arXiv subjects

Artur Polls

Publications and source records attributed to Artur Polls.

53 records · Page 3Linked to original sources

arXiv2010

An analytic parametrization of the hyperonic matter equation of state

An analytic parametrization of the hyperonic matter equation of state based on microscopic Brueckner-Hartree-Fock calculations has been constructed using the realistic Argonne V18 nucleon-nucleon potential plus a three-body force of Urbana type, and three models of the hyperon-nucleon interaction: the Nijmegen soft-core models NSC89 and NSC97e, and the most recent meson-exchange potential of the Juelich group. The construction of this parametrization is based on a simple phase-space analysis and reproduces with good accuracy the results of the microscopic calculations with a small number of parameters. This parametrization allows for rapid calculations that accurately mimic the microscopic results, being therefore, very useful from a practical point of view.

nucl-th↗

arXiv2009

Microscopic calculations of transport properties of neutron matter

We discuss the results of calculations of the shear viscosity and thermal conductivity of pure neutron matter, carried out within the Landau-Abrikosov-Khalatnikov formalism. The probability of neutron-neutron collisions in the nuclear medium has been obtained from a realistic potential, using both the correlated basis function and the $G$-matrix approach. The results of our work indicate that medium modifications of nucleon-nucleon scattering are large, their inclusion leading to a dramatic enhancement of the transport coefficients. On the other hand, the results obtained from the two theoretical schemes appear to be in fairly good agreement.

nucl-th↗

arXiv2009

Density dependence of the nuclear symmetry energy: a microscopic perspective

We perform a systematic analysis of the density dependence of the nuclear symmetry energy within the microscopic Brueckner--Hartree--Fock (BHF) approach using the realistic Argonne V18 nucleon-nucleon potential plus a phenomenological three body force of Urbana type. Our results are compared thoroughly to those arising from several Skyrme and relativistic effective models. The values of the parameters characterizing the BHF equation of state of isospin asymmetric nuclear matter fall within the trends predicted by those models and are compatible with recent constraints coming from heavy ion collisions, giant monopole resonances or isobaric analog states. In particular we find a value of the slope parameter $L=66.5$ MeV, compatible with recent experimental constraints from isospin diffusion, $L=88 \pm 25$ MeV. The correlation between the neutron skin thickness of neutron-rich isotopes and the slope, $L$, and curvature, $K_{sym}$, parameters of the symmetry energy is studied. Our BHF results are in very good agreement with the correlations already predicted by other authors using non-relativistic and relativistic effective models. The correlations of these two parameters and the neutron skin thickness with the transition density from non-uniform to $β$-stable matter in neutron stars are also analyzed. Our results confirm that there is an inverse correlation between the neutron skin thickness and the transition density.

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arXiv2008

Spinodal instabilities of asymmetric nuclear matter within the Brueckner--Hartree--Fock approach

We study the spinodal instabilities of asymmetric nuclear matter at finite temperature within the microscopic Brueckner--Hartree--Fock (BHF) approximation using the realistic Argonne V18 nucleon-nucleon potential plus a three-body force of Urbana type. Our results are compared with those obtained with the Skyrme force SLy230a and the relativistic mean field models NL3 and TW. We find that BHF predicts a larger spinodal region. This result is a direct consequence of the fact that our Brueckner calculation predicts a larger critical temperature and saturation density of symmetric nuclear matter than the Skyrme and relativistic mean field ones. We find that the instability is always dominated by total density fluctuations, in agreement with previous results of other authors. We study also the restoration of the isospin symmetry in the liquid phase, {\it i.e.,} the so-called isospin distillation or fragmentation effect, finding that its efficiency increases with increasing proton fraction and decreases as temperature and density increase. In general, we find that the Brueckner results are comparable to those obtained with the Skyrme and the relativistic mean field models, although the restoration of isospin symmetry is not so efficient in this case.

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arXiv2006

The entropy of a correlated system of nucleons

Realistic nucleon-nucleon interaction induce correlations to the nuclear many-body system which lead to a fragmentation of the single-particle strength over a wide range of energies and momenta. We address the question of how this fragmentation affects the thermodynamical properties of nuclear matter. In particular, we show that the entropy can be computed with the help of a spectral function which can be evaluated in terms of the self-energy obtained in the Self-Consistent Green's Function approach. Results for the density and temperature dependences of the entropy per particle for symmetric nuclear matter are presented and compared to the results of lowest order finite temperature Brueckner--Hartree--Fock calculations. The effects of correlations on the calculated entropy are small, if the appropriate quasi-particle approximation is used. The results demonstrate the thermodynamical consistency of the self-consistent T-matrix approximation for the evaluation of the Green's functions.

