Theory of the Evolution of Superconductivity in Sr$_2$RuO$_4$ under Anisotropic Strain

TL;DR

This paper uses functional renormalization group theory to analyze how anisotropic strain affects superconductivity in Sr$_2$RuO$_4$, revealing a rapid increase in $T_c$ followed by a transition to a spin density wave state, aligning with recent experiments.

Contribution

It provides a theoretical framework explaining the strain-induced evolution of superconductivity and competing orders in Sr$_2$RuO$_4$, highlighting the role of Fermi surface topology changes.

Findings

Rapid increase in $T_c$ with strain

Transition from superconductivity to spin density wave

Agreement with experimental strain effects

Abstract

Sr$_2$RuO$_4$ is a leading candidate for chiral $p$-wave superconductivity. The detailed mechanism of superconductivity in this material is still the subject of intense investigations. Since superconductivity is sensitive to the topology of the Fermi surface (the contour of zero-energy quasi-particle excitations in the momentum space in the normal state), changing this topology can provide a strong test of theory. Recent experiments tuned the Fermi surface topology efficiently by applying planar anisotropic strain. Using functional renormalization group theory, we study the superconductivity and competing orders in Sr$_2$RuO$_4$ under strain. We find a rapid initial increase in the superconducting transition temperature $T_c$, which can be associated with the evolution of the Fermi surface toward a Lifshitz reconstruction under increasing strain. Before the Lifshitz reconstruction is reached, however, the system switches from the superconducting state to a spin density wave state. The theory agrees well with recent strain experiments showing an enhancement of $T_c$ followed by an intriguing sudden drop.