Nature of the spin resonance mode in CeCoIn$_5$

TL;DR

The study reveals that the spin resonance mode in CeCoIn$_5$ is not a sign-changing superconducting indicator but likely results from reduced damping of magnon-like excitations near magnetic quantum criticality.

Contribution

This work challenges the common interpretation of spin resonance modes as evidence of sign-changing superconductivity, providing an alternative explanation for CeCoIn$_5$.

Findings

SRM in CeCoIn$_5$ is not a spin-excitonic bound state.

SRM likely arises from reduced damping of magnon-like modes.

Highlights the need for more rigorous tests of SRMs as indicators of sign-changing superconductivity.

Abstract

Spin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here, we show that for the heavy fermion superconductor CeCoIn$_5$, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn$_5$ likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity.