Cooling clusters of atoms

AS ANYONE who has put the kettle on for a cup of tea knows, adding energy to a system makes it hotter. But German physicists now report that when they add energy to tiny metal clusters, the clusters actually get colder. The findings were reported in the Physical Review Letters.

Fortunately, this isn't the end of physics as we know it. This bizarre property has been seen before in clusters of stars and at sub-atomic levels. But the new results show that clusters of atoms can behave in the same way.

Hellmut Haberland and colleagues from the University of Freiburg investigated the strange cooling behaviour, known as `negative heat capacity', in small clusters of sodium atoms. The atoms were suspended in a stream of non-reactive helium gas, which acted as a 'heat bath' and allowed the team to control the temperature of the clusters.

When researchers heated their clusters using laser light they found that the clusters cooled by about 10 degrees centigrade when they absorbed 1 electron volt of energy. By comparison, to heat 1 millilitre of water by 1 degree, you would need more than 1019 electron volts.

The cooling arises because of fundamental differences between microscopic systems, that contain relatively few atoms, and macroscopic systems. In a lump of bulk material, the number of atoms and molecules located at a surface or interface is minute compared with the total number of atoms in the material.

This means that the surface atoms have little effect on the system's overall energy, and heating causes a continuous conversion of the solid bulk into liquid, as happens when ice melts, for example.

But in tiny clusters, a large number of the atoms are at the surface, and changing any of the remaining 'bulk' atoms into surface or interface atoms through partial melting would require a large amount of energy relative to the cluster's overall energy. By comparison, adopting a completely solid or completely liquid state requires relatively little energy.

So in the temperature range in which clusters melt, they use their internal energy to supplement any heat input in an attempt to achieve complete melting. And using this internal energy causes the clusters' temperature to drop. But negative heat capacity isn't just confined to small clusters.

It can be observed whenever interactions between different parts play an important role in determining the overall energy of the system made up of those parts.

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Section  : Science & Tech
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