCS phase and the superfluid gap in the low--density regime are computed, using a realistic Hamiltonian containing the Argonne AV8' plus Urbana IX three--nucleon interaction. Preliminary results for the EOS of isospin--asymmetric nuclear matter are also presented.

nucl-th↗

arXiv2008

S-pairing in neutron matter. I. Correlated Basis Function Theory

S-wave pairing in neutron matter is studied within an extension of correlated basis function (CBF) theory to include the strong, short range spatial correlations due to realistic nuclear forces and the pairing correlations of the Bardeen, Cooper and Schrieffer (BCS) approach. The correlation operator contains central as well as tensor components. The correlated BCS scheme of Ref. [Nucl. Phys. A363 (1981) 383], developed for simple scalar correlations, is generalized to this more realistic case. The energy of the correlated pair condensed phase of neutron matter is evaluated at the two--body order of the cluster expansion, but considering the one--body density and the corresponding energy vertex corrections at the first order of the Power Series expansion. Based on these approximations, we have derived a system of Euler equations for the correlation factors and for the BCS amplitudes, resulting in correlated non linear gap equations, formally close to the standard BCS ones. These equations have been solved for the momentum independent part of several realistic potentials (Reid, Argonne v_{14} and Argonne v_{8'}) to stress the role of the tensor correlations and of the many--body effects. Simple Jastrow correlations and/or the lack of the density corrections enhance the gap with respect to uncorrelated BCS, whereas it is reduced according to the strength of the tensor interaction and following the inclusion of many--body contributions.

nucl-th↗

arXiv2007

Quantum Monte Carlo calculations of symmetric nuclear matter

We present an accurate numerical study of the equation of state of nuclear matter based on realistic nucleon--nucleon interactions by means of Auxiliary Field Diffusion Monte Carlo (AFDMC) calculations. The AFDMC method samples the spin and isospin degrees of freedom allowing for quantum simulations of large nucleonic systems and can provide quantitative understanding of problems in nuclear structure and astrophysics.

nucl-th↗

arXiv2006

$^1S_0$ superfluid phase--transition in neutron matter with realistic nuclear potentials and modern many--body theories

The $^1S_0$ pairing in neutron matter has been investigated in presence of realistic two-- and three--nucleon interactions. We have adopted the Argonne $v_{8^\prime}$ NN and the Urbana IX 3N potentials. Quantum Monte Carlo theory, specifically the Auxiliary Field Diffusion Monte Carlo method, and Correlated Basis Function theory are employed in order to get quantitative and reliable estimates of the gap. They both fully take into account the medium modifications due to the interaction induced correlations. The two methods are in good agreement up to the maximum gap density and both point to a slight reduction with respect to the standard BCS value. In fact, the maximum gap is about $2.5 \text{MeV}$ at $k_F \sim 0.8 \text{fm}^{-1}$ in BCS and 2.3--$2.4 \text{MeV}$ at $k_F \sim 0.6 \text{fm}^{-1}$ in correlated matter. At higher densities the Quantum Monte Carlo gap becomes close to BCS. In general, the computed medium polarization effects are much smaller than those previously estimated within \emph{all theories}. Truncations of Argonne $v_{8^\prime}$ to simpler forms give the same gaps in BCS, provided the truncated potentials have been refitted to the same NN data set. Differences among the models appear in the correlated theories, most of the reduction being attributable to the tensor force. The three--nucleon interaction provides an additional increase of the gap of about 0.35 MeV.

nucl-th↗

arXiv2002

Structure, rotational dynamics, and superfluidity of small OCS-doped He clusters

The structural and dynamical properties of OCS molecules solvated in Helium clusters are studied using reptation quantum Monte Carlo, for cluster sizes n=3-20 He atoms. Computer simulations allow us to establish a relation between the rotational spectrum of the solvated molecule and the structure of the He solvent, and of both with the onset of superfluidity. Our results agree with a recent spectroscopic study of this system, and provide a more complex and detailed microscopic picture of this system than inferred from experiments.

cond-mat↗

arXiv1996

Momentum distribution of liquid helium

We have obtained the one--body density matrix and the momentum distribution $n(p)$ of liquid $^4$He at $T=3D0^o$K from Diffusion Monte Carlo (DMC) simulations, using trial functions optimized via the Euler Monte Carlo (EMC) method. We find a condensate fraction smaller than in previous calculations. Though we do not explicitly include long--range correlations in our calculations, we get a momentum distribution at long wavelength which is compatible with the presence of long--range correlations in the exact wave function. We have also studied $^3$He, using fixed--node DMC, with nodes and trial functions provided by the EMC. In particular, we analyze the momentum distribution $n(p)$ with respect to the discontinuity $Z$ as well as the singular behavior, at the Fermi surface. We also show that an approximate factorization of the one-body density matrix $ρ(r)\simeq ρ_0(r)ρ_B(r)$ holds, with $ρ_0(r)$ and $ρ_B(r)$ respectively the density matrix of the ideal Fermi gas and the density matrix of a Bose $^3$He.

cond-mat↗