Strain-tuning through a possible van Hove singularity in Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$

The superconducting transition temperature $T_{\mathrm{c}}$ of the tetragonal compound Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ was recently shown to be strongly sensitive to \textless 100\textgreater orthorhombic distortion: $T_{\mathrm{c}}$ increases strongly both when Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ is tensioned and compressed along a \textless 100\textgreater direction. This sensitivity was tentatively attributed to the proximity of one of its Fermi surface sheets to van Hove singularities (vHS): the sections of this sheet that pass closest to the Brillouin zone boundaries are also those perturbed most strongly by \textless 100\textgreater orthorhombic distortion. By increasing the applied orthorhombic distortion to 0.5--1.0{\%} --- a uniaxial pressure almost certainly above 1 GPa --- we have now been able to tune through a sharp peak in both $T_{\mathrm{c}}$ and the upper critical field, $H_{\mathrm{c2}}$. At the peak, $T_{\mathrm{c}}$ more than doubles. The peak is at a strain value consistent with predictions for when the Fermi surface contacts the zone boundary, $i.e.$ the van Hove singularity, although concrete verification will require further measurement. The strength of the enhancement of $H_{\mathrm{c2}}$ may have bearing on the symmetry of the order parameter. The large change in properties at this peak mean that it can almost be considered as a new, unexplored material, opening avenues for future research.