Upper critical field of Sr₂RuO₄ under in-plane uniaxial pressure

In-plane uniaxial pressure has been shown to strongly tune the superconducting state of Sr₂RuO₄ by approaching a Lifshitz transition and associated Van Hove singularity (VHS) in the density of states. At the VHS, T_c and the in- and out-of-plane upper critical fields are all strongly enhanced, and the latter has changed its curvature as a function of temperature from convex to concave. However, due to strain inhomogeneity it has not been possible so far to determine how the upper critical fields change with strain. Here, we show the strain dependence of both upper critical fields, which was achieved due to an improved sample preparation.We find that the in-plane upper critical field is mostly linear in T_c. On the other hand, the out-of-plane upper critical field varies with a higher power in T_c, and peaks strongly at the VHS. The strong increase in magnitude and the change in form of H_c2||c occur very close to the Van Hove strain, and point to a strong enhancement of both the density of states and the gap magnitude at the Lifshitz transition.