Cooling and heat transport in low dimensional phonon systems, superconductors and silicon

Juha Muhonen

    Research output: ThesisDoctoral ThesisCollection of Articles


    Temperatures below 0.1 kelvin can be nowadays routinely attained. The methods for achieving these temperatures rely on either mixing the rare and expensive isotope of helium with the more common isotope (dilution refrigerator) or on adiabatic demagnetisation of paramagnetic salt (ADR). Although both of these methods are mature, they still remain complicated enough to limit the usage only to specialized laboratories. The research done in this thesis revolves around a promising electronic alternative to these techniques; normal metal-insulator-superconductor (NIS) junctions. One of the defining properties of a superconductor is a gap in its electronic density of states. This gap enables it to act as an energy filter for electrons. Because of this property, when a proper bias voltage is applied over a NIS junction the normal metal part will cool down as current passes the junction. The cooling properties of NIS junctions were demonstrated almost two decades ago with cooling powers of the order of one picowatt. At present cooling powers of few hundreds of picowatts have been achieved. This thesis describes research on three areas related to NIS junctions. Firstly we use NIS junctions to cool low dimensional lattice systems, both 1D and 2D. The cooling of a 1D lattice (beam) is interesting for fundamental research. The 2D lattice cooling (membrane) is aimed at bringing NIS devices closer to more widespread use. An electronically cooled membrane would offer a platform on which applications, such as radiation detectors or superconducting electronics, could be integrated. Secondly we focus on the limitations of NIS cooling. In all cooling, one of the main problems is the dissipation of the extracted heat. As the other side of the junction (normal metal) is cooled, the other side (superconductor) is heated with many times larger power. This heat can then weaken the superconducting properties and heat up the phonon system around the junction. These effects act to counter the cooling effect and have been one of the main obstacles in scaling up the cooling power of NIS devices. We study these effects both numerically and experimentally. Thirdly, we study the cooling of silicon with superconducting tunnel junctions. In these superconductor-semiconductor structures the normal metal in a NIS structure is replaced with highly doped silicon. Specifically we study the effects of induced lattice strain to the electron-phonon coupling in silicon and hence to the cooling properties of these structures.
    Translated title of the contributionJäähdytys ja lämmönjohtuminen matala-ulotteisissa hilasysteemeissä, suprajohteissa ja piissä
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    • Kaivola, Matti, Supervising Professor
    • Pekola, Jukka, Thesis Advisor
    Print ISBNs978-952-60-4669-3
    Electronic ISBNs978-952-60-4670-9
    Publication statusPublished - 2012
    MoE publication typeG5 Doctoral dissertation (article)


    • tunnel junctions
    • superconductivity
    • cooling
    • heat transport
    • electron-phonon coupling


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