Superconductivity is supposed to be an all-or-nothing quantum state. Cool a material below a critical temperature, and electrons pair up and enter a state that lets electricity flow without resistance. Warm it back up, and that collective state disappears.\u00a0<\/p>\n
However, experiments on uranium ditelluride suggest that part of the superconducting behavior can survive after the main phase is gone.<\/p>\n
Physicists at the University of Illinois Urbana-Champaign (UIUC) have found evidence that Cooper pairs<\/a>\u2014the paired electrons behind superconductivity\u2014can arrange themselves into a repeating pattern called a pair density wave (PDW).\u00a0<\/p>\n What\u2019s even more interesting is that the pattern persists above the temperature where uranium ditelluride stops being superconducting. These findings provide the first direct evidence that a PDW can exist independently of the main superconducting phase.<\/p>\n \u201cWe were out on a limb when we first suggested it. It\u2019s a very peculiar state, and, although there have been experimental hints, there has been no direct confirmation of the phase\u2019s existence,\u201d Eduardo Fradikin, one of the researchers and a physics professor at UIUC, said<\/a>.<\/p>\n Finding the hidden state<\/p>\n In conventional superconductors<\/a>, electrons form Cooper pairs and those pairs condense into one collective quantum state. Unconventional superconductors can behave differently and host additional electronic phases, including charge density waves (CDWs), where electric charge varies periodically across the material.<\/p>\n PDWs are similar in that they form a spatial pattern, but the quantity being modulated is the density of Cooper pairs. This idea was proposed in 2007, including the striking possibility that these paired electrons could exist above the superconducting transition.\u00a0<\/p>\n However, \u201cPDWs are tricky to analyze in real materials, because they behave like conventional superconductors in some experiments, and like CDWs in other,\u201d Julian May-Mann, one of the study authors and a former graduate student from UIUC, said.<\/p>\n Uranium ditelluride, or UTe\u2082<\/a>, offered an unusual place to look. Once considered an ordinary metal, it was found in 2019 to become superconducting below about 2 kelvins. It is also widely considered a candidate for an unconventional triplet-pair superconductor, in which the paired electrons can carry magnetic moments.<\/p>\n The researchers had previously detected CDWs on the material\u2019s surface using scanning tunneling microscopy, but something did not fit, as magnetic fields could destroy the waves. A normal CDW should not respond so strongly to a magnetic field, prompting the researchers to consider whether a PDW was hiding alongside it.<\/p>\n Better crystals reveal the signal<\/p>\n The researchers first needed cleaner UTe\u2082 crystals because PDWs are extremely delicate and impurities can obscure their signatures. So they produced higher-quality samples using a new molten-flux growth method.<\/p>\n \u201cThanks 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,\u201d Vidya Madhavan, one of the lead researchers and a physics professor at UIUC, said.<\/p>\n They examined the crystals with a vector magnetic-field scanning tunneling microscope. Unlike a conventional setup, the instrument could vary both the strength and direction of the magnetic field\u2014important because UTe\u2082 is anisotropic, meaning its properties depend strongly on direction.<\/p>\n The measurements revealed electronic modes whose responses to temperature and magnetic fields matched theoretical expectations for a PDW, including modes that survived above the superconducting<\/a> critical temperature.<\/p>\n \u201cWe even showed that the modes persist above the temperature at which superconductivity disappears, a telltale theoretical prediction that has not been convincingly observed before now,\u201d Madhavan added.<\/p>\n The researchers also found that a CDW-only explanation conflicted with established principles governing how electronic phases should respond to temperature and magnetic fields. The PDW interpretation provided a more consistent explanation of the observations.<\/p>\n A clue, but not the final answer<\/p>\n The result could reshape how physicists think about unconventional superconductors by suggesting that Cooper pairing can begin before the material develops its full superconducting state.\u00a0<\/p>\n This could help researchers understand why some unconventional materials develop complex phases and potentially guide the search for new superconducting states<\/a>.<\/p>\n \u201cPair density waves are the Cheshire Cat\u2019s grin of superconductivity. 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 are formed beforehand in a different state,\u201d Fradikin explained.<\/p>\n However, there is an important limitation as well. Scanning tunneling microscopy probes the material\u2019s surface, so the researchers cannot yet prove that the same PDW exists throughout the interior.<\/p>\n The next challenge is therefore to determine whether the hidden state extends into the bulk of UTe\u2082 and to establish its nature more firmly.\u00a0<\/p>\n