Nuclear systems under extreme conditions: isospin asymmetry and strong B-fields

TL;DR

This thesis investigates nuclear matter under extreme conditions, including isospin asymmetry and ultra-strong magnetic fields, revealing complex phase behaviors, nuclear shape changes, and superfluidity limits relevant to astrophysical objects like magnetars.

Contribution

It provides new insights into the phase diagram of asymmetric nuclear matter, nuclear shape phenomena in strong magnetic fields, and the critical magnetic field for neutron superfluidity destruction.

Findings

Identification of tri-critical points in the phase diagram.

Observation of nuclear shape changes under strong magnetic fields.

Determination of the magnetic field limit for neutron superfluidity.

Abstract

This thesis explores nuclear systems under extreme conditions of isospin asymmetry and strong magnetic fields. Its first chapter is devoted to the phase diagram of isospin-asymmetrical nuclear matter in the density-temperature plane. Four competing phases of matter are included into the consideration: the unpaired phase, the ordinary Bardeen-Cooper-Schrieffer (BCS) phase, a phase where normal fluid and superfluid separate into distinct domains and the superfluid phase with a non-zero total momentum of Cooper pairs. The evolution of these phases from the weakly BCS limit to the strongly coupled Bose-Einstein-Condensate limit is studied by varying the density from high to low values at various temperatures and asymmetries. The emergence of tri-critical points (including a Lifshitz point) and various intrinsic properties of the condensate (pair wave-function, occupation numbers, quasiparticle spectra) are discussed. The second chapter studies the nuclei $^{12}$C, $^{16}$O, and $^{20}$Ne in a magnetic field $B\approx 10^{17}\,$G by utilizing the SKY3D code, suitably extended to include the magnetic field. Our solutions of time-independent Hartree-Fock equations on a Cartesian three-dimensional grid with a Skyrme interaction demonstrate a number of interesting phenomena, such as level splitting in the magnetic field, spontaneous rearrangement of energy levels, changes in the nuclear shape, etc. The third chapter studies the pairing in low-density neutron matter in the presence of a strong magnetic field. In particular, we find the limiting field for the destruction of the superfluidity of neutrons in the crust and outer core of magnetars. This field is density-dependent but is generally of the order of $B\geq 10^{17}\,$G. The phase diagram of spin-polarized neutron matter is constructed in the case where only the BCS phase is allowed along with the (spin-polarized) normal phase.