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Superconductivity team wins top research prizes



NOBELIST Alexei Abrikosov accepts the 2003 Nobel Prize in physics from Swedish King Carl XVI Gustav.

Awards presented in 2003 to three Argonne scientists highlighted the excellence of Argonne's superconductivity program.

Distinguished Scientist Alexei Abrikosov shared the Nobel Prize in physics.

Materials Science Division Director George Crabtree was awarded the Kamerlingh Onnes Prize at the Seventh International Conference on Materials and Mechanisms of Superconductivity and High-Temperature Superconductors in Rio de Janiero, Brazil.

Senior scientist and Director of the Materials Theory Institute Valerii Vinokur accepted the John Bardeen Prize at the same conference in Brazil, and won a Humboldt Research Prize.

Superconductivity is the transfer of electric current without any loss. Standard copper conductors transfer electricity and create controlled magnetic fields, but they resist current flow, generating heat and wasting energy. Superconductors accomplish the same tasks without resistance or loss.

Abrikosov was awarded the Nobel Prize in physics for developing the theory to explain how magnetic fields penetrate certain superconducting materials. He shared the award with Anthony J. Leggett of the University of Illinois at Urbana-Champaign and Vitaly L. Ginzburg of the Lebedev Institute in Moscow. The three researchers were recognized for their work explaining matter's bizarre behavior at extremely low temperatures.

The distinguished scientist at the Condensed Matter Theory Group in Argonne's Materials Science Division, Abrikosov's research centers on the structure and behavior of solids and liquids, called condensed-matter physics, and he concentrates on superconductivity. He was the first to propose the concept of "type-II superconductors" in 1952 and constructed the theory of their magnetic properties, known as Abrikosov vortex lattices.

When first proposed, his theory was considered controversial, and Abrikosov said "I put it in a drawer, but I did not put it in a wastebasket, because I believed in it."

"Alex's insights and discoveries have launched 50 years of studies into the fundamental nature of superconductivity," said Thomas F. Rosenbaum, the University of Chicago's vice president for research and for Argonne National Laboratory.

The Onnes and Bardeen prizes are awarded once every three years by independent committees. They recognize experimental and theoretical work, respectively, that has provided the most significant insight into the nature of superconductivity. Crabtree's Onnes award acknowledged his pioneering experiments on patterns formed by Abrikosov vortices as they penetrate superconductors. Valerii Vinokur received the John Bardeen Prize for his influential contributions to vortex matter theory.



Alexei Abrikosov in his office.

"To have Argonne recognized in both theory and experiment by the superconductivity community in the same year is truly monumental," Crabtree said.

Argonne's double honor is the result of pioneering experimental and theoretical work that focuses on vortex matter, Crabtree explained. Vortices are whirlpools of electrons circulating around tubes of magnetic flux that form in many superconductors when they are placed in a magnetic field. Each vortex contains one unit, or quantum, of magnetic flux. All electromagnetic properties of superconductors are based on the behavior of these vortices.

Crabtree's award-winning research stemmed from experiments that he and his colleagues performed to see vortex-lattice melting in various temperatures, magnetic fields and degrees of disorder.

Vortex lines in superconductors form a regular array of hexagonal patterns known as lattices. Upon heating the vortex lattices in certain copper oxide superconductors to between 60 and 90 degrees Kelvin (minus 353 and minus 298 degrees Fahrenheit), Crabtree and his colleagues found that the lattices melt, just as ice melts to water.

The vortex liquid above the melting point is a new phase of vortex matter. Its properties are very different from those of the vortex lattice - it flows freely in response to even the smallest driving force. This motion of the vortex liquid produces resistance, a feature exploited in the Argonne experiments to detect its presence. Like other liquids, the vortex liquid moves easily around obstacles and finds meandering paths of least resistance in complicated landscapes.

Vortex lattice melting and the behavior of the liquid phase are strikingly different from anything previously encountered in superconductivity. In high-temperature superconductors, the vortex liquid is the dominant feature of the superconducting state. It provides fertile ground for innovative concepts, experiments and applications.

Vinokur's Bardeen award was the result of a discovery he made with his colleagues; they found that disorder dramatically alters vortex matter resistance. Their theories predicted an effect of disorder on the lattice structure similar to the effect of a bumpy road on a car, a phenomenon they called "dynamic melting."



