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Cor Molenaar

Transport properties of Josephson junction systems

Topology versus geometry

Promotion date: September 5.

Promotor: Prof. dr. ir. Alexander Brinkman

Prof. dr. ir. Hans Hilgenkamp

The Josephson effect occurs when a supercurrent or resistance-less state exists between two superconductors which are separated by a different material. The Josephson effect is a manifestation of a macroscopic quantum state. This makes it possible to do quantum experiments on a tangible scale.

A possible separating material is a topological insulator, which acts as a highway for electrons. A “Majorana” can be created in such a situation. A system behaving in this way makes a different way of performing quantum computation possible.

First generations of topological insulators make it exceedingly difficult to create a Majorana. One of the issues identified in experiments here, is that the junctions are too large, several 100 nm wide. Junctions were created on a different and improved material: Bi2-xSbxTe3-ySe. Their improved Josephson behavior was demonstrated.

Also a macroscopic quantum state was created by combining thousands of Josephson junctions together. This giant collection behaves in spectacular and beautiful ways. We created a model system of a Mott insulator: a class of materials which one expects to be metals instead of insulators. By creating a model system, a tool is created to increase the understanding of these Mott systems.

Your investigations were quite fundamental. Can you explain something more about this line of research?

In superconductors electrons dance like in a waltz, without colliding. A Josephson junction is comparable to a highway in which electrons travel top speed in two directions. Combining the superconductors and the junctions, spectacular phenomena arise: the two worlds are connected and interact, while the music and rhythmic behavior are preserved. Imagine inviting someone to dance, take the highway, transfer instantly to the other world of the adjacent superconductor, whilst not missing a step in the dance. That is the fascinating world in which my research took place.

Different kinds of particle behavior occur at these fascinating interfaces and junctions. The electrons may behave as if a Majorana particle exists, a particle originally from high energy physics, but nowadays also explored in condensed matter physics. By improving the materials of the junctions, and make them more apt as an interface material between two superconductors, the hope is that one day the existence of this particle can be characterized convincingly. As a technical university, we contribute to this type of materials research and hence to these fundamental research developments.

By combining thousands of junctions, another behavior can be actuated: that of a Mott isolator. A lot of basic research is still to be done in that area, and we contribute to that by designing these giant arrays in a clever way.

In order to perform this kind of research, collaboration between the Interfaces & Correlated Electrons and the Nano Electronics groups was necessary. Apart from the intensive discussions with my professors and with my group members, the technicians operating specialized equipment, of both groups, were indispensable, showing the multidisciplinary nature of scientific research and the teamwork involved.

Do you recall some special moments during your PhD work?

The fabrication of the SQUIDs and the junctions was a labor intensive job requiring insight in fabrication techniques and process parameters (like cooling them down properly). Also measurement procedures and understanding the phenomena occurring, required a fair period of time and effort. The moment we succeeded in fabricating the components fitting our experiments, was a very rewarding one.

When working on the Mott isolators, for a long time we were not sure about the phenomena we actually observed. Luckily, some American and Russian theorists were amazed, and very enthusiastic, about the experiments we were able to perform. They helped to make sense out of the results, as they understood the history of the process we had undertaken very well. With their help we were able to make a coherent story out of these discoveries. We would not have been able to do so on our own within the limited period of time of my PhD period, so I am very happy we met them.

Were you able to write some good publications?

Articles appeared in Physical Review Letters B and Applied Physics Letters. Superconductor Science & Technology and Physical Status Solidi also published articles. Now I am still working on some more.

What are your future plans?

Right now I am working in Arnhem at the NRG institute: Nuclear Services for Energy, Environment and Health. I made this choice because after four years of scientific research I now like to solve actual problems for clients, implementing these results on a shorter notice, a half year to a year.

The work and analyses are on expansive and complex systems, which I am working on in a nice team of experts. Also external contacts are part of the job as we analyze and make risk profiles for diverse companies and projects.

What, in your opinion, is important for Mesa+ to stay successful in future?

Being able to use up-to-date lithography and e-beam techniques in house, was very favorable for my work. Sharing facilities and expertise from other groups within Mesa+ was very important. In general, this will be the case even more so in future research.

Also important, is to offer master students opportunities to use nano-lab time for them to built up experience from there. It is a good thing they learn to feel at home within the nano-lab. In this way talented PhD’s will be enthused.