New superconductor study confirms, extends Nobel theory

The behavior of intermetallic superconductors, like the kind used in hospital MRI machines, is even more curious than recent Nobel Prize-winning physicist Alexei Abrikosov had theorized. In newly reported research,* scientists working at the National Institute of Standards and Technology (NIST) Center for Neutron Research have determined that so-called type II superconductors have the equivalent of a multiple personality---at least three distinct physical states, each with its own superconducting behavior. The result should help engineers design new materials for stronger, more efficient superconducting magnets.

Nearly 50 years ago, Abrikosov predicted that superconductors could retain superconductivity in a very strong magnetic field by forming tiny eddies of current. These vortices allow the field to pass through without disrupting the current, until a certain threshold is reached and the resistance-free flow of electrons ceases. Just before the collapse, however, the materials undergo a dramatic spike in current, called the peak effect.

Over a wide range of temperatures and magnetic field strengths, Brown University and NIST scientists tracked the movements of current eddies in a prototype type II superconductor, niobium. Their experiments yielded a phase diagram, a kind of a map that shows how current vortices rearrange in response to changes in temperature and magnetic field.

The study confirmed an earlier set of the team's findings, but also revealed richer, more complex behavior. The recent work verified that the peak-effect jump in current corresponds to an abrupt change in the vortex arrangement--similar to the transformation that occurs when ice melts. They also provide the first experimental confirmation of Abrikosov's prediction that a smooth phase transition occurs for conditions that don't produce the peak effect.

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