Strong Peak in $T_c$ of Sr$_2$RuO$_4$ Under Uniaxial Pressure

TL;DR

This study combines experiments and theory to show that uniaxial pressure significantly enhances the superconducting transition temperature of Sr$_2$RuO$_4$, revealing a peak near a Lifshitz transition and suggesting possible changes in the order parameter parity.

Contribution

It provides new experimental data on strain-induced $T_c$ enhancement and theoretical insights into the electronic structure and order parameter symmetry changes in Sr$_2$RuO$_4$ under uniaxial pressure.

Findings

$T_c$ peaks at 3.4 K under ~0.6% compression

Maximum $T_c$ occurs near a Lifshitz transition

Strain affects the order parameter parity possibilities

Abstract

We report a combined experimental and theoretical study of the dependence of the superconductivity of the unconventional superconductor Sr$_2$RuO$_4$ on anisotropic strain. Novel piezoelectric apparatus is used to apply uniaxial pressures of up to $\sim$1~GPa along a $\langle 100 \rangle$ direction ($a$-axis) of the crystal lattice. $T_c$ increases from 1.5~K in unstrained material to 3.4~K at compression by $\approx$0.6\%, then falls steeply. The $c$-axis upper critical field for the strained $T_c$ = 3.4~K material is a factor of twenty larger than that of the unstrained crystal, whereas the in-plane ($a$-axis) critical field increases by only a factor of three. First-principles electronic structure calculations give evidence that the observed maximum $T_c$ occurs at or near a Lifshitz transition when the Fermi level passes through a van Hove singularity. Finally, we perform order parameter analyses using three-band renormalization group calculations. These, combined with the unexpectedly low in-plane critical field, open the possibility that the highly strained $T_c$=3.4~K Sr$_2$RuO$_4$ has an even- rather than an odd-parity order parameter. Potential implications such as a transition at nonzero strain between odd- and even-parity order parameters are discussed.