A University of Arizona physicist has shown theoretically that strong magnetic fields change a basic property of superconductivity – the property that causes magnets to float, or levitate, above superconductors within weak magnetic fields.
Superconductors are materials that become almost perfect conductors at temperatures approaching absolute zero. Electrical current flows through these substances with almost no electrical resistance. Without electrical resistance, materials will conduct electric current endlessly.
However, physicists have long known that superconductors share another, more fundamental property than zero resistance, the so-called "Meissner effect." The effect is that when superconducting materials within a magnetic field become cold enough to become superconductors, the superconducting materials expel the magnetic field and are no longer magnetic.
The phenomenon of a magnet levitating above a superconductor is a spectacular demonstration of the Meissner effect, which was discovered 75 years ago. It is caused by the superconductor expelling the external magnetic field.
UA physicist Andrei Lebed has found an important new facet to the Meissner effect.
Lebed's theoretical research shows that contrary to conventional belief, some superconductors within strongly magnetic fields will concentrate magnetic lines rather than expel them. Under such conditions, relatively weak magnets may float above the superconductors, but stronger magnets will be attracted to the superconductors and fall.
"Understanding the physical nature of the Meissner effect in superconductors is one of the most important problems because it is intimately related to the possibility of creating superconducting wires for high electrical currents," Lebed said.
As traditionally viewed, the Meissner effect makes it impossible to develop superconducting wires for high electrical currents because magnetism destroys superconductivity. But Lebed's work suggests that superconductors would be stable in strong magnetic fields, so superconducting wires would be possible.
Another potentially important application for this new-found aspect to superconductivity, called the "paramagnetic intrinsic Meissner effect," is that a strong magnetic field could be used to raise temperatures at which materials become superconducting, Lebed said.
Superconductors that work at higher temperatures would be more affordable for technological applications.
Lebed's research was supported by a grant from the National Science Foundation.
His research was published in Physical Review B and reprinted in a virtual journal of the American Physical Society called Applications of Superconductivity.