Superconductor may hide long-sought secret

A new kind of superconductor can’t make up its mind about how to conduct electricity. Current passes through its interior without any resistance, as in a typical superconductor. But its skin behaves like a metal, conducting electricity but with some resistance.

This split personality, described in an upcoming Physical Review Letters, could be the handiwork of something strange hiding on the surface — a two-dimensional entity behaving like a Majorana fermion. First proposed more than 70 years ago, a Majorana fermion is a theoretical type of particle that is its own antiparticle. Electrons and quarks and other particles of matter all have twin antimatter partners.

Some theorists who suspect that neutrinos are their own antiparticles would be excited to find evidence that anything can act like a Majorana fermion, even the surface of the superconductor in the new study. Others hope that such particles could be useful for storing information in new kinds of computers.

“This is great,” says Robert Cava, a chemist at Princeton University who wasn’t involved with the study. “Hopefully it will get people excited about this material.”

Cava and colleagues were the first to create that material, made of copper, bismuth and selenium layered like lasagna. They showed that it’s a superconductor at temperatures within a few degrees of absolute zero. But until now, no one had conclusively proven that this superconductivity is any different from the run-of-the-mill variety discovered in mercury a century ago.

To probe the material, Yoichi Ando of Osaka University and colleagues in Japan injected current into it using a gold wire. This excited electrons at the surface, creating ripples of energy. Conventional superconductors have a dead spot in their surfaces that prevents low-energy, slow-wobbling ripples from forming. But a close look at this material revealed a sea of waves bouncing up and down both quickly and slowly.

Ando says that this pattern of ripples is “unambiguous evidence” of a type of superconductivity never seen before: topological superconductivity, in which electrons become waves molded into a complex shape that resembles the outside of a doughnut. These waves, says Ando, seem to be behaving like exotic two-dimensional particles at the surface of the material — specifically, Majorana fermions.

“This is the best evidence so far for Majorana fermions in a solid material,” says Taylor Hughes, a theoretical physicist at the University of Illinois at Urbana-Champaign. Still, the new experiment didn’t actually reveal the flat fermions themselves — only their supposed influence. No one has yet figured out how to directly detect them in solids. So it’s probably going to take many sources of indirect evidence to make the case that Majorana fermions actually exist in this material, says Hughes.

One way to test the idea would be to use magnetic fields to create swirling vortices on the material’s surface. These vortices should be able to trap a Majorana fermion and reveal it to be located not in a single spot, but stretched between two points like a rubber band.

Being spread out in this way could make Majorana fermions useful for quantum computers. These devices store information in the quantum states of particles, which tend to be fragile. But Majorana fermions can be disturbed only by attacking both of their end points simultaneously, giving them a hardiness that could help future computers solve problems out of the reach of today’s technology, mistake-free.