Andreev tunneling and quasiparticle excitations in mesoscopic normal metal - superconductor structures: Dissertation

Mesoscopic physics deals with systems whose size is between everyday macroscopic scale and the microscopic scale of individual atoms. With mesoscopic structures the flow of single electrons can be controlled. This thesis focuses on the control of single electrons with normal metal - superconductor structures. The emphasis is put on understanding the limitations of the control in the so-called SINIS turnstile, which is a device transporting one electron at a time. By repeating the drive with frequency f, the resulting electrical current in ideal operation I = ef could be utilized as the new definition in the SI unit system. Here e is the elementary charge. In the first part of the thesis, we review the physics of tunnel-coupled normal metals and superconductors and present the operation principle of the SINIS turnstile. We then show parallel operation of ten such devices. This allows one to reach larger currents required for high accuracy measurements. In addition we show that the experimental setup needs to be carefully designed in order to avoid spurious effects due to environmentally assisted tunneling. The second part of the thesis focuses on Andreev tunneling. In this process two electrons tunnel at once in form of a Cooper pair. Andreev tunneling leads to transfer errors, when the tunneling of a single electron is preferred. We discuss the experimental detection techniques of Andreev tunneling based on direct current measurements as well as on electron counting. Furthermore, we show experimentally that by having large enough energy cost for charging the structures, achieved by decreasing the size of the system, Andreev tunneling is suppressed and the accuracy of the turnstile improves. The electron counting techniques allows us to study nontrivial statistics of Andreev tunneling. In the last part of the thesis, excitations in a superconductor are considered. At low temperatures, the number of excitations of a superconductor should diminish exponentially. However, excess excitations in form of broken Cooper pairs are typically present limiting the performance of superconducting circuits. We discuss ways of probing the excitations in the normal metal - superconductor based structures. We investigate the diffusion of the quasiparticles and their relaxation to normal metallic traps or due to recombination into Cooper pairs via electron-phonon interaction.