Direct evidence for the absence of coupling between shear strain and superconductivity in Sr2RuO4

The superconducting symmetry of Sr₂RuO₄ has been intensely debated for many years. A crucial controversy recently emerged between shear-mode ultrasound experiments, which suggest a two-component order parameter, and some uniaxial pressure experiments that suggest a one-component order parameter. To resolve this controversy, we use a new approach to directly apply three different kinds of shear strain to single crystals of Sr₂RuO₄ and investigate the coupling to superconductivity. After characterising the strain by optical imaging, we observe variations of the transition temperature T_c smaller than 10 mK%^-1 as measured by low-frequency magnetic susceptibility, indicating that shear strain has little to no coupling to superconductivity. Our results are consistent with a one-component order parameter model, but such a model cannot consistently explain other experimental evidence such as time-reversal symmetry breaking, superconducting domains, and horizontal line nodes, thus calling for alternative interpretations.