Browsing by Author "Pekola, Jukka, Prof."
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Fluctuations, relaxation and proximity effect in superconducting circuits(Aalto University, 2011) Peltonen, Joonas; Pekola, Jukka, Prof.; O.V. Lounasmaa Laboratory; O.V. Lounasmaa -laboratorio; Kylmälaboratorio; Perustieteiden korkeakoulu; Kaivola, Matti, Prof.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.Item Tunnel junction thermometry and thermalisation of electrons(Aalto-yliopiston teknillinen korkeakoulu, 2010) Holmqvist, Tommy; Pekola, Jukka, Prof.; O.V. Lounasmaa -laboratorio; O.V. Lounasmaa Laboratory; Aalto-yliopiston teknillinen korkeakoulu; Kaivola, Matti, Prof.In this work, normal metal and superconducting tunnel junctions have been studied. In particular applications in low temperature electronic thermometry are investigated. The double oxidation method for achieving high resistance junctions is described and shown to produce high quality devices. A new invention, the single junction thermometer (SJT), is described, and proof of the concept experiments demonstrating the functionality of the device are described. A key feature of the SJT is a well controlled electromagnetic environment surrounding a single junction. It is shown that embedding a single junction within arrays of other junctions is a superior way for controlling the environment as compared to previously proposed methods. Improved thermalisation of electrons is shown to be beneficial for the performance of superconducting RSFQ (Rapid Single Flux Quantum) devices. The so called grey zone of the most basic of these devices, the balanced comparator, is investigated. Improved performance based on metallic cooling fins has been achieved. In the same spirit improved thermalisation of the Coulomb blockade thermometers extends their operation towards lower temperatures. A method employing thick metallic islands was developed and tested to this end.