Superconductivity and Quantum Phase Transitions in Ultrathin Amorphous Pb Films

This dissertation presents a systematic study of superconductivity and superconducting quantum phase transitions in ultrathin homogeneous Pb films. We have explored the superconductor to insulator quantum phase transitions (SITs) down to 80 mK by tuning several parameter viz: disorder, magnetic field, and magnetic impurity. The experiments have been carried out in a unique experimental setup of a modified dilution refrigerator with in situ thin film growth, magnetic impurity deposition, sample rotation and electrical measurement capabilities. The setup enabled us to increase the film thickness and deposit the magnetic impurities (Cr) incrementally, apply a magnetic field, in both perpendicular and parallel directions, up to 8 T, and perform electrical transport measurements at any film thickness and Cr- impurity density, all under ultrahigh vacuum and at low temperatures. The introduction of magnetic impurity as a new tuning parameter for the SIT provided us with a useful model system in which the microscopic mechanism for the destruction of uperconductivity is well understood, the phase boundary is well-defined, and the insulating state is purely fermionic. Using this as a reference and based on the extensive similarities between the disorder (d)-tuned and magnetic impurity (MI)-tuned transitions, we concluded that the disorder (thickness)-tuned SIT is fermionic in nature, contrary to several popular theoretical models. The magnetic field-tuned transition, on the other hand, appears to be qualitatively different from the magnetic impurity or disorder tuned transitions. When increasingly large perpendicular magnetic field is applied to an ultrathin superconducting film, the resistive transition progressively broadens, leading to an insulating state. There is no well-defined phase boundary between the superconducting and insulating state. In fact, in a range of normal state resistance(RN) near the transition, the sheet resistance, R(T) data exhibit reentrance or even double-reentrance. On the insulating side, increasing magnetic field in fact weakens the temperature dependence of R(T). The overall behavior bears many similarities to the disorder-tuned SIT transition in granular films, a model system for a bosonic transition. We observed a giant enhancement ( 13%) of the mean-field critical temperature (TC) of ultrathin homogeneous pure a-Pb films in parallel magnetic fields as large as 8 T. The magnitude of the TC enhancement was found to vary systematically with the film thickness, showing a nonmonotonic behavior. Most importantly, adding paramagnetic impurities (Cr) on the films always leads to suppression and eventual elimination of the TC enhancement. These observations directly contradict the theories based on the idea that the enhancement effect originates from the magnetic field polarizing the paramagnetic impurities. This dissertation also describes an effort to fabricate on-substrate nanoscale metal stencil masks as well as nanowire templates, using a variety of nanofabrication and assembly techniques. These nanostructures will be used for experiments aiming to extend these studies to the one-dimensional limit.