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Surprising Patterns

Research at Victoria University has uncovered surprising patterns in metals bombarded with gas atoms.

Dr Peter Johnson has been working on metals implanted with helium ions. These ions are fired at a metal surface at speeds of more than a million metres per second. Implantation has important applications, such as in the use of implanted nitrogen atoms to make metal surfaces wear-resistant. The helium ions penetrate below the surface of the metal and gather together in very small bubbles. At low temperatures in metals such as copper and molybdenum, the bubbles form a "superlattice". This is similar to the metal's atomic crystal lattice, but on a larger scale.

When Johnson and his team began bombarding gold with helium ions they received a major surprise.

"We found the expected superlattice of helium bubbles, in the same pattern as the gold lattice and with each small bubble 20 times further apart than the gold atoms," he says. "But when we looked again, we found another lattice on top of the first two -- much bigger bubbles another 20 times further apart than the smaller ones. It's a remarkable display of symmetry, an ordered structure on three scales."

The small bubbles contain 300-400 helium atoms each. The large bubbles have about 10 million. The large bubbles are so big they can only form one layer in the thin metal specimens. Scientists worldwide have been intrigued by the findings, and are arguing over how this ordering is caused.