Investigating the quasiparticle dynamics operating in the electrodes of superconducting tunnel junctions using nanosecond phonon pulses

This thesis is concerned with the use of nanosecond phonon pulses to study quasiparticle behaviour in the electrodes of high-quality niobium superconducting tunnel junctions (STJs). This work is part of a collaboration with the Astrophysics Research and Development Division of the European Space Agency (ESA) at ESTEC. STJs are being widely investigated as photon detectors over a broad range of the electromagnetic spectrum. They potentially offer excellent energy resolution, time response and photon counting capabilities. The primary aim of this research was to use phonon pulses to investigate quasiparticle dynamics in STJs and to test this technique as an alternative method of detector characterisation. Experiments were performed in the temperature range 1.20 K - 2.00 K using a helium-4 cryostat. A thin-film metal heater on the back of the substrate was excited to generate a non-equilibrium phonon distribution. The phonons propagate across the substrate and break Cooper pairs in the STJ, simulating photon absorption. Using the phonon technique to complement x-ray absorption experiments is beneficial for a number of reasons. The input energy can be varied continuously over a broad range, the detected signal can be greatly enhanced by repetitive averaging, and the time dependence of the signal can be observed directly. Most importantly the analysis of the detected signal is simplified due to the uniform homogeneous energy deposition in the base electrode of the STJ. In this thesis data from phonon experiments are used to directly determine values for the parameters of an STJ such as the quasiparticle loss and tunnel rates in its electrodes. It is also shown how the input energy, in the form of phonons capable of breaking Cooper pairs, and the corresponding charge output from the device can be determined. These values are then compared with those obtained from x-ray absorption data. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN048580 / BLDSC - British Library Document Supply CentreGBUnited Kingdo