Atomic Steps Steer Quantum Vortices 1,000 Times More Easily

Researchers in Japan have shown that a surface step only one atom high can act like a rail for quantum vortices, allowing the vortices to move roughly 1,000 times more easily in one direction than the other.

The result comes from an ultrathin superconductor built from indium on silicon. In a peer-reviewed study published in Physical Review B, researchers from Japan’s National Institute for Materials Science (NIMS), Hokkaido University, India’s National Institute of Science Education and Research, and the Institute of Science Tokyo measured how Josephson vortices traveled parallel and perpendicular to a regular array of atomic steps.

The contrast reached the order of 10³ at intermediate magnetic fields. Instead of merely trapping vortices—as defects often do in superconductors—the steps created a highly directional transport channel whose behavior could be tuned with temperature and magnetic field.

Why Vortex Motion Matters in a Superconductor

A superconductor carries electrical current without ordinary resistance below a critical temperature. Under a magnetic field, however, magnetic flux can penetrate in quantized whirlpools of circulating current called vortices. When those vortices move, they dissipate energy and produce electrical resistance.

Controlling vortex motion is therefore a central problem in superconducting materials. Conventional engineering often tries to immobilize vortices with defects known as pinning sites. The new experiment explores a different possibility: shaping the surface so vortices can travel easily along one direction while remaining strongly hindered in another.

The team used the atomic-layer superconductor Si(111)-(√7×√3)-In, formed by covering a slightly tilted silicon surface with indium atomic bilayers. The tilt creates parallel terraces separated mostly by 0.31-nanometer-high steps. Those terraces averaged about 89 nanometers wide—close to the 80–100-nanometer transverse size of the Josephson vortex cores in this material.

That matching of length scales is important. Although each step is less than a nanometer high, it crosses the entire elongated vortex core and weakly couples neighboring superconducting terraces like a line of Josephson junctions.

How the Researchers Measured a 1,000-Fold Difference

The researchers first used scanning tunneling microscopy at 0.4 kelvin to confirm the parallel step structure, the superconducting energy gap and the presence of vortices at the steps. They then fabricated two four-terminal measurement configurations so the Lorentz force would push vortices either along the steps or across them.

Moving vortices generate resistance, which gave the team an electrical measure of their mobility. As the samples cooled under an out-of-plane magnetic field, the sheet resistance fell sharply—but at very different rates depending on the direction of vortex motion. The resulting resistance anisotropy, which is proportional to mobility anisotropy, reached approximately 1,000 at intermediate fields.

The measurements also revealed several transport regimes. At relatively high temperatures and low magnetic fields, thermal energy helped vortices creep over pinning barriers. Increasing the field reduced those barriers unevenly. Between roughly 0.10 and 0.20 tesla, the barrier vanished for motion parallel to the steps while remaining finite across them, producing what the authors describe as one-dimensional, pinning-free vortex flow.

At the lowest measured temperatures, thermal activation was no longer sufficient to explain the remaining motion. The researchers concluded that quantum tunneling governed the vortices instead. Their resulting magnetic-field–temperature phase diagram maps where thermal creep, directional free flow and quantum motion dominate.

Atomic Geometry Becomes a Control Surface

The study extends earlier work in which researchers directly imaged Josephson vortices at atomic steps in the same indium-on-silicon system. The new contribution is transport evidence showing just how strongly those steps govern vortex movement across a macroscopic device.

This is a striking example of geometry at one scale controlling behavior at a much larger one. A sub-nanometer step influences a vortex core nearly 100 nanometers across, while the directional effect appears in electrical measurements over current paths measured in fractions of a millimeter.

According to the NIMS research summary, the guiding effect can be adjusted through temperature and magnetic field. That tunability distinguishes the steps from static structural defects and suggests that deliberately patterned surfaces could become a design tool for controlling magnetic-flux transport in future ultrathin superconducting devices.

What This Does—and Does Not—Mean for Quantum Technology

The result does not amount to a new quantum computer or a ready-made qubit architecture. The measurements were performed in ultrahigh vacuum between 0.4 and 4.2 kelvin on a specialized atomic-layer material, and the authors note that more appropriate theoretical descriptions are still needed for quantitative analysis of such a strongly anisotropic system.

Its importance is more foundational. Superconducting circuits depend on maintaining precisely controlled quantum states, while uncontrolled vortex motion can introduce dissipation and noise. A surface that channels vortices predictably—or keeps them away from sensitive regions—could eventually inform the design of superconducting electronics, sensors and components used in quantum systems.

The experiment also offers a clean platform for studying quantum phase transitions in two dimensions. By reducing vortex transport from a two-dimensional problem to an effectively one-dimensional channel over part of the phase diagram, researchers gain a new way to test how disorder, magnetic field and carrier density reshape superconducting states.

The most consequential finding is therefore not simply that atomic steps can pin vortices. It is that a deliberately organized array of the smallest possible surface features can switch between resistance, guided flow and quantum tunneling as external conditions change. The full preprint makes clear that this is a controlled materials-physics demonstration, but it opens a credible route toward engineering quantum motion from the surface up.