Superconductivity in nanocluster films.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Superconductivity in a bulk material is generally characterizing by a sharp drop in the resistance of the sample as a result of formation of a quantum condensate of paired electrons when the temperature is reduced below the critical temperature. However, superconducting behaviour changes dramatically with reducing dimensions of the superconducting structure. Several different behaviours have been reported in this dirty limit such as quantum phase slips, total suppression of superconductivity and a transition from the superconducting state to an insulating state. The question of how small a superconducting object can be before it loses its superconducting behaviour and what are the mechanisms that determine nanomaterials different behaviour at low temperatures are of fundamental importance of understanding the nature of superconductivity at the nanoscale and have the tremendous technological importance. Nanocluster films are ideal for addressing these questions, as they provide independent control of both cluster size and surface coverage and allow the study of local as well as global suppression of superconductivity. In particular, the percolating properties of the groups of clusters play vital role in the superconducting behaviour of the film. In this research clusters were produced in an inert gas aggregation source and deposited in ultra high vacuum. We present results obtained from the electrical characterization of films of ~ 30 nm diameter Pb nanoclusters deposited on SiN substrates at temperatures 10 K and 300 K; these substrates have pre-fabricated gold electrodes to monitor conductance of the percolating nanocluster film. A wide range of samples, with normal state resistances from 30 Ω and 5 MΩ, were prepared. The main focus of the study is on transport measurements for 10 K deposited samples. For low coverage films (above the percolation threshold) with RN ≳ 1 kΩ, low current V(I) and R(T) data show clear evidence of phase slips which appear to occur in the narrow 1 dimensional necks between the clusters. However, high coverage samples (with RN < 1 kΩ) exhibit a Berezinskii-Kosterlitz-Thouless (BKT) transition which is a signature of vortex unbinding in 2D system. Above critical current, it is found that V ~ (I – IC)a, with a ~ 2.1 ± 0.2. The exponent, a, is independent of temperature and particle coverage which is due to the underlying percolating nature of the sample. We have also prepared cluster films with coverage well below the percolation threshold. In these samples coupling between adjacent cluster groups is in the tunnelling regime and the films show a superconductor to insulator (SIT) transition. Furthermore, we have shown the SIT transition can be driven by change in either current or voltage.