Disputas: Dmitry Valerevich Denisov

Prøveforelesning

Bedømmelseskomité

Christian Jooss, Universität Göttingen, Tyskland
Alexander Grishin, Royal Institute of Technology, Sverige
Larissa Bravina, Universitetet i Oslo

Leder av disputas:  Ørjan G. Martinsen

Veileder:  Yuri Galperin, Daniel Shantsev

Sammendrag

This thesis is result of my research in the field of superconductivity carried out during past three years in the Department of Physics, University of Oslo. More precisely it is dedicated to study of magnetic flux properties in thin superconducting films. At present time superconductors are widely used in various micro- and nanoelectronic devices. For example, extremely sensitive magnetic field detectors are used not only for research purposes but also in medicine for scanning electrical activity in the brain; high-frequency generators with a very low noise are used in the cell-phone industry; new supercomputers consists of some superconducting parts. However at certain conditions thermomagnetic instability can arise in such devices leading to serious malfunction. The model describing this instability and methods to avoid it are presented in the thesis. It is shown that thermomagnetic instability results in an abrupt formation of magnetic dendrites. Proposed model is based on solving the thermal diffusion and Maxwell equations taking into account nonlocal electrodynamics in the film and its thermal coupling to the substrate. Within this model the formation of dendrites in thin-film superconductors is fully explained. Threshold magnetic field corresponding to formation of dendrites is theoretically calculated and its dependence on temperature and superconductor dimensions turned out to be in excellent agreement with various experiments. Further detailed comparison of experimental data and theoretical results shows that the model is well suited for predictions of dendrites and it is demonstrated that instability can be suppressed for sufficiently narrow strips, which is of particular importance for design of superconducting electronic devices.

Also it has been recently shown that superconducting films patterned with dots and antidotes allow trapping of significant amount of magnetic flux. Moreover it is possible with the help of special antidote arrays to guide the motion of magnetic vortices over the film area, which opens up a new field often called fluxonics. General problem concerning maximum quantity of flux quanta trapped by one antidote (or simply hole) is described in the second part of the thesis. Current and magnetic field distributions along with flux saturation number are calculated by solving the London equation for the thin-ring geometry. It is found that for large hole radius the thin-film superconductor can trap much less trap than the bulk one, the difference ratio is proportional to square root of the hole radius. In the limit of small hole bulk and film superconductors can trap the equal amount of flux. Knowing the saturation number, one may predict the interaction between vortices and antidot arrays in various experimental configurations.

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