Atomic-scale ‘rails’ guide quantum vortices in superconductors

Researchers at the Research Center for Materials Nanoarchitectonics (MANA), part of the NIMS in Japan, have discovered that atomic-scale surface steps can act as highly effective guides for superconducting vortices. The team found that these quantum objects moved more than 1,000 times more easily along the steps than across them, while the effect could be adjusted by changing either temperature or magnetic field.

The findings provide a new mechanism for controlling vortex motion in two-dimensional superconductors, an area of growing interest for future low-power electronic technologies.

Superconductors are materials capable of carrying electrical current without resistance when cooled below a critical temperature. Because no energy is lost as heat through electrical resistance, they are widely studied for applications ranging from medical imaging systems and scientific instruments to emerging quantum technologies. Yet superconductivity is not always as straightforward as a perfect loss-free state. When exposed to magnetic fields, many superconductors admit magnetic flux in the form of quantised vortices. These tiny quantum structures can move through the material and strongly influence its behaviour. As a result, understanding and controlling their motion has become a major focus of superconductivity research.

According to the MANA team, achieving directional control of vortices in atomically thin superconductors has remained particularly challenging. Such materials are attracting increasing attention because reducing materials to the two-dimensional limit can lead to novel electronic properties that are absent in bulk systems.

A one-atom-high solution

To tackle this challenge, the researchers investigated an ultrathin superconductor whose surface contained regularly arranged atomic steps. Although these steps are only one atom high, they create a subtle landscape capable of influencing the movement of quantum objects.

Using scanning tunnelling microscopy, the team confirmed the presence of parallel atomic steps and directly visualised vortices located along them. The observations suggested that the surface features were acting as preferred pathways for vortex motion.

Electrical transport measurements subsequently revealed the remarkable scale of the effect. Vortices travelled more than 1,000 times more easily along the direction of the atomic steps than across them, indicating an exceptionally strong directional preference. Rather than behaving as passive surface imperfections, the atomic steps effectively functioned as nanoscale rails that channelled vortex movement.  The result demonstrates that an atomic-scale structural feature can exert a profound influence over the movement of a quantum object, linking surface morphology directly to superconducting behaviour.

The researchers found that changing temperature and magnetic field altered how vortices moved through the material. This means the guiding behaviour is not fixed by the structure alone but can be tuned through external operating conditions.

Particularly intriguing behaviour emerged at intermediate magnetic fields. Between approximately 0.10 and 0.20 tesla, the vortices flowed along the atomic steps without being hindered by pinning, a phenomenon referred to as one-dimensional pinning-free vortex flow. In many superconductors, pinning centres trap vortices and restrict their movement. In this case, however, the vortices were able to travel freely along the atomic-scale pathways.

“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,” said Takashi Uchihashi, who led the research team.  The work therefore demonstrates not merely directional transport but a means of controlling that transport through readily adjustable experimental parameters.

Quantum tunnelling emerges at low temperatures

The study also uncovered a transition in the mechanism governing vortex movement. At the lowest temperatures examined, the researchers found that vortex motion was no longer dominated by conventional thermal processes. Instead, it appeared to be governed by quantum tunnelling.

Quantum tunnelling is a fundamental quantum-mechanical phenomenon in which particles or quantum objects can pass through energy barriers that would be insurmountable according to classical physics. Observing tunnelling-driven vortex motion highlights the quantum nature of the system and demonstrates that quantum effects remain important even when vortex transport is constrained along atomic-scale pathways.  The finding adds another layer of scientific interest to the work, connecting surface engineering with the study of quantum transport phenomena in low-dimensional materials.

The research remains fundamentally exploratory, and the scientists have not identified specific commercial applications. However, the findings provide a new strategy for manipulating vortex motion in superconducting systems.  According to the NIMS researchers, controlling the motion of superconducting vortices could be important for the development of future ultra-low-power superconducting technologies. The team further notes that the ability to guide vortices using one-atom-high surface steps opens possibilities for controlling both vortex motion and heat flow in future superconducting devices.

One notable aspect of the work is the simplicity of the underlying mechanism. Rather than relying on complex engineered nanostructures, the effect arises from atomic steps already present on the material surface. This suggests a potentially versatile approach for studying and manipulating vortex behaviour in a variety of ultrathin superconducting systems.

The study, titled “Anisotropic transport of Josephson vortices in atomic-layer superconductors on vicinal surfaces”, adds to the growing body of research exploring how atomic-scale features can shape the behaviour of quantum materials.

By demonstrating that a surface step just one atom high can act as a rail for superconducting vortices, the researchers have revealed an unexpectedly powerful method for directing quantum motion. While practical technologies may still lie in the future, the work provides fresh insight into how the smallest structural features can be harnessed to control some of the most intriguing phenomena in condensed-matter physics.