Number 81 (Story #1), May 27, 1992 by Phillip F. Schewe and Ben Stein|
THE EXISTENCE OF VORTEX GLASS has been verified experimentally by scientists at IBM (C. Dekker et al., Physical Review Letters, 1 June 1992). One of the problems of using high-temperature superconducting materials in magnets is the fact that if the current flowing through the material is too high, the magnetic flux lines (or magnetic vortices) inside the superconductor can move, causing a dissipation of energy and a loss of the superconducting state. Several years ago a theory arose which foresaw that in contrast to this "vortex liquid" state there existed a "vortex glass" state (reached by cooling the material below a critical temperature) in which the vortex lines were frozen in random positions like the atoms in a glassy substance. The IBM scientists (contact Roger Koch of IBM; 914-945-2393) conducted extensive measurements of current and voltage in yttrium-based superconducting films first to verify that a phase transition was taking place, a fact that had been demonstrated in some previous experiments. Unlike previous experiments, however, they probed the nature of the low-temperature phase and found strong quantitative agreement with the vortex-glass model. Specifically, they determined the value of the exponent which characterizes the phase transition. This exponent, which is directly proportional to the height of the potential barrier that keeps the vortices fixed in their positions, was shown to increase with a decrease in current, a key prediction of the vortex-glass model. These new results discount competing theories, including the flux-creep model, which concludes that high-temperature superconductors in magnetic fields will always have a finite value of resistance, even if immeasurably small, at temperatures above absolute zero.