Physicists just found the ‘ghost’ of superconductivity
Physicists have found striking evidence for an unusual form of superconducting behavior in uranium ditelluride, where electron pairs organize into rippling patterns called pair density waves. Even more surprisingly, these patterns appear to survive after ordinary superconductivity disappears, suggesting that the electron pairs responsible for superconductivity may begin forming before the material actually becomes…
Key points
- Physicists at the University of Illinois Urbana-Champaign found evidence that pair density waves in uranium ditelluride persist after the material's superconducting phase disappears.
- The research, published in the Proceedings of the National Academy of Sciences, offers the first direct evidence of this behavior in uranium ditelluride.
- Higher-quality samples grown with a molten flux method and a vector magnetic field scanning tunneling microscope were key to the observations.
Physicists at the University of Illinois Urbana-Champaign's Grainger College of Engineering have found evidence of an unusual superconducting behavior in uranium ditelluride. Their experiments show that Cooper pairs, the paired electrons behind superconductivity, can form uneven patterns called pair density waves (PDWs) even after the material leaves its main superconducting state.
The research, published in the Proceedings of the National Academy of Sciences, provides the first direct evidence that PDWs in uranium ditelluride can remain in the material's ordinary phase after superconductivity disappears. Scientists first predicted PDWs about 20 years ago, and earlier studies observed them alongside superconductivity in other metals.
What the researchers said
Eduardo Fradkin, a physics professor and project co-lead, described pair density waves as "the Cheshire Cat's grin of superconductivity," calling them a vestige that remains once the phase itself is gone. He said that in conventional superconductors Cooper pairs form at the full phase transition, but in this system their observation above the transition point shows they form beforehand in a different state.
Vidya Madhavan, the other project co-lead, said new methods for growing higher-quality samples allowed the team to observe spectral signatures that respond to temperature and magnetic fields as pair density waves should. She said the modes persist above the temperature at which superconductivity disappears, a theoretical prediction not convincingly observed before.
Pair density waves are the Cheshire Cat's grin of superconductivity.
— Eduardo Fradkin
How the finding was made
Uranium ditelluride was generally considered an ordinary metal until 2019, when researchers found it enters a superconducting phase below 2 kelvins. Madhavan's group used scanning tunneling microscopy to identify charge density waves (CDWs) in the material, but found magnetic fields could destroy them, which a CDW alone could not explain.
Collaborators supplied cleaner samples made with a new molten flux growth method. The team examined them with a vector magnetic field scanning tunneling microscope, varying the field's magnitude and direction along with temperature. Some modes continued to exist after the material passed its critical temperature.
The researchers note a limitation: STM imaging only measures effects at a material's surface, so the experiment does not directly reveal the interior. Fradkin said the results still give a strong hint at what is happening inside.
