The topic of this thesis is vortex-physics in multi component Ginzburg- Landau models. These models describe a newly discovered class of supercon- ductors with multiple superconducting gaps, and posses many properties that set them apart from single component models. The work presented here relies on large scale computer simulations using various numerical techniques, but also some analytical methods.
In Paper I, Type-1.5 Superconducting State from an Intrinsic Proximity Effect in Two-Band Superconductors, we show that in multiband supercon- ductors, even an extremely small interband proximity effect can lead to a qualitative change in the interaction potential between superconducting vor- tices by producing long-range intervortex attraction. This type of vortex interaction results in an unusual response to low magnetic fields, leading to phase separation into domains of two-component Meissner states and vortex droplets.
In paper II, Type-1.5 superconductivity in multiband systems: Effects of interband couplings, we investigate the appearance of Type-1.5 superconduc- tivity in the case with two active bands and substantial inter-band couplings. such as intrinsic Josephson coupling, mixed gradient coupling, and density- density interactions. We show that in the presence of these interactions, the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by mixed combinations of the density fields.
In paper III, Semi-Meissner state and nonpairwise intervortex interactions in type-1.5 superconductors, we demonstrate the existence of nonpairwise in- teraction forces between vortices in multicomponent and layered supercon- ducting systems. Next, we consider the properties of vortex clusters in a semi-Meissner state of type-1.5 two-component superconductors. We show that under certain conditions nonpairwise forces can contribute to the forma- tion of very complex vortex states in type-1.5 regimes.
In paper IV, Length scales, collective modes, and type-1.5 regimes in three- band superconductors, we consider systems where frustration in phase differ- ences occur due to competing Josephson inter-band coupling terms. We show that gradients of densities and phase differences can be inextricably inter- twined in vortex excitations in three-band models. This can lead to very long-range attractive intervortex interactions and the appearance of type-1.5 regimes even when the intercomponent Josephson coupling is large. We also show that field-induced vortices can lead to a change of broken symmetry from U (1) to U (1) × Z2 in the system. In the type-1.5 regime, it results in a semi-Meissner state where the system has a macroscopic phase separation in domainswithbrokenU(1)andU(1)×Z2 symmetries.
In paper V, Topological Solitons in Three-Band Superconductors with Bro- ken Time Reversal Symmetry, we show that three-band superconductors with broken time reversal symmetry allow magnetic flux- carrying stable topolog- ical solitons. They can be induced by fluctuations or quenching the system through a phase transition. It can provide an experimental signature of the time reversal symmetry breakdown.
Stockholm: KTH Royal Institute of Technology, 2012. , 74 p.
2013-01-14, FA32, Albanova universitetscentrum, Roslagstullsbacke 21, Stockholm, 10:00 (English)