(1) Formation of high temperature superconducting balls Self-aggregation into a ball is very rare in nature. Application of electric field further destroys space's isotropy. However, we have found that an electric field could drive high Tc superconducting (HTSC) particles into a ball. The experiment uses micrometer-size HTSC powders in liquid nitrogen. When a strong dc field is applied, the dispersed particles quickly form big balls, bouncing between the electrodes. If the temperature is above Tc, the balls disappear. Our current understanding relates this phenomenon to a new positive surface energy induced by surface charges on the HTSC particles. After an electric field is applied, HTSC particles pick up charges from the electrodes. These charges stay at the particles' surface, forming a thin charged layer. When the electric field within the layer is strong enough, it depletes Cooper pairs within the layer. This loss of superconducting condensation energy becomes a positive surface energy. Its minimization leads to the ball formation. As reported by Physics Today (p.9, Feb. 2000), Science News (VI 57, p2 1, Jan.8, 2000), and Physics News Update (Item 464, 1999), this discovery has received great attention. Our research explores the basic science and possible applications of this new property of superconductivity.
(2) Smart Fluids, electrorheological (ER) and magnetorheological (MR) fluids
In research on smart fluids, we have focused on the physical mechanisms, phase transitions, materials, microstructures, and dynamic process. As reported by Nature (V358, 373 (1992)), Scientific American (October, 58 (1993)), (German) Rheology journal (V.3, 284-285, Oct.-Dec. 1993), and New York Times (9/24/1996), our research in this area has received great attention and wide interest. On this project, we have extensive cooperation with industries, such as Ford Motor, Lord Corporation, Nippon Shokubai Co. Ltd. (Japan), Asahi Chemical (Japan), and Bridgestone/Firestone. Based on our understanding about the microstructure of MR and ER fluids, we recently invented a novel approach to change the microstructure of these fluids and produced super-strong MR and ER fluids. These fluids are about 10 times stronger than conventional MR and ER fluids. Their impact will be significant.
(3) Three-dimensional photonic crystals and communication
This research project is focused on 3-D photonic crystals and their application in communication. We have used irreversible ER and MR effect to produce 3-D photonic crystals, which have particles in micrometer size arranged in dielectrics periodically. The metallo-dielectric photonic crystals produced by this method have robust photonic band gaps. The analogy between the propagation of electromagnetic waves in photonic crystals and electron waves in atomic crystals has stimulated the excited research. We expect 3-D photonic crystals will have important applications in lasers, optical communications, quantum computers, etc.
(4) Nonlinear Optics
The goal of this project is to convert laser beams into coherent vacuum ultraviolet (VUV) or soft x-ray radiation by second-harmonic generation (SHG) of nonlinear optical crystals. Up to date, nonlinear optical crystals failed to produce SHG in VUV and x-ray region. The main cause is that these crystals are strongly absorptive in this region. To clarify the issue, we have developed a theory: absorptive nonlinear crystals can produce strong SHG signals under a double resonance condition; however, the conventional configuration does not work; we must use a new configuration, especially use a crystal film instead of bulk crystals. The preliminary experiment seems to support the theory. I am confident that a small VUV and X-ray laser suitable for conventional laboratories will become reality.
"Electrorheological Fluids under Shear",
R. Tao, J. Zhang, Y. Shiroyanagi,X. Tang, and X. Zhang International J. of Modern Physics B, V15, N 6/7, 918-929 (2001).
"Enhance the yield shear stress of magnetorheological fluids", X. Tang, X. Zhang, R. Tao,International J. of Modern Physics B, V15, N.6/7, 549-556 (2001).
"Structures of a Magnetorheological Fluid", G. L. Gulley and R. Tao,International J. of Modern Physics B, V15, N/6/7, 851-858 (2001).
"Path-integral approach to the statistical physics of random systems", R. Tao,J. of Statistical Physics, V103, N3/4, 575-588 (2001).
"Super-strong Magnetorheological Fluids", R. Tao, Journal of Physics:Condensed Matter Physics, V13, R979-R999 (2001).
"Structures of a Magnetorheological Fluid," L. Gulley and R. Tao, International J.of Modern Physics B, V15, N/6/7, 851-858 (2001).
"Path-integral approach to the statistical physics of random systems," R. Tao,J. of Statistical Physics, V103, N3/4, 575-588 (2001).
"Three-dimensional dielectric photonic crystals of body-centered tetragonallattice structure", R. Tao, D. Xiao, Appl. Phys. Lett. 80, 4702-4704 (2002).
"Electric-field induced formation of superconducting granular balls", R. Tao,X. Xu, and Y. C. Lan, in Electro-Rheological Fluids Magneto-Rheological Suspensions(Ed. by G. Bossis, World Scientific, 2002), 607-613.
"Structure-enhanced Yield Shear Stress in Electrorheological fluids", R. Tao,Y. C. Lan, and X. Xu, in Electro-Rheological Fluids Magneto-Rheological Suspensions(Ed. by G. Bossis, World Scientific, 2002), 712-718.
"Electric field induced formation of low temperature superconducting balls", R. Tao,X. Xu, Y.C. Lan, and Y. Shiroyanagi, Physica C, 377/ 3, 357-361 (2002).
"Structure and dynamics of dipole fluids under strong shear," R. Tao,Int. J. of Modern Physics B, V17, N16, 3057-3063 (2003).
"High temperature superconducting ball formation in low frequency ac fields," R. Tao,X. Xu, and E. Amr, Phys. Rev. B. V68, 144505-144511 (2003).
"MgB2 superconducting particles in a strong electric field," R. Tao,X. Xu and E. Amr, Physica C, V389, N3-4, 78-84 (2003).
