A newfound superconducting current travels only along a material’s edge

Superconductors are getting edgy.

For the first time, scientists have spotted
a superconducting current traveling along the edge of a material, like a trail
of ants crawling along the rim of a dinner plate without venturing into its middle.

Normally, such superconducting currents, in which electricity flows without any loss of energy, permeate an entire material. But in a thin sheet of molybdenum ditelluride chilled to near absolute zero, the interior and edge make up two distinct superconductors, physicist Nai Phuan Ong and colleagues report in the May 1 Science. The two superconductors are “basically ignoring each other,” says Ong, of Princeton University.

This distinction between exterior and
interior makes molybdenum ditelluride an example of what are called topological
materials. Their behavior is closely tied to the mathematical field of topology, in which shapes are considered distinct only if one
can’t be molded into another without cutting or melding (SN: 10/4/16). In topological insulators, electric
currents can flow on the surface of a material but not the interior, like a
potato covered in tinfoil (SN: 5/7/10).

Likewise, topological superconductors
are superconducting in their interiors and behave differently on their
surfaces. Although some researchers suspected topological superconductors might
also host superconducting current on their edges, none had yet been found. But
the new observation is “extremely convincing,” says physical chemist Claudia
Felser of the Max Planck Institute for Chemical Physics of Solids in Dresden,
Germany, who was not involved with the research. “It’s really, really super
exciting.”

Molybdenum ditelluride is a metal-like compound
called a Weyl semimetal
(SN: 7/16/15). Its unusual properties
might mean it could harbor Majorana fermions, disturbances within a material that scientists hope to use to create
better quantum computers. Such topological quantum computers are expected to
resist the jitter that impairs quantum calculations (SN: 7/20/17).

In their experiment, Ong and colleagues gradually ramped up the magnetic field on the material. They simultaneously measured how much they could increase the electric current before the superconducting state was lost, a value known as the critical current. As the magnetic field increased, the critical current oscillated, getting larger, smaller, and larger again in a repeating pattern — a hallmark of an edge superconductor.

The oscillation results from the weird
physics of superconductors, in which electrons form partnerships called Cooper
pairs. The pairs act as a unified whole, all taking on the same quantum state, or
wave function, which determines the probability of a particle being found at a
particular location.

A property of the wave function called
the phase is analogous to twists in a party streamer hung around the edges of a
room, Ong says. If connected at the ends, the party streamer can twist once or
twice, but never 1.2 times, for example, because the ends wouldn’t align.
Similarly, the phase must make a full number of twists around the material. The
interplay between the increasing magnetic field and the twisting constraint
causes the critical current to oscillate.

A classic 1960s study known as the Little-Parks
experiment is closely tied to the new work. In that study, a superconductor
shaped like a cylinder exhibited related oscillations in a changing magnetic
field. But in Ong and colleagues’ version, the superconducting current runs
around the edge of a solid chunk of material rather than a physical cylinder.

“It’s a very clever and beautiful way of assessing whether or not there’s an edge current” that is superconducting, says physicist Smitha Vishveshwara of the University of Illinois at Urbana-Champaign, who was not involved with the research.