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.

Tunnel Junctions as Detectors of Noise and Energy Relaxation in Superconducting Circuits

Andrey V. Timofeev

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Information and Natural Sciences for public examination and debate in Auditorium AS1 at Helsinki University of Technology (Espoo, Finland) on the 3rd of September, 2009, at 12 noon.

Overview in PDF format (ISBN 978-952-248-025-5)   [1837 KB]
Dissertation is also available in print (ISBN 978-952-248-024-8)


The experimental work presented in this Thesis addresses fundamental aspects of modern low temperature mesoscopic physics and nano-electronics. It focuses on noise and full counting statistics of electrical charge, and heat transport in superconducting circuits with sub-micron tunnel junctions. The Thesis summarizes the experiments on shot noise asymmetry, heat relaxation in a superconductor, and radiative electronic refrigeration, which are performed at sub-kelvin temperatures, when quantum phenomena of charge and energy transport take place.

The detailed mechanisms of electron transport in mesoscopic conductors can be revealed by studying their noise properties. Yet a sensitive noise detector with a broad bandwidth is required for a complete characterization of fluctuations. In this work, we employ a Josephson junction as an on-chip detector of shot noise and its non-Gaussian statistics in tunnel junctions. The detectable noise bandwidth is determined by the plasma frequency of the detector, which is about 50 GHz in our case. The non-Gaussian component of shot noise is related to the observed asymmetry of the switching rates of the Josephson junction when reversing the polarity of the noise current. The shot noise asymmetry is analyzed with the model of a non-resonant response of the detector to the third order fluctuations.

Thermalization and heat transport is an important issue to secure proper functioning of superconducting mesoscopic systems. We have investigated heat relaxation in a superconductor by injecting hot electrons into it and measuring the energy flux from electrons into phonons. The observations showed strong suppression of the flux at low temperatures, in qualitative agreement with the theory for clean superconductors. The quantitative comparison between the data and the theory suggests presence of an enhanced or additional energy relaxation mechanism.

Finally, we demonstrate the effect of remote electronic refrigeration in a superconducting circuit. We show that matching the circuit impedance enables refrigeration at a distance with a cooling power limited by the quantum of heat conductance. We also observe and analyze the crossover between electromagnetic and quasiparticle heat transport mechanisms in a superconductor. The observed effect can be applied to reduce harmful hot electron effects in mesoscopic devices operating close to the quantum limit.

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

  1. A. V. Timofeev, M. Meschke, J. T. Peltonen, T. T. Heikkilä, and J. P. Pekola. 2007. Wideband detection of the third moment of shot noise by a hysteretic Josephson junction. Physical Review Letters, volume 98, number 20, 207001.
  2. J. T. Peltonen, A. V. Timofeev, M. Meschke, and J. P. Pekola. 2007. 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.
  3. J. T. Peltonen, A. V. Timofeev, M. Meschke, T. T. Heikkilä, and J. P. Pekola. 2007. Detecting non-Gaussian current fluctuations using a Josephson threshold detector. Physica E, volume 40, number 1, pages 111-122.
  4. A. V. Timofeev, C. Pascual García, N. B. Kopnin, A. M. Savin, M. Meschke, F. Giazotto, and J. P. Pekola. 2009. Recombination-limited energy relaxation in a Bardeen-Cooper-Schrieffer superconductor. Physical Review Letters, volume 102, number 1, 017003.
  5. Andrey V. Timofeev, Meri Helle, Matthias Meschke, Mikko Möttönen, and Jukka P. Pekola. 2009. Electronic refrigeration at the quantum limit. Physical Review Letters, volume 102, number 20, 200801.

Keywords: superconductor, tunnel junction, shot noise, quasiparticle relaxation, electronic refrigeration

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© 2009 Helsinki University of Technology

Last update 2011-05-26