Upper Critical Field of Sr$_2$RuO$_4$ under In-Plane Uniaxial Pressure

TL;DR

This study investigates how in-plane uniaxial pressure affects the upper critical fields of Sr$_2$RuO$_4$, revealing significant enhancements near the Van Hove singularity and providing insights into the strain-dependent superconducting properties.

Contribution

The paper demonstrates improved sample preparation enabling precise measurement of upper critical fields under strain, revealing their distinct dependence on $T_c$ and the proximity to the Lifshitz transition.

Findings

Out-of-plane upper critical field peaks at Van Hove singularity.

In-plane upper critical field varies linearly with $T_c$.

Strong enhancement of $H_{c2||c}$ near Lifshitz transition.

Abstract

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 $H_\mathrm{c2||c}$ occur very close to the Van Hove strain, and points to a strong enhancement of both the density of states and the gap magnitude at the Lifshitz transition.