Resonant spin excitations in unconventional heavy fermion superconductors and Kondo lattice compounds

TL;DR

This paper reviews the observation of resonant spin excitations in heavy fermion superconductors and Kondo compounds, highlighting their role in understanding gap symmetry and collective magnetic modes in these complex materials.

Contribution

It provides a comprehensive analysis of spin exciton modes in various heavy fermion and Kondo systems, emphasizing their significance in probing unconventional superconductivity and hidden order phases.

Findings

Spin exciton modes appear below the gap in several heavy fermion systems.

Resonance locations correlate with Fermi surface nesting properties.

The resonance's energy and intensity inform on the gap symmetry and pairing mechanisms.

Abstract

The heavy quasiparticle bands in Kondo materials which originate in the hybridization of f- and conduction electrons exhibit numerous, sometimes coexisting, broken symmetry phases. Most notable are unconventional superconductivity, itinerant small moment antiferromagnetism and hidden order of higher order multipoles of f-electrons which all lead to a gapping of the heavy bands. In rare cases the chemical potential lies within the hybridization gap and the ground state is a Kondo semiconductor without ordering. The dynamical magnetic response of such gapped f-electron systems has been investigated with inelastic neutron scattering. It was found that collective spin exciton modes which are due to residual quasiparticle interactions appear below the threshold of superconducting or hidden order gap or directly the hybridzation gap. The spin exciton resonance is commonly located around a zone boundary vector $Q$ with nesting properties in the normal state. In the superconducting case its appearance gives a strong criterion for the gap symmetry requesting a sign change $\Delta_{{\bf k}+{\bf Q}}=-\Delta_{\bf k}$ due to the coherence factors. Therefore this many body effect with fundamental importance may also be used as a tool to discriminate between proposed gap models. While the spin resonance has been observed for many compounds we restrict our discussion here exclusively to the small group of f-electron superconductors CeCoIn5, CeCu2Si2 and UPd2Al3, hidden order Kondo compounds CeB6 and URu2Si2 as well as the Kondo semiconductor YbB12 .