arXiv subjects

A. Polls

Publications and source records attributed to A. Polls.

At least 91 records · Page 5Linked to original sources

arXiv2000

The Longitudinal and Transverse responses in inclusive electron scattering within the RPA framework

The longitudinal and transverse nuclear responses to inclusive electron scattering reactions are analyzed within the Random Phase Approximation (RPA) framework. Several residual interactions are considered and it is shown that the exchange terms in the RPA make very difficult to find an effective residual interaction capable of reproducing simultaneously the quasielastic peak of both the longitudinal and transverse responses. By means of a simplified model it is illustrated that the residual interaction used in a ring approximation must fulfill some restrictions in order to qualitatively reproduce the full RPA results.

nucl-th↗

arXiv2000

Coherent and Incoherent Dynamic Structure Function of the Free Fermi Gas

A detailed calculation of the coherent and incoherent dynamic structure functions of the free Fermi gas, starting from their expressions in terms of the one- and semi-diagonal two-body density matrices, is derived and discussed. Their behavior and evolution with the momentum transfer is analyzed, and particular attention is devoted to the contributions that both functions present at negative energies. Finally, an analysis of the energy weighted sum rules satisfied by both responses is also performed. Despite of the simplicity of the model, some of the conclusions can be extended to realistic systems.

cond-mat.soft↗

arXiv2000

Finite Temperature Dynamic Structure Function of the Free Fermi Gas

A detailed discussion of the coherent and incoherent dynamic structure function of the free Fermi gas at finite temperature is presented. Their behavior and evolution with the momentum transfer and the temperature is analyzed, while particular attention is paid to the way in which their relative contribution changes with respect to the $\tilde{T}=0$ case. The influence of thermal effects on the lowest order sum rules of the coherent and incoherent responses is also discussed. Finally, the scaling properties of the responses at high momentum transfer as a function of the temperature are also analyzed.

cond-mat.soft↗

arXiv2000

Two-Body Correlations in Nuclear Systems

Correlations in the nuclear wave-function beyond the mean-field or Hartree-Fock approximation are very important to describe basic properties of nuclear structure. Various approaches to account for such correlations are described and compared to each other. This includes the hole-line expansion, the coupled cluster or ``exponential S'' approach, the self-consistent evaluation of Greens functions, variational approaches using correlated basis functions and recent developments employing quantum Monte-Carlo techniques. Details of these correlations are explored and their sensitivity to the underlying nucleon-nucleon interaction. Special attention is paid to the attempts to investigate these correlations in exclusive nucleon knock-out experiments induced by electron scattering. Another important issue of nuclear structure physics is the role of relativistic effects as contained in phenomenological mean field models. The sensitivity of various nuclear structure observables on these relativistic features are investigated. The report includes the discussion of nuclear matter as well as finite nuclei.

nucl-th↗

arXiv1999

Properties of β-stable neutron star matter with hyperons

We present results from many-body calculations for β-stable neutron star matter with nucleonic and hyperonic degrees of freedom, employing the most recent parametrizations of the baryon-baryon interaction of the Nijmegen group. It is found that the only strange baryons emerging in β-stable matter up to total baryonic densities of 1.2 fm^{-3} are Σ^- and Λ. The corresponding equations of state are thence used to compute properties of neutron stars such as the masses, moments of inertia and radii. We also study the possibility of forming a hyperon superfluid and discuss its implications for neutron stars.

nucl-th↗

arXiv1999

A Microscopic Look at Liquid Helium: the 3He Impurity Case

The description of the properties of liquid Helium is a challenge for any microscopic many-body theory. In this context, we study the ground state and the excitation spectrum of one $^3$He impurity in liquid $^4$He at T=0 with the aim of illustrating the power of the correlated basis function formalism in describing heavily correlated systems. The strong interatomic interaction and the large density require the theory to be pushed to a high degree of sophistication. A many-body correlation operator containing explicit two- and thre-particle correlation functions is needed to obtain a realistic ground state wave function, whereas a perturbative expansion including up to two phonon correlated states must be enforced to study the impurity excitation energies. The theory describes accurately the experimental spectrum along all the available momentum range. As empirically shown by the experiments, a marked deviation from the quadratic Landau-Pomeranchuck behavior is found and the momentum dependent effective mass of the impurity increases of $\sim50~%$ at $q\sim1.7~Å^{-1}$ with respect to its q=0 value. Although the main emphasis is given to the Correlated Basis Function theory, we present also comparisons with other methods, as diffusion Monte Carlo, variational Monte Carlo with shadow wave functions and time dependent correlations.

