Superconductors, materials that carry electricity without resistance, can be divided into two broad groups depending on how they react to a magnetic field--or so physicists thought. New experiments show that one well-studied superconductor actually belongs to both groups at the same time. "If the experiment is true, this would add a whole new class of superconductors," says Egor Babaev, a theorist at the University of Massachusetts, Amherst. The advance may not immediately lead to new gadgets and applications, but it suggests that superconductivity, which has already netted four Nobel Prizes, may be an even richer phenomenon than previously thought.
Perhaps nothing demonstrates the weirdness of quantum mechanics quite as starkly as a superconductor. Within such a material, at very low temperatures, pairs of electrons coalesce into one large quantum wave that flows without drag. Because they carry current without resistance, coils of superconducting wire are ideal for generating the high magnetic fields in, say, an MRI machine.
The materials' many other strange properties have inspired a raft of conceptual advances. For example, superconductors abhor a magnetic field. Apply a weak one, and resistance-free electrical currents will circulate within the material to generate an opposing field that, inside the material, will precisely cancel the applied one. But if the applied field is too strong, a superconductor will lose superconductivity.
In a type-1 superconductor, such as lead or tin, this loss happens suddenly as the field surpasses a critical level. A type-2 superconductor, such as niobium, is more complex. It has two critical levels, and if the applied field is stronger than the first but weaker than the second, then the field can penetrate the material, but only by passing through tiny whirlpools of current called vortices.
Now, Victor Moshchalkov of the Catholic University of Leuven in Belgium and colleagues have shown that the superconductor magnesium diboride is in fact both a type-1 and type-2 superconductor, making it the first "type-1.5" superconductor.
How can that be? Superconductors carry electricity without resistance at very low temperatures because their electrons join to make free-gliding pairs. Those electrons typically come from one of the energy "bands" within the material. Physicists have long known that magnesium diboride has two bands that produce superconductivity. One of those bands produces type-1 superconductivity and the other produces type-2 superconductivity. In a paper to be published in Physical Review Letters, Moshchalkov and colleagues argue that the interaction of the two bands yields the new kind of superconductor.
To make that case, the researchers studied the patterns of electrical vortices within a very pure, single crystal of magnesium diboride. The behavior of vortices in fact determines whether a material is a type-1 or a type-2 superconductor. In a type-2 superconductor, the vortices repel each other, so they spread out to make a lattice. In a type-1 superconductor, neighboring vortices attract each other and quickly merge to form ever larger nonsuperconducting stripes and patches.
A type-1.5 superconductor is a little of both. The vortices should repel each other at close distances but attract each other when separated by long distances. That, in turn, should lead to disordered clumps and stripes of vortices separated by large voids, according to the researchers' simulations. And that's exactly what they observed experimentally. The researchers did a series of controlled experiments to rule out other explanations for the weird pattern, such as inhomogeneities in the samples. "We hesitated to come to such an exciting conclusion until all other simpler explanations had been excluded," Moshchalkov says.
If confirmed, the observation opens "an enormous number of possibilities," says Babaev, who had predicted such a state might exist. The vortices in a type-2 superconductor can be induced to form states that are orderly, disorderly, or even flowing like a liquid. The new class of materials would likely exhibit even richer behavior and present new puzzles for theorists to tackle, Babaev says. Hermann Suderow, an experimenter at the Autonomous University of Madrid, notes that there are a few other two-band superconductors, so magnesium diboride may not be the only type-1.5 superconductor.