A research team led by Associate Professor Shinji Kawasaki together with Professor Guo-qing Zheng of the Department of Physics at Okayama University in Japan has now offered a resolution to this puzzle, and the answer is as elegant as it is surprising. Working with high-quality single crystals of CsV3Sb5, the team applied carefully controlled uniaxial strain—stretching the crystal along a single crystallographic direction—while simultaneously measuring its superconducting transition and local electronic properties through nuclear quadrupole resonance. The strain was delivered by a custom-built, piezoelectric-driven strain cell, a delicate apparatus that allows mechanical deformation to be applied in situ at low temperatures without removing the sample from the measurement environment. Their findings, published in Volume 137, Issue 9 of Physical Review Letters on August 28, 2026, indicate that the material does not host one superconducting state but two, and that mechanical strain can pull those two states apart.

The central result is striking in its simplicity. When the researchers stretched the crystal, rather than compressing it, the superconducting transition temperature rose substantially. At zero strain, superconductivity set in at about 3.0 kelvin in their samples. Under tensile strain of +0.90 percent, that onset climbed to 3.6 kelvin—a meaningful enhancement in a material where every tenth of a kelvin counts. What makes this observation remarkable, however, is not the increase itself but what did not change alongside it. The charge density wave, the ordered electronic state that forms at 94 kelvin and competes for the same electrons that superconductivity needs, remained essentially untouched by the strain. Superconductivity, in other words, could be tuned independently of the charge order that has dominated discussions of this material since its discovery.

This decoupling is what sets the Okayama result apart from earlier pressure studies. Hydrostatic pressure, the traditional tool for tuning quantum materials, squeezes a crystal equally in all directions and tends to modify superconductivity largely through its effect on the charge density wave, entangling the two phenomena in any interpretation. Uniaxial strain breaks that entanglement. As Professor Kawasaki put it, strain gives researchers an independent control knob in this material—it enhances superconductivity without changing the bulk charge density wave. That selectivity means the superconducting pairing channel itself can be strengthened while the competing background order remains robust, allowing each ingredient to be studied on its own terms for the first time in this compound.

Even more consequential was what the team found when they examined the superconducting state under the largest tensile strain. The material did not simply become a better superconductor; it underwent two distinct superconducting transitions. The first, at 3.6 kelvin, was associated with a nodal superconducting state, in which the energy gap has zeros and the pairing is unconventional. The second, at 3.0 kelvin, displayed a nodeless state, the hallmark of more conventional pairing. Two superconducting states that are nearly degenerate under ambient conditions—so close in energy and temperature that most experiments see only their blurred superposition—can be separated cleanly when strain is applied. The implication for the field is immediate: the conflicting results that have divided researchers studying CsV3Sb5 may not reflect experimental error at all, but rather the genuine coexistence of two competing pairing states that different techniques weighted differently.

The nuclear quadrupole resonance measurements provided the local, atomic-scale view needed to quantify this separation. Unlike bulk probes that average over the entire sample, nuclear quadrupole resonance is sensitive to the electronic environment surrounding specific nuclei, allowing the researchers to track how the superconducting gap behaves at the level of individual atoms and to distinguish nodal from nodeless character. Through this lens, the team found that the unconventional nodal component of the superconductivity grows dramatically stronger under tensile strain, increasing from a relatively modest contribution of about 10 percent at zero strain to roughly 26 percent at +0.90 percent strain. Stretching the crystal does not merely reveal the hidden nodal state; it actively nourishes it, shifting the balance between the two pairing channels in a controllable way.

For years, different measurements of CsV3Sb5 have pointed toward seemingly different superconducting states, Professor Kawasaki observed, and the new results show that these states can coexist and that uniaxial strain can separate them, giving researchers a direct way to study each state individually. That framing transforms the compound from a source of frustration into an opportunity. A material in which two superconducting channels compete, and in which an external knob can tune their relative strength, is an ideal laboratory for testing theories of unconventional pairing. Theories that predict how lattice geometry, electronic correlations, and multiband structure conspire to produce superconductivity can now be confronted with a system in which the geometry itself is being deliberately deformed.

The broader significance of the work extends well beyond this single compound. Kagome metals like CsV3Sb5 sit at a fascinating intersection of condensed matter physics: their lattice geometry naturally promotes exotic electronic phenomena, including charge density waves, correlated electron behavior, and potentially exotic superconducting states, while their layered structure makes them amenable to the kind of strain engineering the Okayama team employed. The researchers argue that the strain-based approach could serve as a general strategy for separating superconductivity from competing orders—such as charge density waves or spin density waves—in other unconventional superconductors, including iron-based superconductors and heavy-fermion systems. In each of these material families, the entanglement of superconductivity with neighboring ordered phases has historically made it difficult to identify what actually binds electrons into Cooper pairs. A technique that can disentangle those ingredients without destroying either one is a genuinely valuable addition to the experimental toolkit.

