Vorticles and Quasiparticles in Superconducting Films in the Small Order Parameter Amplitude Limit

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Non TechnicalPhysicists can explain why some materials behave as magnets, others as superconductors and others, insulators, do not carry electricity at all. Many technologically promising new materials, however, exhibit a mixture of properties and do not fit within these traditional phases of matter. To gain insight into the origins and manifestations of such mixed phase behavior, Valles' group is performing experiments at ultra-low temperatures on systems perched on the brink of a transition between a superconducting and a non-superconducting phase where mixed behavior should occur. In one case, they are using nano-technology to drive superconductors to become insulators by making them in the form of films thinner than a nano-meter (billionth of a meter) and perforated with nano-meter scale holes. In a second case, they are creating double layered films of a superconductor and a metal that produces a direct competition between these the metal and superconductor phases. These studies are serving as the PhD research training for two graduate students at Brown University. The students will develop expertise in nanotechnology, high sensitivity electrical measurements and low temperature techniques that will make them attractive to industry. In addition, undergraduate physics majors will engage in this research as part of senior thesis projects. TechnicalValles' group is investigating ultrathin superconducting films driven to the brink of a transition to a non-superconducting phase by the reduction of their superconducting order parameter amplitude. The work focuses on two distinguishing microscopic features of the superconductor: magnetic vortices and superconducting quasiparticles. The coexistence and competition between the superconducting and non-superconducting phases depend upon the vortex motion and the quasiparticle distribution. The results potentially will lend insight into similar phase competitions occurring in high temperature superconductors and other technologically promising materials. One set of these ultralow temperature investigations employs ultrathin homogeneous superconducting films patterned with hexagonal arrays of nanoscale size and spaced holes. The holes, which can pin vortices, will be used to detect the influence of vortex motion on the two dimensional superconductor to insulator transition. The second effort uses electron tunneling to characterize anomalies in the density of quasiparticle states that appear in ultrathin superconductor-normal metal bilayers. These states may drive a proposed superconductor to metal quantum phase transition in these composite films. The investigations serve as the PhD research training for two graduate students at Brown. These students will develop expertise in nanotechnology, high sensitivity electrical measurements and low temperature techniques that will make them attractive to industry. In addition, undergraduate physics majors will engage in related senior thesis projects.