Charge transportation in double barrier and magnesiumboride Josephson junctions
Promotion date: 21 February 2003
My research has been on Josephson junctions. A Jospehson junction consists of two superconductors that are weakly coupled, for example by separating the electrodes by a small insulating layer. Electrons can still tunnel through that layer. In my research I used junctions with two of those barriers (thinner than 1 nm) to separate a thin (5 nm) metal from the superconducting electrodes.
Can you tell us something about the topic of your investigations?
My research has been on Josephson junctions. A Jospehson junction consists of two superconductors that are weakly coupled, for example by separating the electrodes by a small insulating layer. Electrons can still tunnel through that layer. In my research I used junctions with two of those barriers (thinner than 1 nm) to separate a thin (5 nm) metal from the superconducting electrodes. In double-barrier junctions interesting physics is going on. An electron that is located inside the middle layer, that only has a thickness of 5 nanometers, acts as an in the well known quantum mechanics example of a particle in a box. Therefore it can only assume fixed energies at the resonance frequencies. There is another phenomenon, called Andreev reflection which leads to electron-hole bound states in Josephson junctions. We studied the interplay between these types of resonances. One important practical advantage of a double barrier-type junction is that there is no need to place a resistance parallel to the junction in a superconducting circuit. In normal Josephson junctions this shunt gives a parasitic inductances at high frequencies. Only when I was writing my thesis I realized where the absence of hysteresis was coming from. Of course I had been thinking about everything during the research, but only when writing I got sufficient overview to see the explanation for the charge transport in double barrier junctions. At the moment I am finishing a final article for the Physical Review Letters B.
Can you give a specific example of something you really liked during your research?
When we heard of a new superconducting material, magnesiumdiboride, we immediately knew that this could be a very valuable material for our research. So we did some research on it. At that time quite a competition was going on. Who would be the first one to grow a MgB2 thin film, and after that who would build the first MgB2 nanodevice and multilayer device. It was a pleasure to be in a group that did very well in this challenging race. It was quite nice, experiencing a drive like that. We even had pizza delivered to the lab at night to be able to keep working. However, you need to keep a close eye on the quality of the research as well. Sometimes people will send in all kinds of nonsense, just to be first to have a publication about a new material.
Did you have the possibility to travel a bit as well?
Yes, quite a lot actually. In the first beginning I spent a couple of months at MIT, in Boston. During the research I went to a couple of conferences as well, of course and we also collaborated with other universities. In the last phase of my research I was specialized quite a bit in some areas and therefore got invited to some review meetings. Recently I went to Florida for an advisory talk, which was a nice trip, and entirely paid for. The american government is investing in double-barrier and MgB2 junctions and I have indicated what the interesting topics are.
Could you tell us some more about the collaboration with the university of Stuttgart?
In Stuttgart, experts are working on electronic structure and lattice vibration calculations. We worked together with them on our investigations on MgB2. They calculated the electron-phonon coupling functions and we used this to calculate the superconducting properties of MgB2. Together, we showed the existence of a double superconducting energy gap and made predictions concerning the influence of this double gap on charge transport in tunnel junctions.
And what did you do when you went to MIT?
I spent the first three months of my PhD-period there. It was offered to me as a way of getting some research experience abroad. I worked on something that was somewhat besides my thesis topic, but the calculations still had to do with Josephson junctions.
It was very interesting to see the difference in culture between research groups here and in the US. At MIT the atmosphere was far more competitive, even internally. A good consequence is that people are far more driven. Because PhD-students are competing internally for a permanent appointment, they are far more easily prepared to spend the night at the lab, for example. On the other hand, here in the Netherlands we can easily spend a week trying to help a colleague on some problem. I think I prefer the Netherlands, because I really like to work in a team, for instance when we were trying to create the first thin film of MgB2.
Can you explain why you liked being a PhD-student? What would you say to someone who is in doubt about becoming one?
In the first place I want to say that you do not need to convince anybody to become a PhD-student. If someone would like to be one, he has to know that for himself. Curiosity and drive to explore are two things a PhD-student needs and if he has those properties, than doing a PhD assignment is a great job. The one big advantage a PhD-student has, is the enormous liberty to do whatever you like. If you get a nice idea, you are free to spend a month investigating it. Something else I liked being a PhD student, is the educational aspect. I have always liked to teach and follow educational courses. When you give education or lectures, you increase your insight in the matter. Even though you thought you knew it quite well, teaching demands an even more thorough understanding. Even if people do not always feel like giving classes, it is always useful.
What are you going to do now?
Well, although I am going to be defend my thesis soon, my contract will not end until September. Since my research was in the same area as my Master’s project I got quite a quick start, so I will finish a bit early. The rest of my time here I am going to spend on research and writing a research proposal. I hope to get funds to go abroad for some years to work as a post-doc. I possibly want to change the field in which I am going to work, but I am not sure what I want to do exactly. The final date for sending in the proposal is in a couple of months, so I still have some time to think. I have been thinking about a more biophysical project, maybe trying to link neural network calculations with measurements on the level of a single neuron. And also I could go on and work on the mechanism of high-Tc superconductivity, which is still not understood. Maybe that this can be a task for me. But I still have time, before I have to decide, we will see.