Quantum effects in the quasiparticle structure of the ferromagnetic Kondo lattice model

Abstract

Abstract.A new "Dynamical Mean-field theory" based approach for the Kondo lattice model with quantum spins is introduced. The inspection of exactly solvable limiting cases and several known approximation methods, namely the second-order perturbation theory, the self-consistent CPA and finally a momentconserving decoupling of the equations of motion help in evaluating the new approach. This comprehensive investigation gives some certainty to our results: Whereas our method is somewhat limited in the investigation of the J < 0-model, the results for J > 0 reveal important aspects of the physics of the model: The energetically lowest states are not completely spin-polarized. A band splitting, which occurs already for relatively low interaction strengths, can be related to distinct elementary excitations, namely magnon emission (absorption) and the formation of magnetic polarons. We demonstrate the properties of the ferromagnetic Kondo lattice model in terms of spectral densities and quasiparticle densities of states.

Kondo lattice model at half-filling

“…In order to map KLM onto an appropriate impurity model, we closely follow the treatment given in Ref. 19, i.e., we introduce fermion operators f iσ to represent the spin operator of the localized f -electron…”

Low-density approach to the Kondo-lattice model

“…For any finite spin, there is a considerable amount of ↓ −spectral weight overlapping with ↑ −states even for very large J. Recently, a DMFT-based approach to the KLM with quantum spins has been proposed [31] which uses a fermionization of the local spin operators. The theory is restricted to S = 1/2 but retains the quantum nature of the spins.…”

“…Due to some reasons, the above-mentioned theories [10,31] are best justified for weak and intermediate couplings J. In this paper, we propose an approximate scheme which mainly aims at the strong coupling regime (JS >> W : W is the bandwidth) being nevertheless perturbationally correct up to order J 2 .…”

“…For finite bandwidth, the mentioned special case is that of a ferromagnetically saturated semiconductor (EuO at T = 0!) [10,31,[33][34][35]. In this situation, an ↑-electron has no chance for a spin-flip, the corresponding quasiparticle density of states, ρ ↑ (E), is therefore only rigidly shifted compared to the "free" DOS [10] and the selfenergy is a constant:…”

Uniform hopping approach to the ferromagnetic Kondo model at finite temperature

“…The impact of quantum spins on the electronic properties has been studied in Ref. 6,7,8. It appears that quantum effects are important for S=1/2 corespins or at T = 0.…”