Feedback spin resonance in superconducting CeCu$_2$Si$_2$ and CeCoIn$_5$

TL;DR

This paper demonstrates that spin resonance modes in heavy-fermion superconductors CeCoIn$_5$ and CeCu$_2$Si$_2$ are magnetic excitons linked to quasiparticles, revealing a $d_{x^2-y^2}$ gap symmetry and unifying their resonance behavior with cuprates.

Contribution

It identifies the resonance modes as magnetic excitons and clarifies the gap symmetry as $d_{x^2-y^2}$, resolving longstanding ambiguities in CeCoIn$_5$ and linking resonance phenomena across different superconductors.

Findings

Resonance modes are magnetic excitons from quasiparticles.

Gap symmetry is $d_{x^2-y^2}$ in both compounds.

Resonance peaks exhibit significant dispersion.

Abstract

We show that the recently observed spin resonance modes in heavy-fermion superconductors CeCoIn$_5$ and CeCu$_2$Si$_2$ are magnetic excitons originating from superconducting quasiparticles. The wave vector ${\bf Q}$ of the resonance state leads to a powerful criterion for the symmetry and node positions of the unconventional gap function. The detailed analysis of the superconducting feedback on magnetic excitations reveals that the symmetry of the superconducting gap corresponds to a singlet $d_{x^2-y^2}$ state symmetry in both compounds. In particular this resolves the long-standing ambiguity of the gap symmetry in CeCoIn$_5$. We demonstrate that in both superconductors the resonance peak shows a significant dispersion away from ${\bf Q}$ that should be observed experimentally. Our results suggest a unifying nature of the resonance peaks in the two heavy-fermion superconductors and in layered cuprates.