Researchers at the University of Illinois at Urbana-Champaign have discovered compelling evidence of pair density waves on the surface of uranium ditelluride. Published in PNAS, the findings reveal these repeating electron patterns persisting above the standard superconducting state, a phenomenon never seen before.
Uncovering Pair Density Waves on Uranium Ditelluride
Scientists predicted the existence of pair density waves, or PDWs, back in 2007. In this exotic state of electron movement, Cooper pairs form specific patterns while moving, resembling ripples on sand. Now, a research team led by the University of Illinois at Urbana-Champaign has spotted these patterns on the surface of the metal uranium ditelluride, with their results published in PNAS.
Uranium ditelluride becomes a superconductor below approximately 2.1 Kelvin, which is about -271 degrees Celsius or -456 degrees Fahrenheit. Researchers are particularly interested in the metal because it is thought to be a rare spin-triplet superconductor, meaning its paired electrons carry a tiny magnetic orientation.
“Pair density waves are the Cheshire Cat’s grin of superconductivity,” says physicist Eduardo Fradkin from the University of Illinois at Urbana-Champaign. “They are the vestige that remains once the phase itself has disappeared.”
Eduardo Fradkin, physicist at the University of Illinois at Urbana-Champaign
Advanced Vector-Magnet Equipment and Temperature Tracking
While signs of PDWs had been detected previously in the metal, the research team took a closer look by utilizing a custom-made, high-quality form of uranium ditelluride. The material was probed using a vector magnetic field scanning tunneling microscope, an instrument capable of measuring magnetic responses in multiple, arbitrary directions.
“A key advantage of our experiment was the newly developed vector-magnet equipment, which provides an unusually large magnetic-field range along multiple directions,” says physicist Zhen Zhu from the University of Illinois at Urbana-Champaign. “By systematically varying both the magnitude and direction of the field, together with temperature, we could track how these modes evolved and build confidence that the behavior we observed was intrinsic.”
Zhen Zhu, physicist at the University of Illinois at Urbana-Champaign
The researchers combined theoretical models to explain the charge patterns they observed both below the critical temperature required for superconductivity and significantly above it.
“Thanks to new methods for growing higher-quality samples, we were able to observe spectral signatures that respond to temperature and magnetic fields exactly as pair density waves should,” says physicist Vidya Madhavan from the University of Illinois at Urbana-Champaign. “We even showed that the modes persist above the temperature at which superconductivity disappears, a tell-tale theoretical prediction that has not been convincingly observed before now.”
Vidya Madhavan, physicist at the University of Illinois at Urbana-Champaign
Implications for Future Superconductor Physics
This discovery does not mean bulk superconductivity occurs in uranium ditelluride at higher temperatures. Instead, it provides physicists with deeper insight into how Cooper pairs function and how they can exist outside of a conventional superconducting state.
Understanding these mechanisms could eventually help researchers figure out the physics required to develop superconductors operating at higher temperatures, potentially making the materials more widely usable in computing, energy grids, and transport.
Researchers must also account for charge density waves, another electron ripple state where some electrons adopt non-uniform patterns. According to Julian May-Mann from Stanford University, understanding how these waves connect to PDWs will remain crucial for interpreting future experimental results because PDWs behave like conventional superconductors in some experiments and like charge density waves in others.