Superconducting/Semiconducting Nanowires, Nanotubes, and Ultrathin FilmsWenhao Wu, Texas A&M University, DMR 0606529

We use membranes with a honeycomb pore array as a “mold” to form nanowires (NW) and nanotubes (NT) by electro/electroless plating. Our most recent discovery is an anomalous proximity effect in which superconductivity in NWs up to 60 m in length is found to depend on whether or not their ends are connected to superconducting electrodes. This length is more than 10 times the length predicted by theories. A paper on this work is under review in Physical Review Letters. We also fabricate semiconducting NTs and study nano semiconductor interfaces formed by infiltrating polymers into the NTs. Such studies potentially can lead to breakthrough applications in renewable energy sources such as solar energy. We also investigate the disappearance of superconductivity in ultrathin films, known as the superconductor-insulator transition (SIT). We observe that the critical resistance at the SIT is h/(2e)2=6.5 k, independent of film thickness and magnetic field in a quite broad parameter space. This is strong evidence for a duality quantum phase transition. A paper based on his work will be submitted for publication in the near future.

Template-based nano-engineering. Clockwise from top left: Top (a) and cross-section (b) views of a home-made membrane; (c) Bi NWs grown using a membrane; (d) and (e) are images of TiO2 NTs grown using a membrane; (f) Cross-section image of TiO2 NTs.

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Top-left: Sn NWs with Sn electrodes are superconducting at the transition temperature of Sn. Bottom-left: Sn NWs with Pb electrodes are superconducting at the transition temperature of Pb. Middle: Field-tuned SIT in ultrathin Be films at the critical resistance h/(2e)2= 6.5 k. Right: With increasing Be film thickness (decreasing normal resistance RN), both the transition temperature Tc and the critical field Bc increase, yet, the critical resistance Rc is constant at h/(2e)2 in a quite broad parameter space.

Education In addition to supporting cutting-edge research, this NSF grant supports the training of undergraduates (2), graduate students (3), and a postdoctoral researcher (1). The training experience is broad and extensive, include such skills as materials synthesis, nanofabrication, measurements and data acquisition, modeling, structure and composition analysis by electron microscopy techniques, vacuum and low-temperature technologies, scientific writing and presentation, etc. The goal is to prepare the students and postdoctoral researchers for careers in science and technology. It is critically important that we have a steady supply of well-trained scientist so that the vitality of science and technology can be maintained in the United States.

Top-left: Graduate student E. Bielejec prepares for the next experiment on our dilution refrigerator. Bottom-left: M. Loth shows excitement during a Friday afternoon mini-Workshop on Quantum Fluid for NSF REU undergraduates. Top-right: Prof. Wu illustrates an electromagnetic bike at a physics open house. Bottom-right: Two kids are fascinated by a levitated magnetic top at a Science Day event.

Superconducting/Semiconducting Nanowires, Nanotubes, and Ultrathin FilmsWenhao Wu, Texas A&M University, DMR 0606529

Outreach and Societal Impact Our group provides outreach opportunities through events such as Science Days at local K-12 schools, Physics Open House, mini-Workshops for SPS and REU undergraduates. We also address some of the specific needs of this nation. One is to encourage women and minority students to pursue research careers in physics. This NSF grant currently supports one female Hispanic graduate student Isabel Schultz who is a first-generation college graduate in her extended family. In another example, Prof. Wu has written a successfully grant proposal to the Department of Education GAANN Fellowship Program with funds to support four women and minority physics graduate students. Lastly, this NSF supported program contributes to our knowledge of electron transport in low dimensions, which is critical for developing future generations of microelectronics. Our nanoscience research potentially can impact very positively on issues such as renewable energy sources.