When warming up causes cooling down

To make a cup of hot tea, you add energy to a pot of water. But try doing the same thing to a tiny ball of atoms, and you may be in for a surprise.

Researchers in Germany have shown that a sodium ball only 5 atoms in diameter can, under the right circumstances, cool down when energized with a laser beam. What’s more, it doesn’t take much injected energy to get a dramatic effect.

Injecting less energy than is found in a single photon of visible light, for example, drops the temperature of the ball by 10C, report Martin Schmidt and his coworkers at the University of Freiburg in the Feb. 12 Physical Review Letters.

“It’s counterintuitive,” admits team leader Hellmut Haberland, “but if things get small, they behave differently.”

The scientists made sodium balls by blowing cold helium gas across the surface of boiling sodium. Then, with the resulting clusters in a vacuum, they used lasers to add energy and measure cluster energies near their melting point of about -13C.

To explain the peculiar temperature tumble, Haberland notes that semimolten states common in the everyday world, such as a glass of water with ice in it, are not observed in these nanometer-scale clusters. When heating ice water, the ice melts before the overall temperature of the water rises. That’s because the added energy goes first into converting the ice to liquid.

On the other hand, nanoclusters near their melting point oscillate between being all solid and all liquid about a billion times per second. In the process, Haberland says, the clusters shunt energy, including that from a laser, to the solid-to-liquid transitions. So, cluster temperatures go down, not up.