Quantum Phase Transitions and Enhancement of Superconductivity by a Parallel Magnetic Field in Two Dimensional Superconductors

This dissertation describes several studies of superconducting phenomena in two dimensions. We have investigated the superconductor-insulator quantum phase transitions (SITs) tuned by several parameters, namely disorder (d), magnetic field (B), and magnetic impurities (MI). The films have been examined through electrical transport measurements and tunneling spectroscopy. Our experiments are carried out in a unique experimental setup which has allowed us to study these three SITs in situ on the same sample. There are two major theoretical frameworks to describe the superconductor-insulator transitions. One theory calls for a transition from a superconductor to a fermionic insulator. The other points to an bosonic insulator with localized Cooper pairs and itinerant vortices. Our experimental capabilities let us make direct comparisons of the SITs and comment on the nature of transition. We consider the MI-tuned SIT a canonical example of an SIT in the fermionic framework. The resistive transitions in the superconducting state are sharp, the phase boundary is well defined, and the insulating state shows weakly insulating behavior. The B-tuned SIT is representative of a bosonic SIT. In this case, the resistive transitions broaden as magnetic field strength is increased and there is no distinct boundary between the superconducting and insulating phases. Transport reveals a likely activated behavior just on the insulating side of the transition indicative of localized superconductivity. At higher magnetic fields, the temperature dependence weakens, possibly to logarithmic, signaling a break up of Cooper pairs into single electrons. Direct evidence of localized Cooper pairs comes via tunneling spectroscopy measurements on d-tuned granular films where full, bulk superconducting energy gaps are measured in the global insulating state. We have performed transport and tunneling spectroscopy measurements of the d-tuned SIT on amorphous Pb films. The transport behavior is qualitatively similar to what we observed for the MI-tuned SIT. We will also show that our observations of the B-tuned SIT on amorphous Pb films are analogous to the d-tuned SIT on granular films. One goal of our studies was to use tunneling spectroscopy to probe the nature of the insulating states of the d-tuned and MI-tuned SITs. We were only able to complete preliminary measurements on the d-tuned transition, however they are consistent with previous experiments in which increasing disorder leads to a decrease of TC and a concomitant suppression of the normal state density of state (DOS). Our in-house designed and fabricated sample rotation system affords us the opportunity to study the same sample in both perpendicular and parallel magnetic field orientations in situ without breaking vacuum. This capability led us to the observation of parallel magnetic field enhanced superconductivity in amorphous Pb films and the 2-D electron gas at the heteroepitaxial interface of the band insulators LaAlO3 and SrTiO3. We found that the mean field TC of the a-Pb films can be enhanced by as much as 13% above the zero-field value. A qualitatively similar field-enhancement effect has been observed in the LaAlO3/SrTiO3 interfacial superconductor. Moreover, our experiments show the field-enhancement is strongly dependent on the film thickness and magnetic impurity concentration. Clearly, the Bardeen Cooper Schrieffer (BCS) and Ginzburg-Landau (GL) theories of superconductivity do not contain the physics which would account for field-enhanced superconductivity. Presently, we do not have a complete theoretical understanding of the enhancement effect. However, our experimental results have placed significant constraints on any viable theoretical model. In this dissertation, several theoretical explanations of parallel field-enhanced superconductivity are discussed and compared with our observations.