In a world where electrons usually collide and lose energy, superconductors allow them to move without resistance. But in uranium ditelluride, scientists have discovered something unexpected: ordered waves of electron pairs persist even after the material loses its superconducting state.
Superconductivity arises at very low temperatures, when electrons form Cooper pairs and pass through the crystal lattice without losses. In ordinary materials, these pairs appear strictly at a phase transition. However, in uranium ditelluride, which becomes a superconductor below 2,1 kelvin, Cooper pairs form repeating spatial patterns — pair-density waves.
Such waves were predicted back in 2007, but there was no direct evidence of their existence outside the superconducting phase. Researchers from the University of Illinois at Urbana-Champaign used a scanning tunneling microscope with a vector magnetic field and high-quality samples of the material. They recorded characteristic electronic modes that respond to temperature and magnetic field exactly as pair-density wave theory predicts.
Most surprising of all: these modes persist at temperatures noticeably above the critical point of superconductivity. Physicist Eduardo Fradkin compared them to the Cheshire Cat's smile — traces that remain after the phase itself has vanished. This does not mean that the material remains a superconductor at higher temperatures, but it shows that Cooper pairs can exist in an organized form independently of the ground state.
Uranium ditelluride is also interesting because it is considered a rare spin-triplet superconductor: the electron pairs in it possess their own magnetic orientation. Observing pair-density waves helps to understand exactly how such unusual pairs form and why they behave differently from those in classical superconductors.
The discovery brings physicists closer to unraveling the mechanisms of unconventional superconductivity and may point the way toward creating materials that work at higher temperatures. Understanding these intermediate states is important for future technologies — from energy-efficient grids to quantum computers.
Pair-density waves show that superconductivity is not simply a matter of switching on or off, but a complex process with intermediate stages that can now be studied directly.
