Role of correlations in spin-polarized neutron matter

Abstract

Background: The possible existence of a phase transition to a ferromagnetic state in neutron matter as origin of the extremely high magnetic fields of neutron stars is still an open issue. Whereas many phenomenological interactions predict this transition at densities accesible in neutron stars, microscopic calculations based on realistic interactions show no indication of it. The existence or non-existence of this transition is a consequence of the different role of nucleon-nucleon correlations in polarized and unpolarized neutron matter. Therefore, to give a definite answer to this issue it is necessary to analyze the behavior of these correlations.Purpose: Using the Hellmann-Feynman theorem we analyze the contribution of the different terms of the nucleon-nucleon interaction to the spin symmetry energy of neutron matter with the purpose of identifying the nature and role of correlations in polarized and unpolarized neutron matter. Methods:The analysis is performed within the microscopic Brueckner-Hartree-Fock approach using the Argonne V18 realistic potential plus the Urbana IX three-body force.Results: Our results show no indication of a ferromagnetic transition as the spin symmetry energy of neutron matter is always an increasing function of density. They show also that the main contribution to it comes from the S=0 channel, acting only in non-polarized neutron matter, in particular from the 1 S0 and the 1 D2 partial waves. Three-body forces are found to play a secondary role in the determination of the spin symmetry energy.Conclusions: By evaluating the kinetic energy difference between the correlated system and the underlying Fermi sea to estimate the importance of correlations in spin-polarized neutron matter, we conclude that non-polarized neutron matter is more correlated than totally polarized one.

Nucleon--Nucleon Correlations and the Isospin and Spin Symmetry Energy

“…At this density, E NM is smaller than E PNM indicating that the system prefers to be non-polarized. Actually, this is true for all densities, and no indication of a ferromagnetic transition has been detected [30].…”

“…On the other hand, to learn on the dependence of the N N correlations on the isospin and spin degrees of freedom, we study the spin-isospin channel decomposition of the expectation value of the interaction energy, and their contribution to the symmetry energy of nuclear matter which is defined as the difference between the energy per particle of neutron matter and symmetric nuclear matter [28]. Finally, we get a further insight into the spin dependence by considering the spin symmetry energy of neutron matter defined as the difference between the energy per particle of polarized and non-polarized neutron matter [30].…”

Hyperons and Neutron Stars

“…Panel (a) of Fig. 3 shows the r-mode instability region for a pure nucleonic (black solid line) and a hyperonic (red dashed line) star with M = 1.27M [55]. The contributions to the bulk viscosity from direct and modified nucleonic Urca processes as well as from the weak non-leptonic process n + n ↔ p + Σ − included in the calculation are shown in the panel (b) of the figure.…”

“…Panel (b): Bulk viscosity as a function of the density for T = 10 9 K and ω = 10 4 s −1 . Contributions direct and modified nucleonic Urca processes as well as from the weak non-leptonic process n + n ↔ p + Σ − are included.Figure adaptedfrom Ref [55]…”

Modification of the masses of the lightest neutral mesons in a hadronic medium under an external magnetic field

“…The intensity of these fields has been estimated around 10 15 G at the star surface, but could reach much higher values in the dense interior of the star. The origin of the magnetism, however, is still under debate [5,6,7]. Furthermore, extreme magnetic intensities are expected in heavy ion collisions [8,9,10].…”