| dc.description.abstract | In this thesis we present three research articles on phase transitions and related phenomena in unconventional and two-dimensional superconductors, within the field of condensed matter physics. It contains an introductory part, where we introduce the field of phase transitions and the models studied in the thesis. We also introduce the numerical framework Monte Carlo simulations, which is employed in all three papers to study fluctuation effects in the models considered.
In paper I we investigate magnetization processes in an unconventional chiral p-wave superconductor. Using a phenomenological Ginzburg Landau model to describe the system, we employ Monte Carlo simulations to study fluctuation effects in the presence of an external magnetic field. We find that close to the upper critical field, a square lattice of single quanta magnetic vortices is stabilized. As the external field is decreased, the single quanta vortices merge into double quanta vortices which form a triangular lattice.
In paper II we investigate the zero field phase transition of the same chiral p-wave model. In the absence of an external magnetic field, this model features a superconducting state which spontaneously breaks time reversal symmetry. We investigate whether time reversal symmetry may be broken independently of the superconducting phase transition, which would result in two sequential phase transitions. Our results show that, for the model parameters considered, the breakdown of time-reversal symmetry always accompanies the superconducting phase transition, resulting in a first-order preemptive phase transition.
In paper III we investigate the superconductor-insulator transition in two dimensions. We study the extended Bose Hubbard model, which includes a repulsive interaction between nearest neighbors, in the limit of large boson occupancy. Using Monte-Carlo simulations to explore the phase diagram, we find four distinct phases. The standard superconducting and insulating phases are intervened by an edge metal phase, characterized by a non-zero edge compressibility. The fourth phase is a critical superconducting phase, which we interpret as a failed supersolid. | en_US |
