Prof. Judy Z. Wu Department of Physics and Astronomy Ph.D in Physics, 1993, University of Houston

Dr. Judy Z. Wu, Professor B080 Malott Hall University of Kansas Dept. of Physics and Astronomy Phone: (785) 864 3240 Fax: (785) 864 5262 Email: jwu@ku.edu

Research Interests

Thin films are the basis for modern microelectronics and a large number of technologies Thin films are also one of the most important tools for the preparation of novel materials. Although application oriented, growth and characterization of thin films has been an important subject for both basic and applied physics research. The focus of our research is on growth, characterization, and applications of various thin films including superconductor, ferroelectric and magnetic materials.

While the size of electronic devices continuously decreasing into nanometer regime as predicted by the Moore?s law, nanosicence has emerged as one of the most prominent research areas in 21st century. One of our new research focuses has been on growth of various nano-matters including nano-particles and nanowires, and development of characterization tools for nano-matter study.

High Tc superconducting films

The discovery of the first high temperature superconuctor (HTS) by Bednoz and Miller in 1986, which won them the Nobel prize, and triggered a worldwide effort in HTS research because of the tremendous potential of these materials in applications. Among many other HTss, Hg-based HTS (HgBa2Can-1CunO2n+2, n=1,2,3,?), or Hg-HTSs, have the highest superconducting transition temperature Tc of 135 K. The highly volatile nature of the Hg-based compounds, however, makes epitaxy of Hg-HTS films the toughest challenge so far in the HTS material research. We have developed several new processes including an alkaline doping assisted process to promote liquid phase formation which accelerates the formation of Hg-HTS phase, and a fast temperature ramping process to bring the processing temperature directly to the window so as to minimize the formation of the impurity phases. Using these new processes, we have demonstrated high-quality Hg-1223 films with Tc>130K and followed with many interesting studies on these films.

Despite the many exciting results on Hg-HTS films that her group achieved early on, two problems hindered further progress: poor run-to-run reproducibility and severe film/substrate reaction. These generic problems associated with epitaxy of volatile compounds in conventional material processing prompted us to invent a non-conventional cation exchange process. This process employs a precursor matrix of similar crystalline structure and chemical composition to the desired material, but without the volatile cations such as Hg. By providing perturbations to the guest cations on the sites near the final sites of the volatile cations, the guest cations can be replaced with volatile cations without collapsing the crystal structure, like an ?atomic surgery? over an existing crystal lattice. The microscopic mechanism of the cation exchange has been a focus of our group in recent years and question we would like to answer is how it occurs at microscopic scale, what are the relevant processing parameters, and can it be applied to design a new material using exiting ones.

Coated conductor research has been a central topic in the HTS research during the past few years. The so-called second-generation HTS tapes are expected to have tremendous potentials for various electrical applications. One of the outstanding problems is the dramatic decrease of Jc with the increasing HTS coating thickness. This problem must be solved for coated conductors to carry large currents. Our group has taken two approaches?bottom up by growing HTS films of different thickness and top down by thinning them using ion milling?to understand the physics related. We have also developed new scheme of making porous HTS thick films to tailor the current flow. In collaboration with researchers at the Air Force research laboratory and Oak Ridge national laboratory, she has developed a microstructure-engineering scheme using vicinal growth induced strain plus Y2BaCuO5 nanoparticle insertion and achieved uniformly porous-structured YBCO thick films with much improved Jc.

Boron nanowires and junctions

Nanoscale devices are promising for next generation electronics. Our focus has been on obtained single crystalline boron nanowire arrays and is studying their electrical transport properties. One thing we want to find out is whether this light and high-temperature semiconductor can have high electrical conductivity in the nanowire or nanotube form as predicted by theoretical calculations. In collaboration with Ames laboratory, Mg has been diffused into these boron nanowires to form MgB2 nanowires and superconductivity was demonstrated at about 35 K. To make devices, we have recently fused boron nanowires together and obtained arrays of fused boron nanowire junctions. Imagining two superconducting nanowires fused together, does it still behave like a Josephson junction? We hope to answer it through our research.

Multi-channel scanning probe microscopy

Correlation of different physical properties at microscopic to nanometer scales is crucial to understanding the basic physics in nano-matters. It should be realized that most existing SPM probes are single-channeled and therefore can only image one physical property at a time. This motivated us to develop multi-channel SPM. The first microwave/optical dual-channel microprobe was demonstrated recently in our lab for simultaneous mapping of microwave and optical properties.

There is no doubt that these multi-channel SPM will provide new tools for material research and nanoscience and we are currently carrying out some exciting applications of these probes. One application is to map electric current flow in HTS coated conductors using the optical/microwave dual-channel microprobe in combination with the standard I-V curve measurement. Three maps can be taken simultaneously during a typical scan over a sample: an optical map for identification of surface defects, a microwave map reflecting the nonuniformity of structure and chemical composition through the HTS layer, and a voltage map showing distribution of the current. This method may provide a unique room-temperature technique for electrical current mapping in long-length HTS coated conductors.