Underscreened Kondo lattice model applied to heavy fermion uranium compounds

Abstract

We present theoretical results for the underscreened Kondo lattice model with localized S = 1 spins coupled to a conduction band through a Kondo coupling, J K , and interacting among them ferromagnetically. We use a fermionic representation for the spin operators and expand the Hamiltonian in terms of bosonic fields. For large values of J K , we obtain a ferromagnetically ordered solution and a Kondo regime with a Kondo temperature, T K , larger than the Curie temperature, T C . This finding suggests a scenario for a coexistence of Kondo effect and ferromagnetic order. In some uranium compounds, such as UTe or UCu 0.9 Sb 2 , this kind of coexistence has been experimentally observed: they order ferromagnetically with a Curie temperature of order T C ϳ 100 K and exhibit a Kondo behavior for T Ͼ T C . The proposed underscreened Kondo lattice model accounts well for the coexistence between magnetic order and Kondo behavior and yields to a new "ferromagnetic Doniach diagram."

Application of theS=1underscreened Anderson lattice model to Kondo uranium and neptunium compounds

“…30: we derive the Green functions and we calculate self-consistently the magnetization M f for the f -electrons, the magnetization m c for the conduction electrons and the two spin-dependent λ σ parameters which describe the Kondo effect, by imposing constraints on the total number of f -electrons and conduction electrons, n f tot = 2, and n c tot = n c , respectively. Having solved the self-consistent equations, we study various properties of the model.…”

“…1 the important effect of the finite f -bandwidth: the band structure is very different from that without any f -bandwidth used in Ref. 30.…”

“…The mean field approach has been previously described in Ref. 30. For the Kondo part H K , it is based on a generalization of a functional integration approach described by Yoshimori and Sakurai for the single impurity case 34 and by Lacroix and Cyrot for the S = 1/2 Kondo lattice case.…”

“…30,31 This model describes the Kondo interaction, J K , between localized S=1 spins and s = 1/2 spins of conduction electrons, and an inter-site ferromagnetic exchange interaction between the f -spins, J H . The mean-field treatment of the UKL model gives an analog of the Doniach phase diagram, and qualitatively accounts for the coexistence of ferromagnetism and Kondo effect.…”

Ordering and criticality in an underscreened Kondo chain

“…Because each nickel thereby has an S = 1 spin and is accompanied by only a single oxygen hole with S = 1/2, this takes the form of an underscreened Kondo lattice model. 7,8 To enable the DMRG analysis, we take this model to be one-dimensional. Though this is obviously a drastic approximation, our model retains the underscreened Kondo physics, and has the symmetries to allow charge ordering of the same type observed in the nickelate materials.…”