The research on superfluidity in Applied Mathematics and Theoretical Physics at ¾ÅÉ«ÊÓƵ covers truly large-scale cosmological superfluid systems, concentrating on neutron star dynamics, as well as micron-sized ultracold atomic superfluids.
A neutron star has a complex structure, with different particle species and states of matter being present in different density regimes. The outer parts of the star form an elastic crust of neutron rich atomic nuclei, permeated by superfluid neutrons.
In the outer core of the star, the protons form a superconducting fluid which co-exists with superfluid neutrons and a relativistic electron gas. Meanwhile, the deep core of the star contains exotic components like hyperons and deconfined quarks.
A model of a realistic neutron star must therefore be based on a multi-fluid description, similar to ones used to model superfluid helium and complex multi-component systems in chemistry.
A key feature of neutron star superfluidity is the so-called entrainment effect. Because of the strong nuclear interaction, a moving neutron is endowed with a virtual cloud of protons (and vice versa). This alters the neutrons' momentum and affects the dynamics of the system. Also, the vortices that inevitably form in a rotating superfluid are likely to be magnetised, and interactions between these magnetised vortices and charged fluid components could provide an important dissipation mechanism for stellar oscillations.
Finally, there will also be an interaction between neutron vortices and magnetic flux tubes in the proton superconductor, which will also have a significant effect on the dynamics. All this indicates that the neutron star problem is much richer than usual laboratory systems.
We are studying how the intricate dynamics of these large-scale superfluid systems affects their electromagnetic and gravitational-wave spectrum. Our current focus is on neutron star free precession, the pulsar glitches and the gravitational-wave driven r-mode instability. We are also interested in understanding the role of the vortex dynamics and superfluid turbulence.
Magnetically or optically trapped neutral atoms at ultra-low temperatures can form a superfluid by undergoing Bose-Einstein condensation or by Bardeen-Cooper-Schrieffer-like fermionic pairing.
The research on fermionic superfluids includes, e.g., the interplay between magnetism and superfluidity in the phase separation of superfluid and normal phases.