It is worth being clear about the practical horizon. CsV3Sb5 itself, with a superconducting transition temperature of a few kelvin, is far too cold to power a grid or an MRI machine, and no result from this study changes that. The value of the work lies in understanding rather than application, at least for now. Knowing that two pairing states coexist and compete in a kagome metal, and that their balance can be shifted by mechanical deformation, provides exactly the kind of mechanistic insight that has historically preceded progress toward higher-temperature superconductors. Lossless power transmission, high-field medical and industrial magnets, and quantum computing technologies all depend on superconductors whose performance is limited by transition temperature and by our incomplete grasp of pairing physics. Every material in which the rules of unconventional superconductivity can be isolated and tested brings the field a step closer to that goal.

The Okayama study thus closes one chapter and opens another in the story of CsV3Sb5. The long-standing debate over whether the material’s superconducting gap is conventional or nodal now has a coherent answer: it is both, with a nodeless state and a nodal state coexisting nearly degenerately until strain pulls them apart. The experimental achievement—applying precise uniaxial strain to fragile single crystals at cryogenic temperatures while performing nuclear quadrupole resonance—demonstrates a level of control that few laboratories have matched. As researchers across the field adopt and adapt strain-tuning techniques, the ability to switch between, and selectively strengthen, competing superconducting states may prove to be one of the most productive tools yet developed for interrogating the quantum materials whose secrets have proved most resistant to ordinary measurement. For a compound that has divided opinion for years, the verdict is unexpectedly unifying: the disagreements were real, and now they can be measured.

Subject of Research: Strain-tuned competing superconducting states in the charge-ordered kagome metal CsV3Sb5

Article Title: Stretching a quantum material uncovers competing superconducting states

Article References: Stretching a quantum material uncovers competing superconducting states. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: kagome metal, CsV3Sb5, superconductivity, uniaxial strain, charge density wave, nuclear quadrupole resonance, nodal gap, nodeless gap, unconventional superconductors, Physical Review Letters, quantum materials, Okayama University

The metaphor that the researchers themselves chose is memorable. Eduardo Fradkin, a physics professor at Illinois Grainger Engineering and a co-lead of the project, compared the phenomenon to the grin of the Cheshire Cat from Alice in Wonderland: the cat disappears, but its grin lingers in the air. In the same way, the pair density wave is the vestige of superconductivity that remains once the phase itself has faded. In a conventional superconductor, Cooper pairs appear only when the full superconducting transition takes place. Here, the experiments show that the pairs are already organized into spatially modulated patterns before the material ever enters its superconducting phase — a scenario that theorists proposed two decades ago but that had never been directly confirmed in a real material.

To appreciate why this matters, it helps to revisit the basic physics of superconductivity. When certain metals are cooled below a critical temperature, their free electrons condense into a collective low-energy quantum state that conducts electricity with zero resistance. But electrons are fermions, and quantum mechanics forbids identical fermions from occupying the same state. The way around this prohibition was explained in 1957 by Illinois physicists John Bardeen, Leon Cooper and Robert Schrieffer in the celebrated BCS theory. Each electron binds weakly to a partner through interactions mediated by the crystal lattice, forming a Cooper pair. Unlike single electrons, these pairs behave as bosons, particles that can happily pile into one shared quantum state. That bosonic condensation is the essence of superconductivity.

BCS theory accounted beautifully for every superconductor known until 1986, when the first unconventional superconductors appeared. These materials violate the assumptions of BCS theory, yet their electrons still pair up and condense, and understanding how remains one of the central open problems in the field. A recurring clue is that unconventional superconductors tend to host other ordered phases below their critical temperature. One common companion is the charge density wave, in which a fraction of the electrons arranges itself into a periodic spatial pattern, producing alternating regions of higher and lower electric charge. In 2007, Fradkin and his colleagues went a step further and theorized a more exotic companion: a pair density wave, in which the superconducting Cooper pairs themselves — normally spread uniformly through the metal — organize into a nonuniform, wave-like pattern.

