Dr Jon Fenton

Biography

• Quantum phase-slips and Josephson junctions
• Low-temperature transport
• Superconductivity
• Nanoscale fabrication

Research interests

I am an experimental physicist, with interests in research in superconductivity at the nanoscale. At present I am particularly interested in investigating quantum phase-slips in superconducting nanowires and seeking to harness this phenomenon to develop useful devices.

I am currently working on a project to study quantum phase-slips in superconducting nanowires with the aim of developing a device which will be a candidate for a new current standard. The research is funded by an EPSRC Postdoctoral Research Partnership Grant between UCL and the National Physical Laboratory (NPL) in Teddington.

For superconducting wires in the presence of fluctuations, there is a finite probability that the phase difference across a small volume of the wire will slip by 2π, causing a brief voltage pulse across the wire. The effects of thermal fluctuations were studied in the 1970s; recent advances in nanofabrication techniques have enabled experimental studies of quantum phase-slips.
Mooij and Nazarov [1] predicted that devices containing quantum phase-slip (QPS) junctions show behaviour which is the exact dual of that of Josephson junction devices. One implication of this is that there is a dual to the Shapiro effect in Josephson junctions exposed to microwaves. In the Shapiro effect in Josephson junctions, steps appear in the IV characteristic at voltages proportional to the microwave frequency and this now provides the basis of the international voltage standard. In QPS junctions in the presence of microwaves, it is expected that steps will appear in the current through the QPS device, at values proportional to the frequency of applied microwaves, suggesting that a current standard might be developed using QPS devices.

In order to achieve such QPS devices using superconducting nanowires, the lateral dimensions of the nanowires must be below 50nm, so that the quantum phase-slip rate is sufficiently large to localise the charge in the wire. (This means that, counterintuitively, the ‘superconducting' wire becomes a perfect electrical insulator for low applied voltages.) I am making use of LCN's Raith 150-TWO electron-beam lithography system to realise these nanowires and carrying out measurements at low temperatures on the wires at both UCL and NPL.

For my PhD work at Birmingham, I studied intrinsic Josephson junction mesa structures in the high-temperature superconductor Bi₂Sr₂CaCu₂O₈. I set up a fast, pulsed measurement system and demonstrated that heating was an important effect needing to be taken into account in low-temperature current-voltage measurements in these systems.

My previous research at UCL has included a project to develop an experimental system for testing a candidate theory for dark energy using low-temperature electrical measurements in the lab. The aim was to measure high-frequency quantum noise in resistively shunted Josephson junctions at temperatures below 1K in an update of a classic experiment by Koch et al. [2], as suggested by Beck and Mackey [3].
I also developed a system for inside LCN's dual-beam FIB/SEM microscope for making low-temperature current-voltage measurements during focussed ion-beam milling, making use of a liquid-helium cooled sample stage.

I have also published work on simulations I have carried out, both on a thermal model related to my PhD work and more recently on thermally activated processes in moderately damped Josephson junctions in Phys. Rev. B in 2008. Most recently I have been making use of simulations to try to better understand QPS devices.

Biographical details:

• 2004-present: Research Fellow in Electronic & Electrical Engineering, UCL / London Centre for Nanotechnology
• 2002-2004: Research Associate in Physics, University of Birmingham
• 2002: PhD in Condensed Matter Physics from University of Birmingham
• 1998: BA, MSci in Natural Sciences (Experimental and Theoretical Physics) from University of Cambridge

Selected publications:

• Felling of individual freestanding nanoobjects using focused-ion-beam milling for investigations of structural and transport properties, W. Li, J.C. Fenton, A. Cui, H. Wang, Y. Wang, C. Gu, D.W. McComb and P.A. Warburton, Nanotechnology 23 105301 (2012).
• Materials for superconducting nanowires for quantum phase-slip devices, J.C. Fenton, C.H. Webster and P.A. Warburton, J. Phys.: Conf. Series 286 012024 (2011).
• Dissipative enhancement of the supercurrent in Tl₂Ba₂CaCu₂O₈ intrinsic Josephson junctions, P.A. Warburton, S. Saleem, J.C. Fenton, M. Korsah and C.R.M. Grovenor, Phys. Rev. Lett. 103 217002 (2009).
• The radio-frequency impedance of individual intrinsic Josephson junctions, J. Leiner, S. Saleem, J.C. Fenton, S. Speller, C.R.M. Grovenor and P.A. Warburton, Appl. Phys. Lett. 95 252505 (2009).
• Skewness variations of switching-current distributions in moderately damped Josephson junctions due to thermally induced multiple escape and retrapping, J.C. Fenton and P.A. Warburton, J. Phys.: Conf. Ser. 150 052052 (2009).
• Monte Carlo simulations of thermal fluctuations in moderately damped Josephson junctions: Multiple escape and retrapping, switching- and return-current distributions and hysteresis, J.C. Fenton and P.A. Warburton, Phys. Rev. B 78 054526 (2008).
• Switchable phase diffusion in intrinsic Josephson junction arrays, J.C. Fenton, M. Korsah, C.R.M. Grovenor and P.A. Warburton, Physica C 460 1407 (2007).
• Critical current suppression in sub-micron intrinsic Josephson junction arrays, J.C. Fenton, M. Korsah, C.R.M. Grovenor and P.A. Warburton, J. Phys.: Conf. Ser. 43 1114 (2006).
• Josephson current suppression in three-dimensional focused-ion-beam fabricated sub-micron intrinsic junctions,
P.A. Warburton, J.C. Fenton, M. Korsah, C.R.M. Grovenor, Superconductor Science & Technology 19 (5): S187-S190 (2006).
• Heating in mesa structures, J.C. Fenton and C.E. Gough, J. Appl. Phys. 94 4665 (2003).
• System for fast time-resolved measurements of c-axis quasiparticle conductivity in intrinsic Josephson junctions of 2212-BSCCO, J.C. Fenton, P.J. Thomas, G. Yang and C.E. Gough, Appl. Phys. Lett. 80 2535 (2002).
• Time dependence of current-voltage measurements of c-axis quasiparticle conductivity in 2212-BSCCO mesa structures, J.C. Fenton, G. Yang and C.E. Gough, Physica C 388 341 (2003).

References
[1] J. E. Mooij, Yu.V. Nazarov, Nat. Phys. 2 169 (2006).
[2] R. H. Koch, D.J. van Harlingen, J. Clarke, Phys. Rev. B 26 74 (1982).
[3] C. Beck, M.C. Mackey, Phys. Lett. B 605 295 (2005).