Superconducting Vortex Experiments Confirm Clogging Is a Nonequilibrium Phase Transition

Physicists studying superconducting vortices driven by direct electrical currents have produced the first experimental proof that clogging behaves as a nonequilibrium phase transition. The investigation, conducted by researchers at the Institute of Science Tokyo and published in Communications Physics, demonstrates that the physical transition from continuous flow to complete obstruction shares critical dynamics with vortex depinning. While earlier numerical simulations and theoretical models had predicted that clogging constitutes a nonequilibrium phase transition characterized by diverging relaxation times near a critical threshold, direct laboratory confirmation had remained unverified across physical systems until this experimental demonstration.

Clogging represents a ubiquitous transport phenomenon that occurs whenever moving particles encounter structural bottlenecks that restrict passage, progressively slowing motion until flow entirely ceases. This phenomenon is observed across a wide range of systems, including granular materials, filtration devices, industrial pipelines, and biological transport networks. Despite its widespread incidence, the physical mechanisms governing the transition from continuous flow to complete clogging remain incompletely understood.

To investigate these underlying dynamics, a research team led by former graduate student Takahide Minemura and Professor Emeritus Satoshi Okuma in the Department of Physics at the Institute of Science Tokyo analyzed vortex motion within amorphous molybdenum-germanium thin films. In these disordered thin films, intrinsic material defects serve as pinning sites capable of capturing passing vortices under an applied direct current. When individual vortices become immobilized at these pinning sites, they create microscopic bottlenecks that subsequently obstruct the passage of trailing vortices.

Distinguishing clogging from conventional pinning required the development of a modified two-step measurement protocol. The researchers applied successive direct current drives oriented in either parallel or opposing directions through the amorphous molybdenum-germanium films while recording the resulting time-dependent voltage signals. By monitoring how voltage evolved over time, the team successfully tracked vortex motion and separated the rapid relaxation process characteristic of collective clogging from the distinctly slower relaxation process associated with conventional defect pinning.

The results revealed that both relaxation times increased greatly and adhered to power-law divergence as the driving current approached a critical value. Furthermore, the critical driving current required to trigger clogging coincided with the critical vortex depinning current within experimental uncertainty. The measured critical exponents governing these scaling behaviors aligned with theoretical predictions established for the two-dimensional directed-percolation universality class, a recognized class of nonequilibrium absorbing phase transitions. These quantitative observations confirm that while clogging and depinning constitute distinct physical processes with separate relaxation rates, they share an identical critical point and exhibit unified critical scaling behavior.

Senior researcher Satoshi Okuma noted that unravelling the transition between steady flow and complete stoppage addresses long-standing challenges across fundamental physics and applied technology, where obstruction can compromise industrial plants, public infrastructure pipelines, and vascular circulation networks. Future investigations may evaluate how these critical dynamics change across alternative material platforms and transport environments, providing systematic principles to predict, mitigate, and control blockages across natural and engineered transport networks.

Previous studies predicted that this transition represents a nonequilibrium phase transition, in which the time required to reach a stable flowing or completely blocked state diverges near a critical point. Okuma explained that although it had been predicted in simulation that the transition from a flowing state to a completely clogged state represents a nonequilibrium phase transition, experimental verification had remained elusive, which was the main motivation of the study.

Beyond superconductivity, the findings provide a foundation for understanding clogging across diverse systems. In Okuma's words, the findings may help guide future studies of flow obstruction in granular materials, colloids, active matter, and other systems in which collective motion can become blocked. In the future, further research could explore how these critical dynamics vary across different materials and transport environments to support more effective strategies for anticipating and controlling clogging in industrial processes, infrastructure, and biological transport systems.

The experimental study, titled “Clogging transition for vortices driven by dc current,” appeared in Communications Physics on September 25, 2026. The work was carried out by researchers in the Department of Physics at the Institute of Science Tokyo, an institution established on October 1, 2024, following the merger of Tokyo Medical and Dental University and Tokyo Institute of Technology under the mission of advancing science and human wellbeing. The authors declared no competing interests.

Subject of Research: Experimental demonstration of clogging as a nonequilibrium phase transition in superconducting vortices

Article Title: Deciphering clogging dynamics

Article References: Deciphering clogging dynamics. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: superconducting vortices, clogging transition, nonequilibrium phase transition, vortex depinning, molybdenum-germanium thin films, critical dynamics, directed percolation, absorbing phase transition, two-step measurement protocol, relaxation times, power-law divergence, Institute of Science Tokyo, Communications Physics, Satoshi Okuma, Takahide Minemura, direct current drive