Superfluid density through a Van Hove singularity: Sr₂RuO₄ under uniaxial strain

Strontium ruthenate (Sr₂RuO₄) is an archetype in the field of unconventional superconductivity; yet, after nearly three decades of strenuous effort, our understanding of superconductivity in Sr₂RuO₄ remains incomplete. To understand how the Van Hove singularity (VHS) impacts the superconductivity, we report on scanning superconducting quantum interference device (SQUID) microscopy measurements of the London penetration depth as the system is strain-tuned through the VHS. We found that the zero-temperature superfluid density increases by ~15%, coinciding with the peak in the superconducting transition temperature, and that the penetration depth varies quadratically with temperature, Δλ(T) ~ T² over the entire strain range. In addition, we performed scanning tunneling spectroscopy to determine the superconducting gap in uniaxially strained samples. Under zero strain, we resolved a superconducting gap with a magnitude of Δ₀ ≈ 350 μeV, and under a uniaxial compression of ≈0.4%, we observed an enhanced gap of Δ₀ ≈ 600 μeV. With a nodal order parameter, an increase in the superconducting gap could bring about an increase in the superfluid density through reduced sensitivity to defects or through reduced non-local effects in the Meissner screening. Our data indicate that tuning to the VHS increases the gap throughout the Brillouin zone, and that non-local effects are likely more important than reduced scattering.