In the world of quantum materials, few compounds have stirred as much debate as the kagome metal CsV3Sb5. This layered crystal, with its vanadium atoms arranged on a lattice of triangles, forms a charge density wave when cooled to 94 kelvins, and at 2,5–3 kelvins it transitions into a superconducting state. Years of experiments produced contradictory results: some measurements pointed to conventional superconductivity with a fully open energy gap, while others indicated nodes where the gap vanishes.
Researchers from Okayama University, led by Associate Professor Shinji Kawasaki and Professor Guo-qing Zheng, applied uniaxial strain to CsV3Sb5 crystals. Using a piezoelectric cell, they stretched the sample along one direction while simultaneously measuring its properties by nuclear quadrupole resonance. The results, published in Physical Review Letters, showed that the material possesses not one but two superconducting states close in energy.
Under a strain of 0,9 percent, the superconducting transition temperature rose from 3,0 to 3,6 kelvins. At the same time, the charge density wave remained virtually unchanged. Unlike hydrostatic pressure, which affects both phenomena simultaneously, uniaxial strain makes it possible to control superconductivity independently.
Under maximum strain, the material exhibited two distinct transitions. The first, at 3,6 kelvins, corresponded to a nodal (unconventional) state; the second, at 3,0 kelvins, to a nodeless (more conventional) one. The share of the nodal component increased from 10 to 26 percent. Thus, strain not only raises the temperature but also shifts the balance between the two pairing channels.
Nuclear quadrupole resonance made it possible to peer into the atomic level and distinguish these states by their local electronic environment. The previously contradictory data from different groups now find an explanation: the two states coexist in a nearly degenerate manner, and different techniques weighted them differently.
The discovery turns CsV3Sb5 from a source of confusion into a convenient model system. Scientists can now study each pairing channel separately by deforming the lattice. This approach could become a general tool for disentangling superconductivity and competing orders in other unusual superconductors.
Although the transition temperature in CsV3Sb5 is still far too low for practical applications, understanding the mechanisms of competition between the two states brings us closer to creating materials with a higher superconducting temperature. Stretching the crystal opens a new way to control quantum properties.