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arXiv2006

Anomalous specific heat jump in a two-component ultracold Fermi gas

The thermodynamic functions of a Fermi gas with spin population imbalance are studied in the temperature-asymmetry plane in the BCS limit. The low temperature domain is characterized by anomalous enhancement of the entropy and the specific heat above their values in the unpaired state, decrease of the gap and eventual unpairing phase transition as the temperature is lowered. The unpairing phase transition induces a second jump in the specific heat, which can be measured in calorimetric experiments. While the superfluid is unstable against a supercurrent carrying state, it may sustain a metastable state if cooled adiabatically down from the stable high-temperature domain. In the latter domain the temperature dependence of the gap and related functions is analogous to the predictions of the BCS theory.

cond-mat.supr-con↗

arXiv2006

Neutrino propagation in dense hadronic matter

Neutrino propagation in protoneutron stars requires the knowledge of the composition as well as the dynamical response function of dense hadronic matter. Matter at very high densities is probably composed of other particles than nucleons and little is known on the Fermi liquid properties of hadronic multicomponent systems. We will discuss the effects that the presence of $Λ$ hyperons might have on the response and, in particular, on its influence on the thermodynamical stability of the system and the mean free path of neutrinos in dense matter.

nucl-th↗

arXiv2006

Deformed Fermi surfaces in ultracold Fermi gases

The superfluid transition in an ultracold two-component atomic Fermi gas is analyzed in the case where the two components have different densities. We describe a superfluid state which spontaneously breaks the rotational-symmetry by deforming the Fermi surfaces of both species into ellipsoidal form. At relatively large hyperfine-spin asymmetries, this deformation is shown to help the appearance of pairing, which in the rotationally-symmetric (BCS) case would be forbidden by Pauli blocking. The prospects for experimental detection of such a deformed Fermi surface phase are discussed.

cond-mat.supr-con↗

arXiv2006

Dynamics of F=1 87Rb condensates at finite temperatures

We investigate the dynamics of a F=1 spinor Bose-Einstein condensate of 87Rb atoms confined in a quasi-one-dimensional trap both at zero and at finite temperature. At zero temperature, we observe coherent oscillations between populations of the various spin components and the formation of multiple domains in the condensate. We study also finite temperature effects in the spin dynamics taking into account the phase fluctuations in the Bogoliubov-de Gennes framework. At finite T, despite complex multidomain formation in the condensate, population equipartition occurs. The length scale of these spin domains seems to be determined intrinsically by nonlinear interactions.

cond-mat.mes-hall↗

arXiv2005

The effect of the in-medium $Θ^+$ pentaquark on the kaon optical potential

The kaon nuclear optical potential is studied including the effect of the $Θ^+$ pentaquark. The one-nucleon contribution is obtained using an extension of the Jülich meson-exchange potential as bare kaon-nucleon interaction. Significant differences between a fully self-consistent calculation and the usually employed low-density $Tρ$ approach are observed. The influence of the one-nucleon absorption process, $K N \to Θ^+$, on the kaon optical potential is negligible due to the small width of the pentaquark. In contrast, the two-nucleon mechanism, $K N N \to Θ^+ N$, estimated from the coupling of the pentaquark to a two-meson cloud, provides the required amount of additional kaon absorption to reconcile with data the systematically low $K^+$-nucleus reaction cross sections found by the theoretical models.

hep-ph↗

arXiv2005

Sum rules of single-particle spectral functions in hot asymmetric nuclear matter

The neutron and proton single-particle spectral functions in asymmetric nuclear matter fulfill energy weighted sum rules. The validity of these sum rules within the self-consistent Green's function approach is investigated. The various contributions to these sum rules and their convergence as a function of energy provide information about correlations induced by the realistic interaction between the nucleons. These features are studied as a function of the asymmetry of nuclear matter.

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arXiv2005

Pairing in a two-component ultracold Fermi gas: phases with broken space symmetries

We explore the phase diagram of a two-component ultracold atomic Fermi gas interacting with zero-range forces in the limit of weak-coupling. We focus on the dependence of the pairing gap and the free energy on the variations in the number densities of the two species while the total density of the system is held fixed. As the density asymmetry is increased, the system exhibits a transition from a homogenous Bardeen-Cooper-Schrieffer (BCS) phase to phases with spontaneously broken global space symmetries. One such realization is the deformed Fermi surface superfluidity (DFS) which exploits the possibility of deforming the Fermi-surfaces of the species into ellipsoidal form at zero total momentum of Cooper pairs. The critical asymmetries at which the transition from the DFS to the unpaired state occurs are larger than those for the BCS phase. In this precritical region the DFS phase lowers the condensation energy of the asymmetric BCS state. We compare quantitatively the DFS phase to another realization of superconducting phases with broken translational symmetry - the single plane wave Larkin-Ovchinnikov-Fulde-Ferrell (LOFF)phase, which is characterized by a non-vanishing center-of-mass momentum of the Cooper pairs. The possibility of the detection of the DFS phase in the time-of-flight experiments is discussed and quantified for the case of $^6$Li atoms trapped in two different hyperfine states.

