Neutron-Rich Nuclei in a Fermi Gas.

Yoshiko Kanada-En’yo, Nobuo Hinohara, Tadahiro Suhara +1 more

Two-neutron correlation in the ${}^{1}S$ channel in quasi-two-dimensional (2D) neutron systems at zero temperature is studied by means of the BCS theory with finite-range effective nuclear forces. The dineutron correlation in low density neutron systems confined in an infinite slab is investigated in a simplified model that neutron motion of one direction is frozen. When the slab is thin enough, two neutrons form a tightly bound dineutron with a small size in the quasi-2D system, and a Bose dineutron gas is found in low density limit. With increase of Fermi momentum, the neutron system changes from the Bose-gas phase to the superfluid Cooper pair phase. The density dependence of the neutron pairing shows the BCS-BEC crossover phenomena at finite low-density region. In the transition region, the size shrinking of neutron pair and enhancement of pairing gap are found. The relation to dineutron correlation at surface of neutron-rich nuclei is also discussed.

FRANCESCO IACHELLO

Selim Jochim

Alexandros Gezerlis, Rishi Sharma

Low-density neutron matter has been studied extensively for many decades, with a view to better understanding the properties of neutron-star crusts and neutron-rich nuclei. Neutron matter is beyond experimental control, but in the past decade it has become possible to create systems of fermionic ultracold atomic gases in a regime close to low-density neutron matter. In both these contexts pairing is significant, making simple perturbative approaches impossible to apply and necessitating ab initio microscopic simulations. Atomic experiments have also probed polarized matter. In this work, we study population-imbalanced neutron matter (possibly relevant to magnetars and to density functionals of nuclei) arriving at the lowest-energy configuration to date. For small-to-intermediate relative fractions, the system turns out to be fully normal, while beyond a critical polarization we find phase coexistence between a partially polarized normal neutron gas and a balanced superfluid gas. As in cold atoms, a homogeneous polarized superfluid is close to stability but not stable with respect to phase separation. We also study the dependence of the critical polarization on the density.

Valery I. Abrosimov, O. I. Davidovskaya

Dipole excitations of a neutron-proton finite Fermi gas are considered within a semiclassical approach based on the kinetic equation. The independent-particle dipole strength functions connected with the external fields, which are exploited in quantum random phase approximation approaches for studying the isoscalar and isovector resonances in nuclei, are found.

Dieter Zawischa