****NON-TECHNICAL ABSTRACT?**** This project consists of experiments on materials patterned with nano-meter scale features that exhibit exotic electrical properties when cooled to near the absolute zero of temperature. The materials consist of films of the element Bi that have been perforated with a nano-meter scale honeycomb array of holes. Without the holes, these films become "superconducting" through a process in which the electrons form pairs, called Cooper pairs that are able to move through the Bi without friction. With the holes, these films can form into the opposite state: an electrical insulator. This insulator, surprisingly, also contains Cooper pairs. The goal of this project is to characterize this state with the objectives of determining the factors that prevent the Cooper pairs from superconducting. Experiments on films patterned with varying hole geometries will determine which geometrical characteristics produce the novel state. In addition, techniques to weaken the Cooper pairs will be employed to test the hypothesis that the Cooper pairing is essential to the formation of the insulator state. Results from this project will enhance our understanding of superconductivity and in particular, the behavior of some high temperature superconductors. In addition, these investigations provide intensive training for graduate and undergraduate students in high technology, research techniques and forefront physics issues and thus, help build the nation?s technological workforce.
?**** This project consists of experiments at dilution refrigerator temperatures on ultrathin films of superconducting materials, patterned with a nano-meter scale honeycomb array of holes. The goal is to characterize a recently discovered, novel insulating state of these films. This state contains Cooper pairs, the primary constituents of superconductors, but the Cooper pairs are localized. The objectives are to determine the factors responsible for the Cooper pair localization. The patterned films are created by depositing Bi onto anodized aluminum oxide templates that are perforated by deep holes. Investigations of films patterned with holes of varying radii and spacing will be performed to determine the geometrical factors influencing the Cooper pair localization. Also, investigations of how the insulating state responds to the introduction of magnetic impurities that can weaken and destroy Cooper pairing will be performed, thereby testing whether the Cooper pairing is essential to the formation of the insulating state These results are expected to impact investigations of other systems near the quantum superconductor to insulator transition and high temperature superconducting materials that exhibit states with finite resistance, such as the pseudo-gap state in under-doped cuprates, which might contain Cooper pairs. The project and its activities also serve as the PhD research training for two graduate students at Brown. It immerses them in frontier issues in condensed matter physics and helps them develop technical expertise that will make them attractive to industry or academia.
Normally a material is characterized as either electrically conducting, insulating or perhaps semi-conducting. Two materials, consequently, are required to make safe household wires. The plastic sheath that prevents shocks and short circuits is an insulator and the encased copper is a conductor. Recent experiments performed around the world have revealed, however, that some materials can be manipulated to be either superconductors or exceptional insulators. How this dual nature comes about has been the subject of intense investigation. What is known to be crucial is that the electrons are able to act in pairs. How this pairing occurs in a metal to lead the free electron motion characteristic of a superconductor is well known. On the other hand, how pairs can form that are immobile, as they need to be in an insulator, has been mysterious. Valles' group conducted a series of experiments at temperatures near absolute zero on a material in a thin film form that was known to be superconducting or insulating depending on its thickness. The films were made of a combination of antimony and bismuth or germanium and lead. They were plated on a surface perforated with a honeycomb array of holes that were spaced 100 nano-meters apart. This hole array in combination with a magnetic field made it possible to detect the existence of the pairs. Their investigations of these films revealed necessary conditions for making a robust insulator with pairs. Specifically, they showed that regular sub-nano-meter variations in film thickness both helped pairs to form and to become immobile. These results were published in a number of papers in the peer reviewed literature and were highlighted in a number of talks in scientific conferences. Intellectual Merit: Knowing these conditions makes it possible to control these dual states, can help guide the development of theories of this exotic paired insulator state and suggest the potential of using these materials as specialized electrical switches. Broader Impacts: These investigations also provided technical training for PhD and undergraduate students that can prepare them to work in high technology industry. They become proficient with electronics, computer analysis of data, and presentation of technical information. In addition, the PI and students made a number of presentations to precollege students both high school and elementary to engage them with the excitement of working at a scientific frontier.
