On the observation of the spin resonance in superconducting CeCoIn_5

TL;DR

This paper critically examines the nature of the spin resonance in superconducting CeCoIn_5, challenging the excitonic interpretation and proposing a magnon-based alternative due to the material's three-dimensionality.

Contribution

The study argues that the spin resonance in CeCoIn_5 is better explained by a magnon scenario rather than a spin exciton, considering the material's three-dimensional properties.

Findings

Resonance is weak and appears only at strong coupling in 3D systems.

The magnon scenario provides a plausible explanation without requiring d_{x^2-y^2} gap symmetry.

Challenges the interpretation of the resonance as evidence for d-wave superconductivity.

Abstract

Recent observation of a resonance spin excitation at (1/2,1/2,1/2) in the superconducting state of CeCoIn_5 [C. Stock et al., Phys. Rev. Lett. {\bf 100} 087001 (2008)] was interpreted as an evidence for d_{x^2-y^2} gap symmetry, by analogy with the cuprates. This is true if the resonance is a spin exciton. We argue that such description is undermined by the three-dimensionality of CeCoIn_5. We show that in 3D systems the excitonic resonance only emerges at strong coupling, and is weak. We argue in favor of the alternative, magnon scenario, which does not require a d_{x^2-y^2} gap.