PhD project
by Søren P. Madsen
Defence:
Defence title:
Vortices in the New Superconductors
Date/Time:
Tuesday, March 14th 2006, 14:00
Place:
Building 348, aud. 227, DTU
Censors:
Associate Professor Mads Peter Sørensen, MAT, DTU
Associate Professor Per Hedegård, KU
Professor Alexey Ustinov, Universität Erlangen-Nürnberg, Tyskland
Chairman:
Professor Jacob Østergaard, CET, Ørsted·DTU
Reception:
There will be a reception after the defence in building 325 room 037, ground floor
Download PhD thesis - 15.4 Mbyte PDF
Projectperiod:
1st of January 2003 to 31st of December 2005
Supervisors:
Main Supervisor:
Prof. Niels Falsig Pedersen, Ørsted·DTU, ELTEK
Supervisors:
Prof. Peter Leth Christiansen, Fysik & IMM, DTU
Prof. Ole Tønnesen, Ørsted·DTU, ELTEK
Introduction:
With the discovery of the new high temperature superconductors, in 1987, it became possible to move superconductors out of the labs and into real life. The discovery also lead many to believe, that superconductivity at room temperature was right around the corner. As is well known, this does not seem to be the case. Nevertheless, many applications for HTSC-materials are known; examples include MR-scanners in hospitals, highly sensitive measurements of magnetic fields and superconductors are even being used as a replacement of normal power cables in power plants.
A superconductor has some unique material properties, zero DC-resistance being only one of them, which are utilized in the practical applications. It is therefore important to understand these materials better, and to explore the unique properties with new applications in mind. This is the reason for the present project.
The project is sponsored by the STVF framework programme "New Superconductors: Mechanisms, processes and products". This programme includes researchers at DTU, Risø and at the University of Copenhagen, exploring different aspects of superconductors; from practical material making to theoretical quantum theory.
Project description:
This project dealt with vortices in stacked long Josephson junctions and in two-gap superconductors. The first part is about Josephson vortices, or fluxons, in stacked long Josephson junctions. The model is related to high Tc superconductors. A possible relationship between the linear and non-linear modes is investigated numerically. The fluxon-solutions can be made to shuttle back and forth in the junctions and they may emit radiation near the junction edge. This radiation is typically in the THz range. The main problem is, however, that the radiated power in a single junction is too small for applications. This is usually solved by stacking more junctions and
getting the fluxons in the different junctions to bunch, radiate coherently, and thus increase the emitted power. The main problem is that the vortices repel each other and therefore prefer to be far apart, preventing coherent radiation. Some different ways of obtaining bunched solutions are discussed. A microwave field is shown to be able to introduce bunching in weakly coupled systems. This may also be done using a cavity instead of a microwave field.
And finally, the very important flux-flow modes are investigated numerically. It is shown, that in some cases the flux-flow modes spontaneously jump from a triangular fluxon-lattice to a square fluxon-lattice, even in stacks with a strong inductive coupling.
The second subject is vortices in two-gap superconductors, such as MgB2. These superconductors are investigated through the two-component Ginzburg-Landau theory. The usual Abrikosov vortex is investigated in the two-component version. The equations are solved in the far-field and the effect of a Josephson-type coupling is considered. The subject of vortex-vortex interaction is briefly discussed in the case of zero Josephson coupling. Due to the added complexity of having two order parameters, new features arise. A texture vortex solution is found analytically and numerically in the two-component theory for the case of zero magnetic field.
The case of non-zero magnetic field is investigated numerically. The textured vortex seems to be unstable in even a small applied magnetic field.
Further information
• ORBIT database
Søren P. Madsen 2006-03-01