Effects of neutron-nuclei interactions in the neutron star crust
At densities greater than neutron drip, the neutron star crust features a plasma of neutron-rich nuclei and electrons, bathed in a gas of unbound neutrons. If the two systems are treated separately, as is typically done, one concludes that the nuclei form a lattice and the neutrons pair up and form a superfluid. The low energy excitations of such a system will feature lattice vibrations and phase fluctuations of the superfluid condensate. Interactions between the unbound neutrons and nuclei affect the properties of the two modes (for example, propogation speed) and introduce a mixing betwen the two. These effects play an important role in determining the transport and structural properties of the inner crust of a neutron star.