Unconventional superconductors can host unusual electronic states, but understanding what drives their superconductivity becomes difficult when different forms of order coexist. The kagome metal CsV3Sb5 has become a particularly debated example. It develops charge density wave order at about 94 K before becoming superconducting at about 2.5 K, yet experiments have offered conflicting views of whether its superconducting gap is conventional or contains nodes. Resolving this question is important because knowing how superconductivity forms can guide the search for better superconducting materials.
Addressing this challenge, a research team led by Associate Professor Shinji Kawasaki, along with Professor Guo-qing Zheng, from the Department of Physics, Okayama University, Japan, investigated CsV3Sb5 using in situ uniaxial strain and nuclear quadrupole resonance measurements. High-quality single crystals were strained along one crystallographic direction using a custom piezoelectric-driven strain cell, while the researchers monitored the superconducting transition and local electronic properties. The study was published in Volume 137, Issue 9 of the journal Physical Review Letters on August 28, 2026.
The team found that stretching the crystal, rather than compressing it, substantially increased the superconducting transition temperature. At zero strain, the transition began at about 3.0 K, while tensile strain of +0.90% raised it to 3.6 K. Remarkably, the charge density wave remained essentially unchanged under strain, showing that superconductivity could be tuned independently of the material's existing charge order. This decoupling offers researchers a new way to study the two phenomena separately.
A closer look at the superconducting state revealed an even more striking result. Under the largest tensile strain, the material underwent two superconducting transitions. The first occurred at 3.6 K and was associated with a nodal superconducting state, while a second transition appeared at 3.0 K and showed a nodeless state. The findings indicate that two distinct superconducting states, which are nearly degenerate under ambient conditions, can separate when strain is applied, helping explain why previous experiments reached different conclusions about the material's superconducting nature.
"For years, different measurements of CsV3Sb5 have pointed toward seemingly different superconducting states," says Prof. Kawasaki. "Our results show that these states can coexist and that uniaxial strain can separate them, giving us a direct way to study each state." The researchers further found that the unconventional nodal component becomes much stronger under tensile strain, increasing from a relatively small contribution of 10% at zero strain to about 26% at +0.90% strain.
"Strain gives us an independent control knob in this material—it enhances superconductivity without changing the bulk charge density wave," says Prof. Kawasaki. This distinction is important because hydrostatic pressure changes superconductivity largely through its effect on charge order. The results instead show that uniaxial strain can selectively strengthen a superconducting pairing channel while leaving the competing background state robust. The approach could therefore be useful beyond CsV3Sb5, providing a general strategy for separating superconductivity from competing orders such as charge or spin density waves in other unconventional superconductors, including iron-based and heavy-fermion systems.
Overall, the study resolves an important piece of the long-standing debate surrounding superconductivity in CsV3Sb5 by showing evidence for two competing pairing states: a nodeless state and a nodal state. Although CsV3Sb5 itself has a transition temperature too low for practical applications, the experimental strategy provides a broader pathway for understanding and tuning quantum materials. Such knowledge could ultimately support the development of higher-temperature superconductors for lossless power transmission, medical and industrial magnets, and quantum computing technologies.