cond-mat.supr-con↗

arXiv2005

Bulk and single-particle properties of hyperonic matter at finite temperature

Bulk and single-particle properties of hot hyperonic matter are studied within the Brueckner-Hartree-Fock approximation extended to finite temperature. The bare interaction in the nucleon sector is the Argonne V18 potential supplemented with an effective three-body force to reproduce the saturating properties of nuclear matter. The modern Nijmegen NSC97e potential is employed for the hyperon-nucleon and hyperon-hyperon interactions. The effect of the temperature on the in-medium effective interaction is found to be, in general, very small and the single-particle potentials differ by at most 25% for temperatures in the range from 0 to 60 MeV. The bulk properties of infinite matter of baryons, either nuclear isospin symmetric or a beta-stable composition which includes a non-zero fraction of hyperons, are obtained. It is found that the presence of hyperons can modify the thermodynamical properties of the system in a non-negligible way.

nucl-th↗

arXiv2005

Ferromagnetic instabilities in neutron matter at finite temperature with the Skyrme interaction

The properties of spin polarized neutron matter are studied both at zero and finite temperature using Skyrme-type interactions. It is shown that the critical density at which ferromagnetism takes place decreases with temperature. This unexpected behaviour is associated to an anomalous behaviour of the entropy which becomes larger for the polarized phase than for the unpolarized one above a certain critical density. This fact is a consequence of the dependence of the entropy on the effective mass of the neutrons with different third spin component and a new constraint on the parameters of the effective Skyrme force is derived in order to avoid such a behaviour.

nucl-th↗

arXiv2003

$K^-/K^+$ ratio in heavy-ion collisions at GSI with an antikaon self-energy in hot and dense matter

The $K^-/K^+$ ratio produced in heavy-ion collisions at GSI energies is studied. The in-medium properties at finite temperature of the hadrons involved are included, paying a special attention to the in-medium properties of antikaons. Using a statistical approach, it is found that the determination of the temperature and chemical potential at freeze-out conditions compatible with the ratio $K^-/K^+$ is very delicate, and depends very strongly on the approximation adopted for the antikaon self-energy. The use of an energy dependent $\bar{K}$ spectral density, including both s and p-wave components of the $\bar{K}N$ interaction, lowers substantially the freeze-out temperature compared to the standard simplified mean-field treatment and gives rise to an overabundance of $K^-$ production in the dense and hot medium. Even a moderately attractive antikaon-nucleus potential obtained from our self-consistent many-body calculation does reproduce the ``broad-band equilibration'' advocated by Brown, Rho and Song due to the additional strength of the spectral function of the $K^-$ at low energies.

nucl-th↗

arXiv2002

The antikaon nuclear potential in hot and dense matter

The antikaon optical potential in hot and dense nuclear matter is studied within the framework of a coupled-channel self-consistent calculation taking, as bare meson-baryon interaction, the meson-exchange potential of the Jülich group. Typical conditions found in heavy-ion collisions at GSI are explored. As in the case of zero temperature, the angular momentum components larger than L=0 contribute significantly to the finite temperature antikaon optical potential at finite momentum. It is found that the particular treatment of the medium effects has a strong influence on the behavior of the antikaon potential with temperature. Our self-consistent model, in which antikaons and pions are dressed in the medium, gives a moderately temperature dependent antikaon potential which remains attractive at GSI temperatures, contrary to what one finds if only nuclear Pauli blocking effects are included.

nucl-th↗

arXiv2000

Partial wave contributions to the antikaon potential at finite momentum

The momentum dependence of the antikaon optical potential in nuclear matter is obtained from a microscopic and self-consistent calculation using the meson-exchange Jülich $\bar{K}N$ interaction. Two self-consistent schemes are discussed, which would lead to substantially different predictions for the width of ${\bar K}$ nuclear bound states. The effect of higher partial waves of the $\bar{K}N$ interaction, beyond the L=0 component, is studied and found to have moderate but non-negligible effects on the ${\bar K}$ nuclear potential at zero momentum. At momenta as large as 500 MeV/c, relevant in the analysis of heavy-ion collisions, the higher partial partial waves modify the ${\bar K}$ optical potential by nearly a factor of two.

nucl-th↗