Paul Ching-Wu Chu

views updated

Paul Ching-Wu Chu

A Chinese-American experimentalist in solid-state physics, Paul Ching-Wu Chu's (born 1941) leadership in superconductivity research in 1986-1987 led to revolutionary advances world-wide in material swhich carry electric current without resistance at high temperatures.

Paul Ching-Wu Chu was born in China's Hunan Province on December 2, 1941. In 1949 he was taken to Taiwan where he grew up and received his formative education. Both his family and his country were supportive of his youthful interests in radios, electronics, and transistors. By the time he finished high school in Taichung, central Taiwan, he knew enough of the physical sciences to want to become an experimental physicist.

Chu attended Cheng-Kung University from 1958 to 1962, where he obtained his B.S. degree. After a year's service as an officer in the Nationalist Chinese Air Force, he migrated to the United States, where he earned his Master's degree in physics at Fordham University in the Bronx (1965). Then he moved to the University of California at San Diego in order to study solid-state physics in the laboratory of Bern T. Matthias (1918-1980), where he earned his Ph.D. in 1968. Matthias, an expert in superconductivity, exerted a strong influence on Chu, counseling him to think daringly within his discipline. Known to his American friends by his baptismal name, Paul, Chu published his first scientific papers in 1967 and 1968 on the relationships of high pressure and low-temperature superconductivity in various metallic compounds.

What is Superconductivity

The Dutch physicist Kamerlingh Onnes first made liquid helium in 1908 and, by means of a series of experiments utilizing that discovery, discovered the phenomenon of superconductivity in 1911. Superconductivity is the ability of electric currents to float through certain materials completely untouched, without an ounce of energy loss. Until 1986, superconductivity had only been seen at extremely cold temperatures, a few dozen degrees above the absolute zero mark. As a technology, that rendered it practically useless because of the enormous expense required to cool something to those frigid depths. Supercold and superconducting go hand in hand because superconductivity is a state of matter that's frozen beyond solid. Take a familiar substance, like water. Hot water molecules careen around in steamy disarray; cooler water gets organized enough to flow, or stay put in a glass; really cold water can freeze rock solid. The atoms in superconductors line up in an even more ordered array—so well ordered, in fact, that they all behave like a single atom. When electricity flows through this super-crystalline arrangement, it doesn't collide with atoms in the metal, scattering its energy this way and that, as normal currents do. It doesn't waste its energy as heat. So the current loses none of its punch.

But just as ice won't freeze above 32 degrees Fahrenheit, materials won't superconduct above their transition temperature, which is different for each material. Like a freezing point, it is the temperature at which the transformation from solid to superconducting takes place.

Traditional (pre-1986) superconductors all had transition temperatures below about 40 degrees above absolute zero. The only way to get something that cold is to surround it with liquid helium, the coldest liquid that can be created on Earth. But liquid helium is so expensive that any efficiency savings from superconductivity are quickly eaten up in cooling costs.

The miracle of 1986 was twofold: First, the new materials were ceramics, and no one could figure out how a ceramic could carry electricity at all—much less without resistance. Second the materials became superconducting at temperatures "warm" enough to be cooled with liquid hydrogen, which physicists point out is cheaper than beer.

What Can Superconductivity Do?

Since 1986, dozens of new high-temperature super-conducting materials have been discovered. The one currently holding the record for the highest transition temperature was created by Chu, who hosted the anniversary workshop; it becomes superconducting at 164 degrees above absolute zero. Still, high temperature alone does not make a practical superconductor. The applications with the most potential to benefit from superconductor technology require strong electric currents and powerful magnets; nuclear magnetic imaging, for example, or power supply cables and motors.

But many of the new materials can't carry a strong current without destroying their own superconductivity—melting, in effect. Others fall apart in the presence of a strong magnetic field. Others can carry currents so high that they would evaporate a traditional copper wire. But unfortunately, Chu says, they are very unstable. "Instability and [high temperature superconducting] go hand in hand," Chu said.

Some applications of superconductors—like magnetically levitating trains—rely on still another property of these magic materials. A true superconductor strongly repels magnetic fields. A magnet brought near a superconductor floats on an invisible cloud of magnetic force like a boat on water. Trains riding on such clouds should be able to fly from city to city at 500 mph or more, but for now, only the Japanese are seriously pursuing such a project.

High-temperature superconducting is especially useful in applications that require exquisite sensitivity. Because superconducting atoms move in lockstep, it's very easy to pick up even the most minute variations in magnetic fields. Thus, IBM is already developing mine detectors for the Navy that can pick up magnetic fields smaller than those created by a moving paper clip.

Indeed, the most developed applications to date are supersensitive magnetic field detectors (call SQUIDS, for Superconducting Quantum Interference Devices) that are used in everything from geology to oil prospecting. Developers see huge markets in medicine, for example, in high-temperature devices for listening to the magnetic fluctuations of the heart and brain. Of course, none of these promises will become reality until a host of technological problems is overcome. Cooling systems have to get better, and cheaper, and high-temperature superconducting components have to get rugged enough to stand up to the hard knock of everyday use and mass production. Researchers who make these materials are still hoping that the theorists—whose job it is to explain how they work—will offer some guidance in the future. Without a workable theory, they've had to rely on guesswork and intuition. "I really have no idea what the mechanism is behind high-temperature superconducting," Chu said.

