Nanocircuits with Superconductivity: Nonequilibrium studies

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Juha Voutilainen Name of the doctoral dissertation Nanocircuits with Superconductivity: Nonequilibrium studies Publisher School of Science Unit Low Temperature Laboratory Series Aalto University publication series DOCTORAL DISSERTATIONS 60/2012 Field of research Engineering Physics, Theoretical and Computational Physics Manuscript submitted 28 February 2012 Manuscript revised 20 April 2012 Date of the defence 18 May 2012 Language English Monograph Article dissertation (summary + original articles) 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.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 signifant 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. 1