1. Superconducting phases of strongly-interacting matter in large magnetic fields
- Author
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Evans, Geraint Wyn, Schmitt, Andreas, and Andersson, Nils
- Abstract
The large magnetic fields of neutron stars and produced in heavy-ion collisions motivate investigation into the response of strongly-interacting matter to extreme magnetic forces beyond just theoretical interest. Furthermore, the varying temperature T, baryon chemical potential μ
B , and aforementioned magnetic fields B, of these systems leads to questions concerning the phase structure of Quantum Chromodynamics (QCD) at large. At low temperatures, superconducting phases become a possible candidate for the ground state in the μB -B plane. This thesis investigates two scenarios where these phases emerge at T=0, with an emphasis on the type-II regime. The first scenario concerns type-II superconductivity at large μB . At asymptotically high baryon density the ground state of QCD is a colour superconductor where gluonic fields can experience a Meissner effect. In the two-flavour pairing (2SC) and colour-flavour locked (CFL) colour-superconducting phases, a small admixture of the photon with a gluon is also expelled which means an applied external magnetic field will experience a slight Meissner effect. Therefore, these phases act as very weak electromagnetic superconductors. Previous works have shown that with massless quarks the type-II 2SC phase is preferred in a certain parameter region where the magnetic defects are domain walls. We introduce corrections for a finite strange quark mass in a Ginzburg-Landau approach, and find that the domain wall defects are replaced by a cascade of multi-winding flux tubes, among other changes to the phase diagram. Due to the emergence of a second colour-superconducting condensate emerging in the core, the magnetic flux is confined into "rings'' where both condensates are depleted, forming pipe-like structures. In the second scenario, μB is low enough such that the presence of nucleons is not yet energetically favourable. It was previously shown that, using Chiral Perturbation Theory and incorporating the chiral anomaly via a Wess-Zumino-Witten term, the ground state in this region above a certain critical magnetic field is a Chiral Soliton Lattice (CSL) of neutral pions - an inhomogeneous phase consisting of a series of domain walls. It was further shown that the CSL becomes unstable to charged pion fluctuations at an even higher second critical field. We argue this instability corresponds to a second order phase transition to a type-II superconducting vortex lattice phase and construct this phase in the chiral limit. We find the type-II vortex lattice phase is preferred and has a non-zero, inhomogeneous baryon number density, leading to a two-dimensional crystalline structure. Preceding the presentation of these works, the μB -B plane of the QCD phase diagram is reviewed such that the results can be placed in the wider context of this plane.- Published
- 2023