Imagine flipping a dial to make a material behave like a perfect conductor, a complete blocker, or something intriguingly in-between that could help us to better understand the puzzling quantum world. That’s the idea behind a new study in Physical Review Letters, co-authored by FZU researcher Filip Křížek within an international collaboration.
Superconductors – materials that carry electrical current with zero resistance below a certain temperature – are becoming central to next-generation electronics, sensing, and quantum computing. Yet, as research has matured, scientists have discovered a richer landscape of quantum phases hiding around the superconducting state. Probing these phases in real, complex materials can be difficult, and purely “on/off” superconductivity isn’t always the most convenient knob for experiments.
To gain finer control, researchers turn to Josephson junction arrays – grids of tiny superconducting islands separated by ultrathin barriers. These engineered systems can emulate quantum states that are otherwise hard to access and even harder to model. Historically, however, their flexibility has been limited by how much you can tune them once they’re built.
In the new work, the team created a hybrid semiconductor–superconductor lattice – think of it as an electronic switchboard – that combines the perfect flow of superconductors with the natural tunability of semiconductors. This architecture lets the researchers “rewire” how the islands talk to each other in real time, guiding the system smoothly among superconductor–insulator, superconductor–metal, and metal–insulator regimes. In practical terms, they can set not just whether electricity flows, but how it flows.
One especially captivating stop on this dial is the anomalous metal phase – a paradoxical state that conducts like a metal yet resists both the total lock-up of an insulator and the perfect glide of a superconductor, even near absolute zero. It’s a proving ground for quantum fluctuations and disorder, and a potential pathway to quantum devices that are both powerful and robust.
“This novel device concept allows us to tune the system into the anomalous metal phase, an exotic regime where current flows without becoming perfectly lossless or shutting off – a stable, low-temperature ‘middle ground’ shaped by quantum fluctuations,” says Filip Křížek, researcher at the Department of Spintronics and Nanoelectronics at Cukrovarnická. “Being able to dial in this behavior on demand gives us a clean, controllable platform for quantum simulation to test theories and, ultimately, to engineer more dependable quantum components. I am glad that we could use our local know-how in material development to enable such an interesting study.”
By turning puzzling phases into accessible, tunable settings, the study opens a practical route to explore – and exploit – quantum matter with unprecedented precision.
Sasmal, S., Efthymiou-Tsironi, M., Nagda, G., Fugl, E., Olsen, L. L., Krizek, F., ... & Vaitiekėnas, S. (2025). Voltage-tuned anomalous-metal to metal transition in hybrid Josephson junction arrays. Phys. Rev. Lett. 135, 156301. DOI: https://doi.org/10.1103/xbm4-37cf