Purdue Physics Colloquium

Rongjia Tao
Professor of Physics
Southern Illinois University

Title: "Formation of Superconducting Balls"

Abstract:
Superconducting granular balls, a new phenomenon, have been observed. In nature, there are very few cases that granular particles could aggregate together by themselves to form a round ball. Application of electric field further destroys space's isotropy. Therefore, it is against common sense that an electric field could drive high Tc superconducting (HTSC) particles together to form a round ball. However, this is our recent discovery. When a strong electric field is applied to a suspension of micron-sized HTSC particles in liquid nitrogen, the HTSC particles quickly aggregate themselves together into macroscopic balls. The ball, about 0.25 mm across and containing several million particles, was quite sturdy; surviving constant heavy collisions with the electrodes. The experiment has established that this effect is uniquely associated with the superconductivity. Metal particles in this situation would bounce between the two electrodes to transport charges or form chains along the field direction (in ac field). The same HTSC particles, above their superconducting transition temperature, do not ball up. 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. (http://www.aip.org/enews/physnews/1999/split/pnu464-1.htm)

Fig. SEM picture of a ball of Bi2Sr2CaCu2O8+x particles. The diameter is 0.25 mm and particles are closely packed. The ball is trapped in an ac field first. After it is removed from liquid nitrogen, it becomes very fragile although the granular friction still holds the ball.

Research Interests:

Brief Bio:

Professor of Physics Southern Illinois University, 1993-present

Department Chair 1994 -1999

Visiting Professor of Physics Princeton University, 1997

Associate Professor of Physics Southern Illinois University, 1991-93

Assistant Professor of Physics Southern Illinois University, 1989-91

Assistant Professor of Physics Northeastern University, 1985-89

Research Assistant Professor of Physics University of Southern California, 1984-85

Research Fellow University of Cambridge, 1984

Research Associate University of Washington, 1982-84

Professor of Physics	Southern Illinois University, 1993-present
Department Chair	1994 -1999
Visiting Professor of Physics	Princeton University, 1997
Associate Professor of Physics	Southern Illinois University, 1991-93
Assistant Professor of Physics	Southern Illinois University, 1989-91
Assistant Professor of Physics	Northeastern University, 1985-89
Research Assistant Professor of Physics	University of Southern California, 1984-85
Research Fellow	University of Cambridge, 1984
Research Associate	University of Washington, 1982-84