Strong Increase in Ultrasound Attenuation Below T$_\mathrm{c}$ in Sr$_2$RuO$_4$: Possible Evidence for Domains

TL;DR

Ultrasound attenuation measurements in Sr$_2$RuO$_4$ reveal a sevenfold increase below T$_c$, suggesting the presence of domain walls and a two-component superconducting order parameter, challenging existing theories.

Contribution

This study provides the first ultrasound attenuation evidence supporting domain walls and a two-component order parameter in Sr$_2$RuO$_4$, highlighting inhomogeneous superconductivity.

Findings

Sevenfold increase in ultrasound attenuation below T$_c$

Attenuation increase is specific to compressional sound

Results suggest inhomogeneous, two-component superconducting state

Abstract

Recent experiments suggest that the superconducting order parameter of Sr$_2$RuO$_4$ has two components. A two-component order parameter has multiple degrees of freedom in the superconducting state that can result in low-energy collective modes or the formation of domain walls -- a possibility that would explain a number of experimental observations including the smallness of the time reversal symmetry breaking signal at T$_\mathrm{c}$ and telegraph noise in critical current experiments. We perform ultrasound attenuation measurements across the superconducting transition of Sr$_2$RuO$_4$ using resonant ultrasound spectroscopy (RUS). We find that the attenuation for compressional sound increases by a factor of seven immediately below T$_\mathrm{c}$, in sharp contrast with what is found in both conventional ($s$-wave) and high-T$_\mathrm{c}$ ($d$-wave) superconductors. We find our observations to be most consistent with the presence of domain walls between different configurations of the superconducting state. The fact that we observe an increase in sound attenuation for compressional strains, and not for shear strains, suggests an inhomogeneous superconducting state formed of two distinct, accidentally-degenerate superconducting order parameters that are not related to each other by symmetry. Whatever the mechanism, a factor of seven increase in sound attenuation is a singular characteristic with which any potential theory of the superconductivity in Sr$_2$RuO$_4$ must be reconciled.