Argonne scientists win top superconductivity awards

ARGONNE, Ill. (June 19, 2003) — Two top prizes in the field of superconductivity have been awarded to scientists from the U.S. Department of Energy's Argonne National Laboratory.

George Crabtree, director of Argonne's Materials Science Division, received the Kamerlingh Onnes Prize for pioneering experiments on vortex matter - patterns formed by units of magnetic flux as they penetrate superconductors. Valerii Vinokur, a senior scientist and director of the Materials Theory Institute in the Materials Science Division, received the John Bardeen Prize for influential contributions to vortex matter theory.

"This is a wonderful recognition of our program in superconductivity," said Crabtree. "The two prize committees work completely independently, so the double honor is especially significant. Never before has the same institution been honored twice in the same year."

Both prizes are given every three years, the Onnes prize for experimental work and the Bardeen for theoretical work that has provided significant insight into the nature of superconductivity.

Argonne's prize-winning work focuses on vortex matter. If a superconductor is placed in a magnetic field, the field penetrates it in the form of electromagnetic lines, similar to lines made by iron shavings on a piece of paper when a magnet is placed underneath it.

The lines squeeze into an array of narrow streams, known as vortices, each containing exactly one unit, or quantum, of magnetic flux. All electromagnetic properties of superconductors are based on the behavior of these vortices.

The vortex lines form a regular array of hexagonal or cylindrical patterns known as lattices. Crabtree and his associates designed experiments that varied the temperature and degree of disorder to determine if the vortex lattice behaves like ordinary matter when heated. Would it melt into a liquid state or would the vortex lines disorder and become hopelessly entangled, like a bowl of spaghetti?

They found that the vortex lattice indeed melts at a temperature between 60 and 90 Kelvin (-183 degrees Celsius). They detected melting by the force necessary to move the vortices. In the frozen state, the vortices were fixed in place when a small force was applied, while in the liquid the vortices moved no matter now small the driving force.

"We thought the experiment showed melting to a true liquid rather than to an entangled ball of vortex lines," said Crabtree. "But we had to be sure, so we measured the magnetic moment of the vortex system." Just at the melting point, they found a tiny jump in the magnetic behavior, indicating a sudden absorption of heat, meaning it melted into a true liquid, much as ice melts to water.

Vinokur and his colleagues discovered 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."

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

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 the disorder. But if it's driven at some in-between speed, disorder takes the lattice apart and it "melts."

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

Both Vinokur and Crabtree stressed that Argonne's programs in vortex matter are strongly collaborative efforts. "Success requires a critical mass of excellent scientists all working together. We are fortunate to have such a group at Argonne. It is one of the greatest strengths of the national laboratory system," they said.

Crabtree shared his award with Professor Eli Zeldov of the Weizmann Institute of Science, Israel, and Vinokur shared the Bardeen prize with Professor David Nelson of Harvard and Professor Anatoly Larkin of the University of Minnesota. They attended the Seventh International Conference on Materials and Mechanisms of Superconductivity and High Temperature Superconductors in Rio de Janeiro, Brazil last month to receive their prizes.

The nation’s first national laboratory, Argonne National Laboratory conducts basic and applied scientific research across a wide spectrum of disciplines, ranging from high-energy physics to climatology and biotechnology. Since 1990, Argonne has worked with more than 600 companies and numerous federal agencies and other organizations to help advance America's scientific leadership and prepare the nation for the future. Argonne is operated by the University of Chicago as part of the U.S. Department of Energy's national laboratory system.

For more information, please contact Steve McGregor (630/252-5580 or media@anl.gov) at Argonne.