Mr Dominic Walliman
Dominic completed his PhD in 2010 and joined D-Wave Systems Inc., working on their quantum annealing quantum computers. He writes a series of award winning children’s science books called Professor Astro Cat.
Dominic also makes internet videos explaining science which you can find here:
https://www.youtube.com/user/dominicwalliman
His website is: http://dominicwalliman.com
Here's what Dominic did for his PhD ...
Research Topics:
Intrinsic effects in ultra narrow tracks of High Tc Superconductors
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Research Overview
Hello my name is Dominic, welcome to my page. I am a PhD student currently researching the intrinsic properties of ultra-narrow track superconductors. I am looking for evidence of thermal and quantum phase slips in a range of superconducting materials in a hope to add to our knowledge of this phenomenon. You may not know what phase slips are, so here is a brief introduction.
Phase Slips
In a superconducting material, the electrical resistance drops sharply to zero at the transition temperature. This is not true however, for superconducting tracks that have very small lateral dimensions. If a track is fabricated to have a width and thickness less than a coherence length, a finite resistance below the transition is seen.
Schematic diagram of a track with width and thickness less than a coherence length.
The importance of reducing the superconductor to dimensions less than a coherence length is to suppress the superconductivity in every dimension but its length. This means that the wavefunction of the superconductor can be described in one dimension.
One can consider the phase of this wavefunction, which will match that of the superconductors at each end. When no current is passed through the track the phase difference across the track will be constant, however, when a current is applied the phase difference will evolve with time.
This seems incompatible with a steady state because, to be superconducting, a material needs to maintain a coherent phase. The resolution of this is that phase slip events occur. This is where the increasing phase difference causes the amplitude of the wavefunction to go to zero over a small distance to allow the phase to slip by 2π
Schematic diagram of a phase slip event (modified from [1])
Fabrication
I fabricate ultra-narrow tracks using a variety of methods. I predominantly start with superconducting thin films and then fabricate these to create the narrow tracks. The growth of these thin films is dependant on the material to be measured and I use a variety of methods including thermal or electron beam evaporation, sputtering and pulsed laser deposition. The fabrication of these films to make the ultra narrow tracks is also done by a variety of methods that are material dependant. These methods include photolithography, electron beam lithography, argon ion beam milling and focused gallium ion beam milling (FIB).
An example of a narrow track of Niobium fabricated using an FIB
Measurement
To see if my samples exhibit phase slips I measure their resistance temperature characteristics. I do this with a four terminal electrical contact to eliminate series resistances from the electronics. As the sample is cooled I take a series of current-voltage characteristics, from which I determine the resistance of the track. I can then monitor the resistance below the transition, and see if the sample exhibits phase slips.
A series of R-T characteristics of narrow tracks in YBCO (from reference 2)
References
[1] Modified from Tinkham, M. Introduction to Superconductivity 2nd edn, p.289, (New York, McBraw-Hill, 1996)
[2] P. Mikheenko, X. Deng, S. Gildert, M. S. Colclough, R. A. Smith, and C. M. Muirhead, Phys. Rev. B 72, 174506 (2005)