AN INTERNATIONAL team of physicists has created the world鈥檚 smallest
racetrack. It鈥檚 made from semiconductor rings so tiny that just a single
electron races around inside each one. Such minuscule rings, just 50 nanometres
across, could perhaps form the basis of a high-capacity computer memory, storing
a digital 1 when the electron is going round the circuit and a 0 when at rest.
According to Mauro Boero of Cambridge University, the research is 鈥渆xtremely
interesting . . . It opens a new way of storing information in a dot by means of
尘补驳苍别迟颈蝉补迟颈辞苍.鈥�
The rings are a variation on the technology of quantum dots鈥攃onducting
islets so small that they can contain at most one or two mobile electrons. One
way researchers make quantum dots is to spray indium arsenide molecules on a
gallium arsenide surface and allow the molecules to clump together into droplets
like steam condensing on a mirror.
But researchers at the University of Munich and the University of California,
Santa Barbara, found that the dots changed shape if they covered the droplets
with a layer of gallium arsenide as they formed. 鈥淲e discovered a spontaneous
reformation of the quantum dot into a volcanic crater shape,鈥� says J枚rg
Kotthaus of the University of Munich. Kotthaus says that during the growth of
the dots, indium first concentrates in the centre, causing a depression, but
then diffuses outwards, causing the dots to change into tiny rings.
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In conventional quantum dots, electrons can move around in many directions,
occupying a variety of energy levels. In the rings, however, the researchers
found that the electrons were confined to fewer energy levels. So at first, when
they applied a magnetic field to the rings, nothing happened. But when they
upped the field to a hefty 8 tesla, the electrons suddenly all started moving
around the rings in the same direction.
鈥淭his is like storing a bit: the ground state [energy level] has changed from
one which doesn鈥檛 contain a swirl to one that does,鈥� Kotthaus says. But before
the rings make a useful computer memory, they will have to find a way to get the
electrons moving at less than 8 tesla, the sort of field produced by room-sized
hospital NMR machines.