cond-mat↗

arXiv1999

Strange nuclear matter within Brueckner-Hartree-Fock Theory

We have developed a formalism for microscopic Brueckner-type calculations of dense nuclear matter that includes all types of baryon-baryon interactions and allows to treat any asymmetry on the fractions of the different species (n, p, $Λ$, $Σ^0$, $Σ^+$, $Σ^-$, $Ξ^-$ and $Ξ^0$). We present results for the different single-particle potentials focussing on situations that can be relevant in future microscopic studies of beta-stable neutron star matter with strangeness. We find the both the hyperon-nucleon and hyperon-hyperon interactions play a non-negligible role in determining the chemical potentials of the different species.

nucl-th↗

arXiv1999

Correlations derived from Modern Nucleon-Nucleon Potentials

Various modern nucleon-nucleon (NN) potentials yield a very accurate fit to the nucleon-nucleon scattering phase shifts. The differences between these interactions in describing properties of nuclear matter are investigated. Various contributions to the total energy are evaluated employing the Hellmann - Feynman theorem. Special attention is paid to the two-nucleon correlation functions derived from these interactions. Differences in the predictions of the various interactions can be traced back to the inclusion of non-local terms.

nucl-th↗

arXiv1999

Beyond Gross-Pitaevskii:local density vs. correlated basis approach for trapped bosons

We study the ground state of a system of Bose hard-spheres trapped in an isotropic harmonic potential to investigate the effect of the interatomic correlations and the accuracy of the Gross-Pitaevskii equation. We compare a local density approximation, based on the energy functional derived from the low density expansion of the energy of the uniform hard sphere gas, and a correlated wave function approach which explicitly introduces the correlations induced by the potential. Both higher order terms in the low density expansion, beyond Gross-Pitaevskii, and explicit dynamical correlations have effects of the order of percent when the number of trapped particles becomes similar to that attained in recent experiments.

cond-mat↗

arXiv1998

Binding energy of $Λ$ hypernuclei from realistic YN interactions

s- and p-wave $Λ$ single-particle enrgies are obtained for a variety of $Λ$ hypernuclei from the relevant self-energy constructed within the framework of a perturbative many-body approach. Results are presented for present realistic hyperon-nucleon interactions such as Jülich B and Nijmegen SC models. Effects of the non-locality and energy-dependence of the self-energy on the bound states are investigated.

nucl-th↗

arXiv1998

Isospin symmetry breaking nucleon-nucleon potentials and nuclear structure

Modern nucleon-nucleon (NN) potentials, which accurately fit the nucleon-nucleon scattering phase shifts, contain terms which break isospin symmetry. The effects of these symmetry violating terms on the bulk properties of nuclear matter are investigated. The predictions of the charge symmetry breaking (CSB) terms are compared with the Nolen-Schiffer (NS) anomaly regarding the energies of neighboring mirror nuclei. We find that, for a quantitative explanation of the NS anomaly, it is crucial to include CSB in partial waves with $L>0$ (besides $^1S_0$) as derived from a microscopic model for CSB of the NN interaction.

nucl-th↗

arXiv1998

3He impurity excitation spectrum in liquid 4He

We evaluate microscopically the excitation spectrum of the 3He impurity in liquid 4He at T=0 and compare it with the experimental curve at the equilibrium density. The adopted correlated basis perturbative scheme includes up to two independent phonons, intermediate correlated states and the correlation operator is built up with two- and three-body correlation functions. The experimental spectrum is well described by the theory along all the available momentum range. A marked deviation from the simple Landau-Pomeranchuck, quadratic behavior is found and the momentum dependent effective mass of the impurity increases of about 50% at q around 1.7 A^-1 respect to its q=0 value. No signature of roton-like structures is found.

