Solving the mysteries of melting

A team of US researchers have made a discovery in a superconductor that may lead physicists closer to answering a fundamental question: how do solids melt?

The researchers, led by Brown University physicist Sean Ling, are said to be the first to show that swirling electrons, tightly packed in quantised vortices, can literally melt or freeze just like ice or water.

Ling and colleagues from the US National Institute of Standards and Technology worked with a single crystal of niobium; a metal commonly used in magnets for MRI machines that acts as a superconductor at low temperatures.

The team took a cylinder of niobium 1-inch long by one-half inch in diameter and put it in a magnetic field, which causes the formation of vortices formed by billions of Cooper pairs of electrons. The metal was then heated to see whether the vortex lattice would melt.

To create photographic evidence of melting, Ling’s team beamed billions of neutrons through the top of the cylinder. At the bottom of cylinder, the light formed spots when the vortices were solid. The light then formed a ring when the vortices had melted into a liquid. The vortices were frozen solid at 4.1 degrees Kelvin and melted at 4.8 degrees Kelvin.

‘This is of fundamental interest to physicists because it provides a playground for studying the phenomenon of melting,’ said Ling. ‘It’s a common phenomenon, but understanding of this has evaded generations of physicists. Now we can actually use superconductors as a model system to study the mechanism of melting – it paves the way for future studies.’

The experiment also showed that the melting occurs at the same time as the superconductor’s ‘peak effect’, a sudden increase of critical current followed by a sudden decrease. The higher current-carrying capability is believed to be caused by a softening of the vortex lattice.

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