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Variation of the Superconductor Order Parameter in Quench-Condensed Granular Films

Abstract

This thesis investigates the superconductor order parameter through variation of amplitude and phase. Quench condensed two dimensional granular Pb films probe the phase, and a proximity effect of normal metal Ag on top of a granular Pb film probes the amplitude. Experiments in this thesis show that these grains are superconducting locally even though the film may be globally insulating due to poor coupling. The granular film can be thought of as a two-dimensional array of resistively shunted Josephson junctions. Phase stiffness is explored via varying the Josephson coupling between superconducting grains. The overlap between order parameters of the grains can range from very small to very large. The larger the overlap, the stiffer the coupled grains are to resist phase fluctuations. Severe phase fluctuations can destroy superconductivity. The amplitude is related to the density of Cooper pairs and this is related to the density of states and the energy gap. The proximity effect reduces the density of Cooper pairs by allowing a larger volume for the pairs to roam in. Analogies can be made between the variation of coupling and the suppression of the order parameter amplitude in a granular Pb + Ag system and a high T c system using the phase diagram of Emery-Kivelson theory.

Superconductivity is explored below critical temperatures of about 7 Kelvin in two dimensional quench condensed Pb films measured in a disordered granular state, in a uniform state, in proximity to a normal metal, and in combinations of the above. Much is known about low temperature superconductors in three dimensions, but interesting questions remain in two dimensions. In two dimensions, there is a transition from an insulating state to a superconducting state that is not a sharp transition. In between lie interesting temperature dependent sheet resistance behavior. Non-insulating disordered two dimensional granular films below a bulk critical temperature exhibit a fascinating linear relation between the logarithmic sheet resistance and the temperature, logR = logRo + mT, where Ro is the normal state sheet resistance above Tc. The resistive transition is wide in temperature range near the insulating side of the transition and decreases as coupling between the grains is increased. This is unusual behavior since the resistance is not a power law of temperature and not many properties vary with temperature in the numerator of the exponential. If the film is insulating, it exhibits activated behavior. With the films having a wide resistive transition, it is not clear whether the film at 0 K will exhibit insulating behavior, superconducting behavior, or in between behavior. The result depends on the coupling of grains in the disordered granular film.

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