The Superconductivity of Sr$_2$RuO$_4$ Under $c$-Axis Uniaxial Stress

TL;DR

This study explores how applying uniaxial stress along the c-axis affects the superconducting properties of Sr$_2$RuO$_4$, revealing complex behavior that challenges simple models and emphasizing the role of three-dimensional effects.

Contribution

It provides the first experimental investigation of c-axis uniaxial stress on Sr$_2$RuO$_4$ and compares results with three-dimensional theoretical models.

Findings

Record c-axis compression of 3.2 GPa achieved.

Superconducting transition temperature $T_c$ is suppressed under stress.

Upper critical field $H_{c2}$ increases, indicating proximity to Van Hove singularities.

Abstract

Applying in-plane uniaxial pressure to strongly correlated low-dimensional systems has been shown to tune the electronic structure dramatically. For example, the unconventional superconductor Sr$_2$RuO$_4$ can be tuned through a single Van Hove singularity which results in a strong enhancement of both $T_\text{c}$ and $H_\text{c2}$. Out-of-plane ($c$ axis) uniaxial pressure is expected to tune the quasi-two-dimensional structure even more strongly, causing it to approach two Van Hove singularities simultaneously. Here we achieve a record value of $3.2$ GPa compression along the $c$ axis of Sr$_2$RuO$_4$. Although the rise in $H_\text{c2}$ shows that we are indeed approaching the van Hove points, $T_\text{c}$ is suppressed, a result that contradicts expectations based on simple two-dimensional models. As a first attempt to take the third dimension into account, we present three-dimensional calculations in the weak interaction limit, and discuss the extent to which they are consistent with observation. Our experimental results highlight the importance of out-of-plane effects in low-dimensional systems in general and provide new constraints on theories of the pairing interaction in Sr$_2$RuO$_4$.