One-Atom-High Steps Steer Superconducting Vortices 1,000 Times More Easily, Japan Finds
NIMS physicists found that tiny whirlpools of current in an ultrathin superconductor glide along rows of atomic steps like trains on rails, a possible route to lower-power superconducting devices.
Physicists in Japan have found that steps on a crystal surface just one atom high can act as rails that guide tiny quantum whirlpools through a superconductor, letting them slide more than 1,000 times as easily along the steps as across them.
The work comes from a team led by Takashi Uchihashi at the Research Center for Materials Nanoarchitectonics, known as MANA, part of Japan's National Institute for Materials Science in Tsukuba. The results, with Wenxuan Qian as first author, were published July 30 in Physical Review B and publicized by the institute this week.
Superconductors carry electric current with zero resistance, but when a magnetic field penetrates them it does so in the form of vortices: tiny whirlpools of circulating current, each carrying a fixed quantum of magnetic flux. When those vortices move, they dissipate energy and can destroy the zero-resistance state, so how and where they move strongly shapes how a superconductor behaves. Engineers usually try to pin vortices in place. Being able to steer them in a chosen direction instead could open up new kinds of ultralow-power superconducting electronics, but doing so in extremely thin, two-dimensional superconductors has been difficult.
Uchihashi's group studied an atomic-layer superconductor grown on a so-called vicinal surface, a crystal cut at a slight angle so that its surface forms a regular staircase of parallel, one-atom-high steps. Using a scanning tunneling microscope, the researchers confirmed the steps and directly imaged vortices sitting along them. The steps act as weak links in the superconductor, and the vortices trapped there are a type called Josephson vortices.
The team then measured electrical resistance in different directions using a four-terminal setup. At intermediate magnetic fields, vortices moved more than 1,000 times as easily along the steps as across them. Between about 0.10 and 0.20 tesla, the vortices flowed freely along the steps without getting snagged by defects, forming what the team calls one-dimensional, pinning-free vortex flow. At the lowest temperatures, their motion was governed by quantum tunneling, meaning the vortices slipped past barriers they did not classically have enough energy to cross.
"Our study shows that atomic-scale steps can act as effective rails that guide superconducting vortices, and that this guiding effect can be tuned simply by changing the temperature or magnetic field," Uchihashi said.
That tunability is the key point. A built-in, atomic-scale track whose strength can be adjusted with ordinary knobs like temperature and field gives researchers a way to control where vortices go and, with them, where heat and energy flow inside a device. The team says the approach could help in designing future superconducting technologies that use less power while processing information more efficiently. The paper is titled "Anisotropic transport of Josephson vortices in atomic-layer superconductors on vicinal surfaces."