cond-mat.supr-con↗

arXiv1998

Hyperon properties in finite nuclei using realistic $YN$ interactions

Single-particle energies of the $Λ$ and $Σ$ hyperons are obtained from the relevant self-energies. The latter are constructed within the framework of a perturbative many-body approach employing present realistic hyperon-nucleon interactions such as the models of the Juelich and Nijmegen groups. The effects of the non-locality and energy dependence of the self-energy on the bound states are investi- gated. It is also shown that, although the single-particle hyperon energies are well reproduced by local Woods-Saxon hyperon-nucleus potentials, the wave functions from the non-local self-energy are far more extended. Implications of this behavior on the mesonic weak decay of $Λ$ hypernuclei are discused.

nucl-th↗

arXiv1998

Analysis of exchange terms in a projected ERPA Theory applied to the quasi-elastic (e,e') reaction

A systematic study of the influence of exchange terms in the longitudinal and transverse nuclear response to quasi-elastic (e,e') reactions is presented. The study is performed within the framework of the extended random phase approximation (ERPA), which in conjuction with a projection method permits a separation of various contributions tied to different physical processes. The calculations are performed in nuclear matter up to second order in the residual interaction for which we take a (pi+rho)-model with the addition of the Landau-Migdal g'-parameter. Exchange terms are found to be important only for the RPA-type contributions around the quasielastic peak.

nucl-th↗

arXiv1997

Phaseshift equivalent NN potentials and the deuteron

Different modern phase shift equivalent NN potentials are tested by evaluating the partial wave decomposition of the kinetic and potential energy of the deuteron. Significant differences are found, which are traced back to the matrix elements of the potentials at medium and large momenta. The influence of the localisation of the one-pion-exchange contribution to these potentials is analyzed in detail.

nucl-th↗

arXiv1997

Modern nucleon-nucleon potentials and symmetry energy in infinite matter

We study the symmetry energy in infinite nuclear matter employing a non-relativistic Brueckner-Hartree-Fock approach and using various new nucleon-nucleon (NN) potentials, which fit np and pp scattering data very accurately. The potential models we employ are the recent versions of the Nijmegen group, Nijm-I, Nijm-II and Reid93, the Argonne $V_{18}$ potential and the CD-Bonn potential. All these potentials yield a symmetry energy which increases with density, resolving a discrepancy that existed for older NN potentials. The origin of remaining differences is discussed.

nucl-th↗

arXiv1997

Description of recent large-$q$ neutron inclusive scattering data from liquid $^4$He

We report dynamical calculations for large-$q$ structure functions of liquid $^4$He at $T$=1.6 and 2.3 K and compare those with recent MARI data. We extend those calculations far beyond the experimental range $q\le 29\Ain$ in order to study the approach of the response to its asymptotic limit for a system with interactions having a strong short-range repulsion. We find only small deviations from theoretical $1/q$ behavior, valid for smooth $V$. We repeat an extraction by Glyde et al of cumulant coefficients from data. We argue that fits determine the single atom momentum distribution, but express doubt as to the extraction of meaningful Final State Interaction parameters.

cond-mat↗

arXiv1997

Momentum distributions in ^3He-^4He liquid mixtures

We present variational calculations of the one-body density matrices and momentum distributions for ^3He-^4He mixtures in the zero temperature limit, in the framework of the correlated basis functions theory. The ground-state wave function contains two- and three-body correlations and the matrix elements are computed by (Fermi)Hypernetted Chain techniques. The dependence on the ^3He concentration (x_3) of the ^4He condensate fraction $(n_0^{(4)})$ and of the ^3He pole strength (Z_F) is studied along the P=0 isobar. At low ^3He concentration, the computed ^4He condensate fraction is not significantly affected by the ^3He statistics. Despite of the low x_3 values, Z_F is found to be quite smaller than that of the corresponding pure ^3He because of the strong ^3He-^4He correlations and of the overall, large total density ρ. A small increase of $n_0^{(4)}$ along x_3 is found, which is mainly due to the decrease of ρrespect to the pure ^4He phase.

cond-mat.stat-mech↗