Electrostatic control of quasiparticle transport in superconducting hybrid nanostructures

A flow of electric current in a metal is the result of the collective motion of mobile conduction electrons within a relatively static background formed by ionized atoms. An electric current of 1 ampere used in everyday appliances corresponds to a flow rate of about 6*1018 electrons per second. In this thesis, I have studied experimentally and theoretically certain metallic nanostructures where electric charge can be measured and transported at a precision of one electron. Before this thesis, single-electron effects in hybrid structures consisting of superconductors (S) and normal metals (N) had not been thoroughly investigated. Many of the new results presented in this thesis concern the SINIS-type single-electron transistor. This structure consists of superconducting source and drain electrodes with a normal metallic island in between. The island is contacted to the electrodes via tunnel junctions (I). Due to a phenomenon konwn as the Coulomb blockade, the electric and heat currents through the transistor can be significantly altered by changing the gate charge by a fraction of the elementary charge. Several physical phenomena in the SINIS transistor were observed for the first time in the experiments of this thesis: We showed that the cooling power incident on the normal metal of the transistor can be modulated by the gate charge. We also demonstrated that the SINIS transistor can be used as an electron turnstile. The electric current through the turnstile is equal to the product of the elementary charge and the frequency of an external driving signal. A device that realizes this current-frequency-dependence with a sufficiently high accuracy could be used in electrical metrology in the future. As an important technological advance in the study of hybrid structures, we demonstrate that single-electron tunneling events between a superconductor and a normal metal can be detected in real time with a capacitively coupled single-electron transistor. By counting individual electrons, electric currents less than 1 attoampere can be measured, which is impossible with traditional room-temperature electronics. By measuring the rate of electron tunneling events, we were able to study the coupling of high frequency microwaves and so-called nonequilibrium quasiparticles to the measured samples. Finally, we have determined the distribution of heat dissipated in the process of charging a metallic island by a single electron.