The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Fluctuations, Relaxation and Proximity Effect in Superconducting Circuits

Joonas Peltonen

Doctoral dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the School of Science for public examination and debate in Auditorium AS1 at the Aalto University School of Science (Espoo, Finland) on the 11th of November 2011 at 12 noon.

Overview in PDF format (ISBN 978-952-60-4351-7)   [5776 KB]
Dissertation is also available in print (ISBN 978-952-60-4350-0)

Abstract

Mesoscopic physics investigates structures smaller than the everyday macroscopic scale but larger than the scale of individual atoms, with properties that can often only be explained in terms of the laws of quantum mechanics. A typical mesoscopic electrical component is a tunnel junction, formed by a thin insulating oxide layer separating two metallic electrodes. In this thesis, various mesoscopic circuits containing sub-micron tunnel junctions between normal (N) and superconducting (S) metals are studied experimentally at sub-kelvin temperatures. An emphasis is placed on the influence of electrical fluctuations on the systems, as well as the strong connection between electrical and thermal transport in them.

We first demonstrate that a Josephson tunnel junction between two S electrodes functions as an on-chip detector of current fluctuations in a wide band of frequencies, potentially useful for studying charge transport in various mesoscopic systems. The lifetime of the zero-voltage state in a current-biased junction is very sensitive to the fluctuations in the bias current. We are able to observe the non-Gaussian nature of the shot noise generated by electrons tunneling across another nearby tunnel junction coupled to the detector junction.

Several of the experiments in this thesis probe how the transport close to a transparent interface between a superconducting and a normal conducting electrode is modified by the phenomenon of superconducting proximity effect. We present direct measurements of electron overheating in a normal metal weak link between two superconductors, explaining the routinely observed hysteretic current-voltage characteristic. Electronic temperature is probed locally by contacting the N island to an additional S electrode via an oxide barrier (I), thereby forming an NIS tunnel junction. Connecting the S electrodes of such a proximity SNS weak link into a closed loop, we further demonstrate use of the structure as a sensitive magnetometer with low dissipation. We probe also the electronic thermal conduction of short S wires between two N terminals. Due to the inverse proximity effect, the thermal conductance is found to be strongly enhanced beyond the value for a bulk superconductor.

We consider theoretically the prospects for rectifying thermal fluctuations by an NIS junction in a suitable electromagnetic environment, thereby realizing a Brownian refrigerator. Finally, we report the observation of increased cooling power in a voltage-biased NIS junction in small applied magnetic fields. This is attributed to enhanced relaxation of the hot electrons injected into the S lead of the junction.

This thesis consists of an overview and of the following 11 publications:

  1. J. T. Peltonen, A. V. Timofeev, M. Meschke, and J. P. Pekola. Detecting current noise with a Josephson junction in the macroscopic quantum tunneling regime. Journal of Low Temperature Physics, volume 146, numbers 1-2, pages 135-159, January 2007.
  2. A. V. Timofeev, M. Meschke, J. T. Peltonen, T. T. Heikkilä, and J. P. Pekola. Wideband detection of the third moment of shot noise by a hysteretic Josephson junction. Physical Review Letters, volume 98, number 20, 207001, May 2007. © 2007 American Physical Society (APS). By permission.
  3. J. T. Peltonen, A. V. Timofeev, M. Meschke, T. T. Heikkilä, and J. P. Pekola. Detecting non-Gaussian current fluctuations using a Josephson threshold detector. Physica E: Low-dimensional Systems and Nanostructures, volume 40, number 1, pages 111-122, October 2007.
  4. H. Courtois, M. Meschke, J. T. Peltonen, and J. P. Pekola. Origin of hysteresis in a proximity Josephson junction. Physical Review Letters, volume 101, number 6, 067002, August 2008. © 2008 American Physical Society (APS). By permission.
  5. M. Meschke, J. T. Peltonen, H. Courtois, and J. P. Pekola. Calorimetric readout of a superconducting proximity-effect thermometer. Journal of Low Temperature Physics, volume 154, numbers 5-6, pages 190-198, March 2009.
  6. F. Giazotto, J. T. Peltonen, M. Meschke, and J. P. Pekola. SQUIPT - Superconducting Quantum Interference Proximity Transistor. Nature Physics, volume 6, number 4, pages 254-259, April 2010.
  7. M. Meschke, J. T. Peltonen, J. P. Pekola, and F. Giazotto. Tunnel Spectroscopy of a Proximity Josephson Junction. arXiv:1105.3875, 5 pages, submitted for publication, May 2011. © 2011 by authors and © 2011 American Physical Society (APS). By permission.
  8. J. T. Peltonen, P. Virtanen, M. Meschke, J. V. Koski, T. T. Heikkilä, and J. P. Pekola. Thermal Conductance by the Inverse Proximity Effect in a Superconductor. Physical Review Letters, volume 105, number 9, 097004, August 2010. © 2010 American Physical Society (APS). By permission.
  9. J. V. Koski, J. T. Peltonen, M. Meschke, and J. P. Pekola. Laterally proximized aluminum tunnel junctions. Applied Physics Letters, volume 98, number 20, 203501, May 2011. © 2011 American Institute of Physics (AIP). By permission.
  10. J. T. Peltonen, M. Helle, A. V. Timofeev, P. Solinas, F. W. J. Hekking, and J. P. Pekola. Brownian refrigeration by hybrid tunnel junctions. Physical Review B, volume 84, number 14, 144505, October 2011. © 2011 American Physical Society (APS). By permission.
  11. J. T. Peltonen, J. T. Muhonen, M. Meschke, N. B. Kopnin, and J. P. Pekola. Magnetic-Field-Induced Stabilization of Non-Equilibrium Superconductivity. arXiv:1108.1544, 5 pages, submitted for publication, August 2011. © 2011 by authors and © 2011 American Physical Society (APS). By permission.

Keywords: superconductivity, tunnel junctions, Josephson effect, proximity effect

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Last update 2012-07-23