Superconductivity in Sr₂RuO₄ Insensitive to Static Shear Strain

The superconducting order parameter in Sr₂RuO₄ remains unresolved after decades of research [1, 2]. A key recent controversy has emerged between uniaxial pressure and shear-mode ultrasound experiments. Uniaxial pressure studies along the [110] direction have shown a smooth evolution of the superconducting transition temperature (T_c), consistent with a one-component superconducting order parameter [3]. In contrast, shear-mode ultrasound experiments have revealed discontinuities in the elastic moduli, interpreted as evidence pointing toward a two-component order parameter [4, 5]. To address this controversy, we developed a new experimental approach in which three distinct types of shear strain are applied directly to high-quality single crystals of Sr₂RuO₄ [6]. After characterizing the applied strain fields via optical imaging down to low temperature, we measured the strain-induced variations in T_c using low-frequency magnetic susceptibility. We observed T_c variations smaller than 10 mK/%, indicating that shear strain has little to no coupling to the superconducting state. These results support a one-component order parameter but do not fully explain all observations. Our findings place new constraints on the order parameter and underscore the need for theoretical models that can reconcile all known experimental signatures in Sr₂RuO₄.

[1] Y. Maeno, S. Yonezawa, and A. Ramires, J. Phys. Soc. Jpn. 93, 062001 (2024).

[2] Y. Maeno, A. Ikeda, and G. Mattoni, Nat. Phys. 20, 1712–1718 (2024).

[3] F. Jerzembeck, Y. S. Li, G. Palle, et al., Phys. Rev. B 110 (6), 064514 (2024).

[4] S. Benhabib, C. Lupien, I. Paul, et al., Nat. Phys. 17 (2), 194-198 (2024).

[5] S. Ghosh, A. Shekhter, F. Jerzembeck, et al., Nat. Phys. 17 (2), 199-204 (2021).

[6] G. Mattoni, T. Johnson, A. Ikeda, et al., arXiv:2509.10215.