NEXT time you order a cold drink, spare a thought for the ice cubes in it. They may look ordered and stable, but in fact they are mildly frustrated. Metaphorically speaking, it鈥檚 not unlike the frustration we feel when we can鈥檛 decide which drink to order. It鈥檚 all about indecision. The structure of water is such that when it freezes, its hydrogen atoms find it impossible to settle on one particular configuration, until at absolute zero 鈥� still unsettled 鈥� they freeze into a chaotic state.
The physics of frustration could lead to all kinds of new technologies. But why does it happen, and why does it lead to chaos? After all, the third law of thermodynamics stipulates that at absolute zero, any system must form a perfectly ordered structure. As the temperature drops, the atoms should stop jittering about and settle into a neat pattern. But inside the structure of ice, hydrogen atoms move randomly about a lattice of oxygen atoms, with two hydrogen atoms for every oxygen. When the ice reaches absolute zero, the hydrogens are supposed to pick a position to freeze tidily into. But since there are many possible arrangements, the hydrogen atoms can鈥檛 decide, and freeze into a disordered mess.
Do any other substances experience frustration? Quite a few, but only one kind is known that experiences it in quite such an extreme way: rare magnetic materials called pyrochlores. With pyrochlores, it鈥檚 electrons rather than hydrogen atoms that get frustrated. Electrons have a property called spin, which is related to their angular momentum. They spin around their axes in one of two directions, 鈥渟pin-up鈥� or 鈥渟pin-down鈥�. At normal temperatures, the spins are scattered about randomly, some up and some down. If the magnet is chilled, the electrons should settle into an ordered pattern 鈥� by forming pairs of ups and downs, for instance. But the geometric structure of the pyrochlore makes this ordering impossible. Imagine three electrons meeting in a corner, trying to form pairs. There is no way for them to decide which neighbour to pair up with, so they become frustrated and spin about chaotically in a state known as 鈥渟pin ice鈥�.
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鈥淓xotic spin-ice states hold huge potential for new technologies鈥�
The exotic, complex spin-ice states of pyrochlores hold immense potential for new technologies. They are helping researchers understand everything from superconductivity to diseases caused by protein misfolding. But their most useful application so far is in spintronics, the development of technologies that exploit the spin of electrons as well as their charge. Binary computer language, for instance, can be encoded in spin, and this could lead to the development of magnetic random-access memory (MRAM) 鈥� with huge capacity for data storage 鈥� and eventually to immensely powerful quantum computers. That鈥檚 something worth celebrating with an ice-cold drink.
