Speaker
Description
Neutron stars are fascinating astrophysical objects, containing matter at densities that exceeds the density of atomic nuclei. Among the most puzzling phenomena associated with them are pulsar glitches. Pulsars are rapidly rotating neutron stars, emitting beams of radiation from their magnetic poles and acting as the most precise clocks in the Universe, even surpassing the accuracy of atomic clocks on Earth. Occasionally, however, they exhibit sudden increases in their rotational frequency, events known as glitches.
These unexpected spin-ups are thought to arise from complex interactions between different internal components of the star. Several theoretical models have been proposed to explain glitches, most of them relying on superfluidity in parts of the star [1].
This talk will focus on the neutron stars crust, which is thought to be a lattice of nuclear clusters immersed in an electron gas. The transition from the outer to the inner crust is where neutrons begin to "drip" from nuclei, eventually leading to a superfluid neutron gas between the nuclear clusters [2].
The exotic phases within the inner crust are central to the star thermal and hydrodynamic behavior. This talk focuses on a quantum mechanical description of the inner crust. A central aspect is determining the superfluid density, which is a quantity directly related to the effective mass of nuclear clusters moving through the neutron gas.
I will present the Hartree-Fock-Bogoliubov calculations with Bloch boundary conditions which we used to model the inner crust [3, 4, 5]. In order to get the most reliable results, modern energy density functionals have been implemented, together with a realistic pairing interaction.
For the extraction of the superfluid density, a framework based on a Galilean transformation is constructed, allowing one to get this quantity within a time-independent calculation.
Together with fully self-consistent numerical results, I will present the expression for the superfluid density which we derived in the Bardeen-Cooper-Schrieffer approximation [6]. This allows one to evaluate this quantity directly from the single-particle band structure, thus offering a benchmark for complete results and providing insight into the interplay between periodicity and superfluidity. In this framework, we showed the importance in the inner crust of the so-called geometric contribution to the superfluid density, which is known to be relevant also in ultra-cold atoms [7] and high-temperature superconductors [8].
Our results suggest that about 90% of the neutrons are effectively superfluid, making possible to explain glitches with models that involve the crust only.
[1] P. W. Anderson and N. Itoh, Nature 256, 25–27 (1975).
[2] N. Chamel and P. Haensel, Liv. Rev. Relativity 11, 10 (2008).
[3] G. Almirante and M. Urban, Phys. Rev. C 109, 045805 (2024).
[4] G. Almirante and M. Urban, Phys. Rev. C 110, 065802 (2024).
[5] G. Almirante, T. Kaskitsi, and M. Urban, Phys. Rev. C (2026).
[6] G. Almirante and M. Urban, Phys. Rev. Lett. 135, 132701 (2025).
[7] S. Peotta and P. Torma, Nat. Commun. 6, 8944 (2015).
[8] M. Iskin, Phys. Rev. B 109, 174508 (2024).