Materials in a superconducting state are of great interest to scientists. This is due to rare properties (mainly the ability to conduct electricity with zero resistance, without heat loss) that promise to transform computing, energy networks and transportation. This phenomenon occurs when free electrons condense into a low-energy quantum state at very cold temperatures, pairing up into what are called Cooper pairs so that they can pass through materials without interference. In 2007, scientists predicted the existence of a more exotic type of electron motion, called pair density wave (PDW), where Cooper pairs form specific shapes. patterns as it moves, like ripples in sand. Now, a team led by researchers at the University of Illinois at Urbana-Champaign has found compelling evidence of PDW on the surface of the metal uranium ditelluride, and their results have been published in PNAS. Additionally, the patterns were found to persist outside the standard superconducting state, something that has never been seen before. “Pair density waves are the Cheshire cat’s smile of superconductivity,” says physicist Eduardo Fradkin of the University of Illinois at Urbana-Champaign. “They are the vestige that remains once the phase itself has disappeared.” In conventional superconductors, Cooper pairs form when the full phase transition occurs, but in this system, their observation in PDWs above the transition point shows that they formed beforehand in a different state. Uranium ditellide becomes a superconductor below about 2.1 Kelvin (around -271 °C or -456 °F) and is of particular interest to researchers as it is believed to be a rare ‘spin triplet’ superconductor, in which the paired electrons also have a small magnetic orientation. Signs of PDW have previously been detected in the metal, and here the researchers took a closer look using a custom-made, high-quality form of uranium ditellide. probed using a vector magnetic field scanning scanning microscope (STM), which can measure magnetic responses in multiple and 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 of the University of Illinois at Urbana-Champaign. “By systematically varying both the magnitude and direction of the field, along with temperature, we were able to track how these modes evolved and build confidence that the behavior we observed was intrinsic.” The researchers added theoretical models to explain the charging patterns they were seeing below the critical temperature required for superconductivity, and also at a point 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 of the University of Illinois at Urbana-Champaign. “We even show that the modes persist above the temperature at which superconductivity disappears, a revealing theoretical prediction that has not been convincingly observed until now.” frameborder=”0″ enable=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” enablefullscreen> This does not mean that bulk superconductivity is occurring in uranium ditelluride at higher temperatures, but it does help physicists understand how Cooper pairs work and how they can exist outside the conventional superconducting state. It could also help researchers better understand the physics needed to develop superconductors at higher temperatures. These advances could eventually make these materials more usable. There is Another electron wave state called charge density waves (CDWs), where some electrons adopt non-uniform patterns. Understanding how these waves are connected to PDWs will be crucial to interpreting experimental results in the future. “PDWs are difficult to analyze in real materials, because they behave like conventional superconductors in some experiments, and like CDWs in [others]” says physicist Julian May-Mann of Stanford University. “Confirming the existence of a PDW requires both high-quality experimental data and careful theoretical analysis.” The research has been published in PNAS. This article was fact-checked by Peter Dockrill and edited by Peter Dockrill. While we take pride in our process, we are human. If you spot an error, please let us know.
