Robust Upward Dispersion of the Neutron Spin Resonance in the Heavy Fermion Superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$

TL;DR

This study reveals a robust upward dispersing neutron spin resonance in the heavy fermion superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$, challenging the conventional spin-exciton interpretation and suggesting a magnon-like excitation.

Contribution

It demonstrates the upward dispersion of the neutron spin resonance in Ce$_{1-x}$Yb$_{x}$CoIn$_5$ and compares experimental results with theoretical models, providing new insights into the nature of magnetic excitations in heavy fermion superconductors.

Findings

Resonance exhibits ring-like upward dispersion.

Dispersion remains robust against Yb-doping.

Results are inconsistent with the spin-exciton downward dispersion model.

Abstract

The neutron spin resonance is a collective magnetic excitation that appears in copper oxide, iron pnictide, and heavy fermion unconventional superconductors. Although the resonance is commonly associated with a spin-exciton due to the $d$($s^{\pm}$)-wave symmetry of the superconducting order parameter, it has also been proposed to be a magnon-like excitation appearing in the superconducting state. Here we use inelastic neutron scattering to demonstrate that the resonance in the heavy fermion superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$ with $x=0,0.05,0.3$ has a ring-like upward dispersion that is robust against Yb-doping. By comparing our experimental data with random phase approximation calculation using the electronic structure and the momentum dependence of the $d_{x^2-y^2}$-wave superconducting gap determined from scanning tunneling microscopy for CeCoIn$_5$, we conclude the robust upward dispersing resonance mode in Ce$_{1-x}$Yb$_{x}$CoIn$_5$ is inconsistent with the downward dispersion predicted within the spin-exciton scenario.