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A. Ariando (promotion date: 12 May 2005)

Josephson Junction Arrays with d-wave-induced π-phase-shifts

Promotion date: 12 May 2005

A. Oriando

In general it is about the behaviour of Josephson junctions connecting high- and low-temperature superconductors, and the possibilities of using such a structure as new components in superconducting (quantum)-electronics.There is rich physics involved in this kind of structure, and there are many possibilities for applications. Josephson junctions consist of two superconductors that are separated by for example a thin insulating barrier layer. These two superconductors are then only weakly coupled, meaning that the wave functions of the electrons in the two superconductors are only slightly overlapped. In this case, interestingly, electrons can still tunnel from one superconductor to the other through this barrier layer, even without any voltage introduced. This current can flow simply because of a difference in phase of the electron wave functions between the two superconductors.

What was your thesis about?

In general it is about the behaviour of Josephson junctions connecting high- and low-temperature superconductors, and the possibilities of using such a structure as new components in superconducting (quantum)-electronics.

Why?

There is rich physics involved in this kind of structure, and there are many possibilities for applications. Josephson junctions consist of two superconductors that are separated by for example a thin insulating barrier layer. These two superconductors are then only weakly coupled, meaning that the wave functions of the electrons in the two superconductors are only slightly overlapped. In this case, interestingly, electrons can still tunnel from one superconductor to the other through this barrier layer, even without any voltage introduced. This current can flow simply because of a difference in phase of the electron wave functions between the two superconductors.

Until now, most of Josephson junctions are fabricated using two superconductors of the same materials or similar properties. Now, in our group, we have been trying to combine two different superconductors, low temperature metallic superconductors and high temperature cuprate superconductors. They are different in nature. While the phase of wavefunction in metallic superconductors is the same in all direction in space, in cuprate superconductors it changes by π when you turn by 90 degree.

Now you can think of making a ring structure combining metallic and cuprate superconductors, so that when going around this ring, you will see this π phase change. Unfortunately, or maybe I should say fortunately, nature can not accept this. In superconductors, a condition should be fulfilled that the phase can only change by a multiple of 2π when going around a close loop. Simply put, this intrinsic phase change leads to unique phenomena, such as the spontaneous generation of magnetic flux. This flux appears purely because of the different quantum symmetries, and not relies on the application of bias currents or on magnetism in the materials involved.

So your work was purely fundamental.

Partly. The spontaneously generated flux is naturally a degenerate two state system. So, the polarity of the flux can be either pointing down or up, and this polarity can also be set by the action of an external field. In collaboration with people in IBM Yorktown in USA, we have demonstrated this possibility, and been working on this direction now.

Is it not a drawback of the device that you have to cool down one half to low temperatures and to heat up the other half to a high temperature?

No, no, no. Even the high temperature superconductors operate at temperatures that are well below zero. They become superconducting at about 90 Kelvin. Low temperature superconductors need even colder conditions becoming superconducting around 10 Kelvin.

What were your main findings?

We have been the first group in the world that can make this structure with high quality and reproducibility. That is quite an achievement by itself, I think.

On the other hand, I have been doing also experiments where we use this technique to test the symmetry of the electron wavefunction of other cuprate superconductors. In that class of superconductors there is still a controversial, and we finally came to a success result and published it in Physical Review Letters. This is crucial information for theorists, who want to develop a theory for high temperature superconductors. There is still no theory that can explain the high temperature superconductors. I myself am now working on superconducting memory, which we can possibly combine with qubits, the basic element of the quantum computer.

What are quantum computers?

Quantum computers are still pretty far off. Simply put, they will be much faster and also smarter than present day computers, with more simultaneous processing.

You are now a Post-doc with Nanoned. Are they involved in this?

Yes, a group in Delft which is involved in NanoNed is working on qubits, the basic element of the quantum computer. Here in Twente we are working on the memory. It is a close collaboration.

You are from Indonesia. How did you get here?

During my bachelor study in Indonesia, Professor Horst Rogalla from the Low Temperature Division once visited our lab, and suggested to me in 1999 to do my Masters here in Twente.
I applied. I came here for the interview, stayed a week and fortunately was accepted. It is a long way from Indonesia.

Do you like the Netherlands?

In general, I really like living in the Netherlands, especially in a small city like Enschede. Less stress, clean environment, and well-managed systems and infrastructures. I also have my family here. We enjoy life in the Netherlands.

In retrospect; what did you like best about your PhD?

The freedom. I like that we have a freedom to decide the direction of our own research. I am also very pleased that our results got published in high level journals like Nature and Physical Review Letters.

For the summary of the thesis, click here.