Superconducting phases of strongly-interacting matter in large magnetic fields.

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 μ_B-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 μ_B. 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, μ_B 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 μ_B-B plane of the QCD phase diagram is reviewed such that the results can be placed in the wider context of this plane.

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Identifiers

ORCID for Geraint Wyn Evans: orcid.org/0000-0001-6106-4567

ORCID for Andreas Schmitt: orcid.org/0000-0003-2858-4450

ORCID for Nils Andersson: orcid.org/0000-0001-8550-3843

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