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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Nanocircuits with Superconductivity: Nonequilibrium Studies

Juha Voutilainen

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 18th of May 2012 at 12 noon.

Overview in PDF format (ISBN 978-952-60-4624-2)   [2098 KB]
Dissertation is also available in print (ISBN 978-952-60-4623-5)

Abstract

This thesis concerns low-temperature nanoelectronics and, in particular, nonequilibrium phenomena which come about when the concept of temperature loses its significance in nanoscale electric circuits. Under more conventional circumstances, these nonequilibrium phenomena are smothered by thermal motion or die away at large scale but they become essential for a field that is looking for applications in structures smaller than one micron, at temperatures lower than one degree above the absolute zero. The appearance of superconductivity – the technologically-interesting phenomenon of dissipationless transport of electric current – precisely at low temperatures further boosts the chance of finding these applications.

With my collaborators, I consider five different types of setups where nanoconductors are coupled to superconducting electrodes in typical low-temperature operating conditions which lead to the formation of a nonequilibrium state. Using the established theoretical methods of the field, I estimate how the nonequilibrium state affects the properties of the conductor, for example, in metallic conductors which have acquired superconducting properties due to superconducting proximity effect and in graphene, a one-atom thick film of carbon.

As a result of our work, it is possible to predict in detail when nonequilibrium phenomena should become observable. These phenomena include rectification of electric current and the enhancement of supercurrent due to incoherent noise in the electromagnetic environment of the conductor. In particular, our studies imply that the effect of the nonequilibrium state in actively-researched radiation detectors based on electron heating has been so far underestimated, whereas in graphene the equilibrium state is preserved under more extreme conditions than would be expected from the behavior of metallic conductors of the same size. Nevertheless, we observe that the the nonequilibrium phenomena bring about significant effects in all the studied setups which means that these phenomena must be taken into account when designing any applications in the field of low-temperature nanoelectronics.

This thesis consists of an overview and of the following 5 publications:

  1. Juha Voutilainen, Tero T. Heikkilä, and Nikolai B. Kopnin, Nonequilibrium phenomena in multiple normal-superconducting tunnel heterostructures, Physical Review B, 72, 054505 (2005). © 2005 American Physical Society (APS). By permission.
  2. Nikolai B. Kopnin and Juha Voutilainen, Nonequilibrium charge transport in quantum SINIS structures, Physical Review B, 75, 174509 (2007). © 2007 American Physical Society (APS). By permission.
  3. Juha Voutilainen, Matti A. Laakso, and Tero T. Heikkilä, Physics of proximity Josephson sensor, Journal of Applied Physics, 107, 064508 (2010). © 2010 American Institute of Physics (AIP). By permission.
  4. Juha Voutilainen, Aurelién Fay, Pasi Häkkinen, Janne K. Viljas, Tero T. Heikkilä, and Pertti J. Hakonen, Energy relaxation in graphene and its measurement with supercurrent, Physical Review B, 84, 045419 (2011). © 2011 American Physical Society (APS). By permission.
  5. Juha Voutilainen, Pauli Virtanen, and Tero T. Heikkilä, Absorption of heat into a superconductor-normal metal-superconductor junction from a fluctuating environment, eprint arXiv:1202.3358, 5 pages + 3 pages of supplementary information (2012). © 2012 by authors.

Keywords: nanoelectronics, superconductivity, nonequilibrium

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© 2012 Aalto University

Last update 2012-10-31