Theory of Crystalline Electric Field and Kondo Effect in Pr Skutterudites

TL;DR

This paper investigates the Kondo effect in Pr skutterudites by analyzing low-lying crystalline electric field levels and conduction band interactions, proposing a mechanism for near-degenerate singlet and triplet states influencing magnetic properties.

Contribution

It introduces a model combining point-charge interaction and hybridization to explain the near-degenerate pseudo-quartet and its complex exchange interactions in Pr skutterudites.

Findings

Nearly degenerate pseudo-quartet formed by singlet and triplet states.

Different exchange interactions (ferromagnetic and antiferromagnetic) within the pseudo-quartet.

Renormalization group analysis shows various ground states depending on parameters.

Abstract

Possible Kondo effect in Pr skutterudite is studied with attention to characteristic features of low-lying crystalline electric field (CEF) levels and the conduction band. A mechanism for the small CEF splitting between a singlet and a triplet is proposed as combination of the point-charge interaction and hybridization of 4f with ligand p states. Provided 4f^3 configurations dominate over 4f^1 as intermediate states, p-f hybridization favors the triplet, while point-charge interaction favors the singlet. For realistic parameters for hybridization as well as 4f^1 and 4f^3 levels, these singlet and triplet can form a nearly degenerate pseudo-quartet. It is found that one of two spin 1/2 objects composing the pseudo-quartet has a ferromagnetic exchange, while the other has an antiferromagnetic exchange with conduction electrons. The magnitude of each effective exchange depends strongly on a parameter characterizing the triplet wave function under the T_h symmetry. It is argued that differences of this parameter among Pr skutterdudites are responsible for the apparent diversity of their physical properties. Numerical renormalization group is used to derive the renormalization flows going toward singlet, doublet, triplet or quaret according to the CEF splitting and exchange interactions.