One of the most striking predictions of that 2007 theory was that pair density waves should be able to exist above the superconducting critical temperature, implying that Cooper pairs can form and order even while the metal as a whole is not superconducting. Fradkin admits the idea was bold. The state is peculiar, he noted, and although there had been experimental hints over the years, no direct confirmation of the phase’s existence had been achieved. Part of the difficulty is diagnostic: as Julian May-Mann, a former Illinois graduate student who performed the study’s theoretical analysis, explained, pair density waves behave like conventional superconductors in some experiments and like charge density waves in others, so confirming one requires both high-quality data and careful theoretical interpretation.

The material that finally revealed the state is uranium ditelluride, which until 2019 was regarded as an ordinary metal. That year, researchers discovered a superconducting phase below 2 kelvins, and subsequent studies led physicists to suspect that uranium ditelluride is an elusive triplet-pair superconductor — one in which the paired electrons carry magnetic moments, unlike the spin-canceling pairs of BCS superconductors. The only confirmed triplet-pair super-phase in nature is superfluid helium-3, a system studied extensively by the late Illinois physicist Anthony Leggett, who won the Nobel Prize for that work. Fradkin cautions that the question is not completely settled, but he notes that the consensus is that uranium ditelluride belongs to this rare class.

The experimental thread began in the laboratory of Vidya Madhavan, a physics professor and department head at Illinois Grainger Engineering and the project’s other co-lead. Her group had identified charge density waves in uranium ditelluride using scanning tunneling microscopy, but they noticed something deeply strange: the waves could be destroyed by magnetic fields. A charge density wave is simply a collective electronic state modulated in space, and there is no reason for it to respond to magnetic fields, let alone be wiped out by them. When the experimentalists brought the data to Fradkin and his students, the theorists predicted that such behavior could occur if a pair density wave were also present in the system — and, as Madhavan put it, pair density waves were the best explanation available for such an unusual response.

Proving it required overcoming a formidable experimental obstacle: pair density waves are delicate and form only in highly regular crystals, and impurities in earlier samples had obscured the signals entirely, like trying to spot a light in a cloud of fog. Collaborators supplied higher-quality uranium ditelluride crystals grown with a new molten flux method, and the team examined them with a vector magnetic field scanning tunneling microscope, an instrument that probes how material surfaces respond to magnetic fields applied in arbitrary directions. That capability proved essential because uranium ditelluride is strongly anisotropic — its superconducting upper critical field depends heavily on direction — so the researchers needed to sweep the field’s magnitude and orientation systematically while varying temperature. Zhen Zhu, a postdoctoral research associate who carried out the experiments, emphasized that tracking how the observed modes evolved under these combined controls built confidence that the behavior was intrinsic to the material rather than an artifact.

The payoff came in the form of modes that responded to temperature and magnetic field exactly as pair density waves should, including their destruction under magnetic effects. Most tellingly, some of these modes continued to exist above the critical temperature, in the temperature range where the main superconducting phase no longer existed. May-Mann pointed out that foundational principles of condensed matter physics constrain how different phases can appear and disappear as temperature changes or fields are applied, and any explanation based on a charge density wave alone clashes with those principles. The pair density wave interpretation, by contrast, provides a consistent account of everything the team observed. In Fradkin’s framing, the pairs are formed beforehand, in a different state, and the pair density wave is simply the grin that outlasts the cat.

There are caveats, and the researchers are candid about them. Scanning tunneling microscopy characterizes only a material’s surface, and it is possible for the interior of a crystal to behave differently from its boundary. Even so, Fradkin argues that the surface results give a very strong hint at what is happening inside the bulk, and the team is optimistic that the finding opens new research directions, both for confirming pair density waves in other unconventional superconductors and for exploring how triplet pairing reshapes the landscape of quantum phases. For Zhu, the satisfaction lies in the convergence: the temperature dependence, the magnetic-field dependence and the improved sample quality all told the same story at once. The work was supported by the U.S. Department of Energy’s Office of Science, and its contributors include Yudi Huang, Kaiming Liu, Zheyu Wu, Shanta Saha, Johnpierre Paglione, Alexander Eaton, Andrej Cabala and Michal Vališka. What began as a theoretical proposal sketched on a limb twenty years ago now stands as measured fact — and the grin, it turns out, was there all along, waiting for instruments sharp enough to see it.

Subject of Research: Pair density wave order and Cooper pairing above the superconducting critical temperature in uranium ditelluride

Article Title: The Cheshire Cat’s grin: Cooper pairs found above superconducting critical temperature in pair density waves

Article References: The Cheshire Cat’s grin: Cooper pairs found above superconducting critical temperature in pair density waves. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: superconductivity, Cooper pairs, pair density wave, uranium ditelluride, unconventional superconductors, charge density wave, scanning tunneling microscopy, triplet pairing, BCS theory, condensed matter physics, critical temperature, quantum materials