For decades, scientists have searched for magnetic monopoles--particles that, unlike traditional magnets, have just a north or south pole. This week in Science, two teams of condensed matter physicists independently report observations of the next best thing: tiny ripples in solid materials that act like the elusive particles. The find does not end the quest for bona fide monopoles, but it may herald the discovery of other weird "quasiparticles" in solids, as well as provide the basis for new technologies.
Monopoles would be the magnetic equivalent of electrically charged particles, and there are several reasons physicists would like to see them. In 1931, famed British theorist Paul Dirac argued that the existence of monopoles would explain the quantization of electric charge: the fact that every electron has exactly the same charge and exactly the opposite charge of every proton. In the 1980s, theorists found that the existence of monopoles is a basic prediction of "grand unified theories," which assume that three forces--the electromagnetic, the strong force that binds the nucleus, and the weak force that causes a type of radioactive decay--are all different aspects of a single force.
Now, two teams led by Tom Fennell of the Institute Laue-Langevin in Grenoble, France, and Jonathan Morris of the Helmholtz Centre Berlin for Materials and Energy have spotted analogs of monopoles in crystals called "spin ices," in which magnetic ions arrange themselves like the hydrogen ions in ice. The magnetic ions sit at the tips of four-sided pyramids or tetrahedra connected corner to corner (see diagram). At temperatures near absolute zero, they should organize themselves by a simple rule: In each tetrahedron, two ions point their north poles inward toward the center and two point outward.
Flaws in this pattern are the monopoles. If one ion flips--perhaps because it gets energized by the thermal energy in the crystal--it leaves one tetrahedron with three ions pointing inward and the neighboring tetrahedron with only one ion pointing inward (see figure). The two imbalanced tetrahedra act like north and south magnetic poles, respectively. If nearby spins also flip, the imbalances can shift independently from one tetrahedron to the next, so that the north and south poles end up connected only by a string of ions that point from one to the other. Thus the imbalanced tetrahedra become magnetic monopoles.
To detect such monopoles, Fennell and colleagues shined polarized neutrons on their sample of the spin ice holmium titanate and measured the scattering at various angles to reveal the underlying pattern of two ions facing in and two facing out. They then showed that, as the sample warmed, the scattering changed just as computer models predicted it would if monopoles were emerging, the team reports.
Morris and colleagues studied a similar material, but they dug up different evidence of monopoles. This team applied a magnetic field to stretch out the strings connecting the imbalanced tetrahedra in the spin ice dysprosium titanate; it then used neutron scattering to reveal the presence of the strings and, hence, the monopoles at their ends. The researchers also warmed the sample and showed that the heat needed to raise its temperature by a fixed amount--its specific heat--varied as if the solid was filled with a "gas" of monopoles pushing and pulling one another. "Essentially, there is a gas of north and south magnetic poles that are interacting just like plus and minus electric charges," Morris says.
The monopole quasiparticles are only variations in the pattern of ions in the system and not real particles, so they don't bear on grand unified theories or charge quantization. But the results suggest that other weird quasiparticles may exist in such solids, Fennell says. It's also conceivable that such monopoles might form the basis for a magnetic version of electronics, he says. "It's one of those unpredictable things where you don't know what people might think of," Fennell says.
"It's a beautiful observation," Peter Holdsworth, a theorist at the École Normale Supérieure de Lyon in France, says of the work. Shivaji Sondhi, a theorist at Princeton University, predicts a flurry of activity to probe the interactions between monopoles. "I would like to see the experiment in which a single monopole comes traipsing through [the detector]," he says, "and somebody will probably do that someday."