A History of Chu's Work

In 1926 W. H. Keesom first produced solid helium by using high pressures at low temperatures. It was Matthias who carried on this tradition, looking for more and better superconductive materials and ever higher critical temperatures (T_c) at which these materials might show superconductivity. Matthias found more than a thousand such compounds and achieved a record T_c of 18.3 K (Kelvin) in a nickel/tin alloy in 1953. Using his basic approach, a new record of 23.2 K was obtained by John Gavaler and Lou Testardi in metallic niobium/germanium films in 1973, seven years before his death. Chu's inspiration, like Keesom's, came largely from his mentor. But whereas Matthias had been constrained by having to work with the very expensive cryogenics of liquid helium, Chu, in his early research, was able to use the much cheaper but dangerous cryogenics of liquid hydrogen. The holy grail, a cryogenic of benign liquid nitrogen, which is cheaper than milk or beer, remained the animating motive for superconductivity research throughout the 1970s and 1980s.

After two years with the technical staff of Bell Laboratories in New Jersey, Paul Chu was appointed to his first academic post at Cleveland State University in Ohio. There, in 1973, he became a U.S. citizen and rose to the rank of full professor by 1975, creating an energetic research team in superconductivity, magnetism, and dielectrics. In 1978, with J. A. Woolam, he co-edited the book High Pressure and Low Temperature Physics. This work, plus more than 50 refereed scientific papers, led him to be wooed by and won to the Department of Physics at the University of Houston in 1979.

For the better part of the next decade Chu and his colleagues worked steadily toward realizing his dream of fabricating a superconducting material that could perform above the unbelievably high temperature of 77 K. Methods of all sorts were tested and their results appraised, but the 1973 record remained unbroken for 13 years. A 50 K leap to a "practical" high temperature above 77 K, the boiling point of liquid nitrogen, seemed insurmountable to many scientists. But then, in 1986-1987, everything changed and high temperature superconductivity (HTS) was born.

Johannes Georg Bednorz and Karl Alex Müller at the Zürich research center of the IBM Corporation published a report in the September 1986 issue of Zeitschrift für Physik that they had observed "possible high T_c" superconductivity at 35 K in a La₂Ba₁Cu₄ O (or lanthanum 214) mixture. A worldwide race to understand the chemistry and physics of this phenomenon started. Research teams around the world such as Bell Labs in New Jersey and laboratories in Tokyo, Zürich, Berkeley, Houston (of course), and many other cities were trying to understand the occurrence of superconductivity in the La214 compound. Chu's group chemically purified the compound, isolated the superconducting phase, and then exerted great physical pressure on it, thereby successfully raising the T_c into the mid-50s K by the end of 1986.

By January 12, 1987, they had unambiguously established superconductivity above 77 K for the first time. Since physical pressure had such a phenomenal effect on T_c, they decided to recreate the same effect by substituting smaller atoms for the lanthanum and barium in the compound. It seemed reasonable to substitute calcium for barium, but that didn't work. Another attempt at substitution, this time using strontium, was successful. Lanthanum was easily replaced by yttrium. Thus, by the end of January 1987, together with M. K. Wu (his former student, then at the University of Alabama at Huntsville), Chu's group was convinced that they had stabilized and observed unambiguously superconductivity above 90 K in their Y₁Ba₂Cu₃ O (or Yttrium 123) compound. Within a few weeks, replacement of yttrium with nearly all the rare earth elements demonstrated that a whole new class of superconductors had been discovered.

For the first time, superconductivity research could proceed using cheap liquid nitrogen. The copper-oxygen layers in these ceramics seems to be where the superconductivity is occurring. Unusual electrical and magnetic behavior, suggesting the existence of superconductivity at much higher temperatures, has subsequently been reported. However, these anomalies fail to meet the four criteria to prove unambiguously that they were superconducting. Those criteria are zero electrical resistivity, the Meissner effect, high stability, and reliable reproducibility. By the end of 1987, despite anomalies and a prevailing pessimism that any further discoveries would be made, Chu was confidently predicting HTS well above 100 K:

High T_c is a real possibility, and known applications pose great promises and challenges as well to all of us. But I think the area of novel applications tailored to the unusual properties of this class of materials will hold even greater promise. Indeed, the year 1987 is a "super" year in physics. We have witnessed superconductivity, [a] supernova, [the] superstring (theory), and superconducting supercollider (authorization). Let us enjoy these super-events in physics.

Almost immediately, in the spring of 1988, superconductivity up to 125 K was observed in a thallium compound (TICaSrCuO) by S. Sheng and A. Hermann.

Bednorz and Müller won the 1987 Nobel Prize for Physics, but Chu won respect and support as he continued to work toward "super high," and "super tech"—that is, room-temperature superconductivity and superconducting technology, respectively.

Room-temperature superconductivity was reported by the French research team of Jean-Pierre Bastide and Serge Contreras of the National Institute of Applied Science in Lyon in December of 1996. Room-temperature conductivity represents a jump of 200 degrees Fahrenheit, and researchers are conducting experiments to verify the French results.