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Distributed October 30, 2003
Contact Ricardo Howell

A team of researchers from Brown University and the National Institute of Standards and Technology has developed a new phase diagram for type-II superconductors. The research, reported in a recent issue of Physical Review Letters, confirms the seminal theory of type-II superconductors predicted by one of the winners of this year’s Nobel Prize in physics, and unravels behavior long suspected to exist in these materials.

PROVIDENCE, R.I. — Research reported by a team of scientists from Brown University and the National Institute of Standards and Technology (NIST) working at the NIST Center for Neutron Research (NCNR) provides important insight into the complex and technologically important relationship between superconductivity and magnetic fields.

Superconductivity – the resistance-free passage of electric current – and magnetic fields are antagonists: Increasing the magnetic field eventually extinguishes the superconductive state.

The study, presented in a recent issue of Physical Review Letters, provides evidence for a new phase transition in type-II superconductors, explaining in diagrammatic detail how the dissolution of the superconductive state takes place. This new map for type-II superconductors expands upon the landmark theory of type-II superconductors by Alexei Abrikosov, a winner of this year’s Nobel Prize in physics.

Nearly 50 years ago, Abrikosov predicted that some superconductors (called type-II) could retain superconductivity in a very strong magnetic field by forming tiny eddies of current, called Abrikosov vortices, that allow the field to pass through the superconductor without disruption until the field reaches a certain threshold level. This property enabled the development of powerful superconductive magnets for magnetic resonance imaging and other important applications.

Conducting its research at the NCNR during the last four years, the team led by Brown physics professor Sean Ling succeeded in mapping the complete set of complex events that underlie Abrikosov vortex behavior in a prototype superconductor, niobium.

In a finding that was hailed as a “milestone contribution,” the team reported in 2001 that type-II materials undergo a phase transformation akin to ice melting into a liquid. The team demonstrated that this structural transformation was related to a peak effect that had been found in many type-II materials. Scientists had debated the origin of this peak effect until the Brown-NIST experiment directly showed the structural transformation in the Abrikosov vortex lattice.

At temperatures near absolute zero in a magnetic field, Abrikosov vortices in a type-II superconductor position themselves in a lattice-like arrangement. But as the niobium sample nears its critical temperature, the team determined that this rigid arrangement softens, becoming almost gelatin-like, and the maximum possible current the superconductor can carry suddenly increases. Just as suddenly, however, true superconductivity collapses as the once orderly arrangement of vortices becomes an amorphous jumble resembling the disordered structure of liquid water.

But when a French-UK study had trouble reproducing the result, the team, including Brown Ph.D. student Sang Ryul Park, Ling, and NIST physicist Jeff Lynn, chose to conduct an exhaustive re-examination of the problem. Over a wider range of temperatures and magnetic-field strengths, the team analyzed how the NCNR’s beams of chilled neutrons interacted with the vortices as they moved and re-arranged themselves.

The experiments confirmed the earlier results, but also uncovered richer and more complicated behavior that had gone undetected. The new studies show that the material undergoes several types of phase transitions.

In the presence of a small magnetic field, the team found a smooth transition directly into the Abrikosov vortex lattice. Increasing temperature will eventually cause the superconductive state to vanish, without the expected peak-effect spike in current and without an abrupt onset of melting.

“Abrikosov’s theory predicted this smooth, continuous transition, but the prediction was not widely accepted,” Ling said. “In this low field regime, Abrikosov’s bold prediction turns out to be exactly right.”

At higher magnetic fields, the vortex structure softens and locks in, producing the peak effect in the critical current. Then, in a transition that Abrikosov did not predict, the vortices suddenly abandon their lattice positions and disorder ensues. At a certain temperature and magnetic field strength, all three of these phases may exist simultaneously.

“Almost a half century after Abrikosov’s landmark paper, we finally know for sure that there is a real melting transition in type-II superconductors, in addition to a smooth transition predicted by Abrikosov,” Ling said.

One fundamental result of the new work is a model phase diagram for type-II superconductors, which includes the so-called high-temperature superconductor materials first discovered in 1987. Knowing how the structure and arrangement of the vortex lattice changes in response to temperature and field should aid efforts to engineer new materials that will improve superconductive magnets, Ling said.

This research was supported by the National Science Foundation and the Galkin Fund.

The research team includes Ling; Brown Ph.D. student Sang Ryul Park; Sungmin Choi of the Korea Advanced Institute of Science and Technology; and Daniel Dender and Jeff Lynn of the NIST Center for Neutron Research.

As a non-regulatory agency in the U.S. Department of Commerce’s Technology Administration, NIST develops and promotes measurement, standards and technology to enhance productivity, facilitate trade and improve the quality of life. For more information, visit www.nist.gov or contact Mark Bello in NIST Public and Business Affairs: (301) 975-3776.

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