Underscreened Kondo Necklace

TL;DR

This paper introduces the underscreened Kondo necklace model with spins greater than 1/2 to explore the competition between Kondo screening and magnetic order, revealing an antiferromagnetic phase at low temperatures.

Contribution

It extends the Kondo necklace model to higher spins and analyzes the phase diagram, showing the dominance of antiferromagnetic order in the underscreened regime.

Findings

Low-temperature phase is always an x-y antiferromagnet.

Effective Hamiltonian in strong coupling limit is an antiferromagnetic x-y model.

Degree of moment compensation varies with Kondo coupling and spin S.

Abstract

It has been suggested recently by Gan, Coleman, and Andrei that studying the underscreened Kondo problem may help to understand the nature of magnetism in heavy fermion systems. Motivated by Doniach's work on the S=1/2 Kondo necklace, we introduce the underscreened Kondo necklace models with S>1/2. The underscreened Kondo necklace is the simplest lattice model on which the competition between Kondo spin compensation, and magnetic ordering due to an RKKY-type interaction can be examined. We used the mean-field approximation to determine the phase diagram, and found that the low-temperature phase is always an x-y antiferromagnet. This contention is further supported by the derivation of the exact form of the effective hamiltonian in the limit of very large Kondo coupling: it is found to be an antiferromagnetic x-y model for the residual (S-1/2)-spins. In general, the degree of moment compensation depends on both the Kondo coupling, and on S.