Superconducting penetration depth through a Van Hove singularity: Sr₂RuO₄ under uniaxial stress

A plethora of experiments in Sr_2⁢RuO₄ have reached conflicting conclusions about the symmetry of the superconducting gap. To probe the gap's structure in k space, we use strain to continuously tune the band structure through a Van Hove singularity (VHS) while imaging the superconductivity with scanning superconducting quantum interference device microscopy. We find that the superfluid density peaks at the VHS and that the temperature dependence of the penetration depth is 𝑇² quadratic over the entire measured range of strain ɛ. These results are consistent with a gap structure that has vertical line nodes, experimentally confirming that nonlocal effects in the Meissner screening can lead to 𝑇² behavior and clarifying the nature of the low-energy excitations in Sr₂⁢RuO₄.