Stretching a Quantum Material Splits Its Superconductivity in Two

In the crowded landscape of quantum materials, few compounds have generated as much confusion—and as much excitement—as the kagome metal cesium vanadium antimonide, known to researchers as CsV3Sb5. This layered crystal, whose vanadium atoms sit on a geometrically frustrated lattice of corner-sharing triangles, develops a charge density wave at around 94 kelvin and then slips into a superconducting state at roughly 2.5 kelvin. For years, experiments probing the nature of that superconductivity have returned contradictory answers: some measurements suggested a conventional, fully opened energy gap, while others detected nodes, points or lines where the superconducting gap vanishes entirely. That disagreement matters far more than an academic squabble might suggest, because the shape of the superconducting gap is a fingerprint of the pairing mechanism, and the pairing mechanism is precisely what researchers need to understand if they hope to design superconductors that work at higher, more useful temperatures.

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