Crystals get in a twist
May 23, 2002
A Möbius strip is easy to make from a single strip of paper by simply twisting one end before joining the two ends together. Now physicists in Japan have created the one-sided structures from single crystals of niobium selenide. The exotic crystal forms could eventually be used to explore topological effects in quantum mechanics and to construct new electronic devices (S Tanda et al 2002 Nature 417 397).
Niobium selenide crystals are usually synthesized by heating niobium and selenium in a sealed quartz tube until long whiskers or ribbons form. Satoshi Tanda and co-workers at Hokkaido University in Sapporo have modified this crystal-growth process by creating a large temperature gradient across the tube, which allows the selenium to exist as both liquid droplets and a gas.
The Hokkaido team found three different crystal forms – rings, Möbius strips and figures-of-eight. As the ribbon-shaped crystals grow, they wrap around the selenium droplets, due to the surface tension of the liquid, until they eventually form a perfect seamless ring. Möbius strips are more difficult to produce because of the half-twist involved.
However, the low symmetry of the monoclinic niobium selenide crystals encourages the ribbons to both twist and bend, while rotating droplets may also help. Meanwhile, the figure-of-eight structures form if the ribbons circle the droplet twice. Tanda and co-workers are extending the technique to other materials, including tantalum selenide and tantalum sulphide.
About the author
Valerie Jamieson is Features Editor of Physics World