VORTICES Magnetic fields penetrate superconductors in concentrated tubes called vortices. Each vortex consists of a tube of magnetic field surrounded by a circulating superconducting current that flows with zero resistance.

"If you drive fast enough," Vinokur explained, "it's easy on the car. If you drive very slowly, pay attention to every bump, it's also easy on the car. But you'll discover some intermediate speed that is very unpleasant for you and the car."

Vinokur said the same effect occurs with a vortex lattice. Pushed hard enough it can overcome any disorder in the superconductor. Pushed very slowly, it will adjust to disorder. But if it is driven at some in-between speed, disorder takes the lattice apart and it "melts."

In "first gear," the vortex lattice slowly creeps across bumps due to disorder. Vinokur and his colleagues discovered that the energy barriers controlling this slow motion exhibit a universal "scaling" behavior as a function of the driving force, which is a general feature common to all disordered systems. This finding was crucial to understanding phenomena at the melting point, a feature early emphasized by Vinokur.

These prizes have never before been awarded to two scientists at the same institution in the same year. Vinokur said that he feels this award marks the recognition of ground-breaking ideas that no one previously accepted.

"I was thrilled when I looked at the previous recipients of the prize," Vinokur said. "They were scientists whose contributions to the field of theoretical physics in the last 50 years were determining and crucial for its further development. I was honored to be recognized as on a par with people like that."

Abrikosov won the John Bardeen Award in 1991, the first year it was awarded.

Later in 2003, Vinokur was recognized by the Alexander von Humboldt Foundation for his work in superconductivity and nanophysics. Nanomaterials - measured in billionths-of-a-meter - behave differently when compared to their more traditional counterparts. The value, quality and international impact of the nanophysics program Vinokur established at Argonne played an important role in Vinokur's recognition.

Vinokur plans to use his award money to spark collaboration between German scientists and the Materials Theory Institute he heads.

These distinctions come on top of the many honors Argonne has received for its superconductivity program through the years. Currently, Argonne's superconductivity scientists are among the world's most often cited and are invited to give talks all over the world.

Both Vinokur and Crabtree emphasize that Argonne's programs in vortex matter are collaborative efforts. "Success requires a critical mass of excellent scientists all working together," Crabtree said. "We are fortunate to have such a group at Argonne."

"Exchange of ideas is highly encouraged at Argonne," Abrikosov said. "It is useful to have both theorists and experimenters working together. Argonne is unusual in that the environment is conducive to joint work."

Abrikosov explained that when he began working at the laboratory, he had not imagined the number of experiments being performed. He said that he felt proud that he was able to help the scientists here make sense of their experiments and discoveries.

Collaboration is key

Both Crabtree and Vinokur attribute the laboratory's exceptional superconductivity program to close interactions between theorists and experimentalists.



AWARD WINNERS George Crabtree (left) and Valerii Vinokur are other members of Argonne's winning high-temperature superconductivity team.

"Collaboration is simply integral to this program," Crabtree said. "If experimentalists and theorists did not meet daily in the hallways and over coffee, we would not have the same level of teamwork."

The dramatic development of vortex physics at Argonne over the last 10 years echoes similar breakthrough progress at the laboratory across the field of superconductivity. The discovery of high-temperature superconductors in 1986 shook the foundations of the field. Within months, the highest temperature at which a material became superconducting jumped from 23 K (minus 418 F) to 93 K (minus 292 F) and later rose to 160 K (minus 172 F). Traditional concepts that had guided the field for 30 years were suddenly outmoded.

"It was a magic time, a once-in-a-lifetime opportunity," said Crabtree. "We saw the enormous promise of the field and the chance to make a lasting contribution."

Argonne leaped ahead, not only in vortex matter but also in synthesizing new materials, developing innovative experiments and advancing creative theory.

The laboratory's materials synthesis program devised new methods for controlling the subtle variations in composition and structure that govern superconducting properties. These forefront materials enabled state-of-the-art experiments at Argonne and around the world. Laboratory scientists explored exotic features of the high-temperature superconducting state, such as its vanishing density of superconducting electrons along certain crystallographic directions, and its ability to carry large electrical currents at zero resistance even in high magnetic fields.

"It is the integration of materials, experiments and theory under one roof that gives Argonne its scientific and technological power," said Crabtree. "This is a defining feature of national laboratories that few other research institutions share."

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