Fine structure of spectra in the antiferromagnetic phase of the Kondo lattice model

TL;DR

This paper investigates the detailed spectral features of the antiferromagnetic phase in the Kondo lattice model, revealing inelastic scattering effects and spin resonances that influence optical properties and are relevant for models with itinerant f-electrons.

Contribution

It introduces the concept of spin resonances in the spectral function of the antiferromagnetic Kondo lattice model, highlighting effects beyond the simple hybridization picture and their dependence on temperature and magnetic field.

Findings

Identification of spin resonances inside the bands near the quantum phase transition.

Spin resonances are absent in models with localized f-electrons and ferromagnetic Kondo coupling.

Optical conductivity features include a threshold, plateau, and a secondary peak related to spin resonances.

Abstract

We study the antiferromagnetic phase of the Kondo lattice model on bipartite lattices at half-filling using the dynamical mean-field theory with numerical renormalization group as the impurity solver, focusing on the detailed structure of the spectral function, self-energy, and optical conductivity. We discuss the deviations from the simple hybridization picture, which adequately describes the overall band structure of the system (four quasiparticle branches in the reduced Brillouin zone), but neglects all effects of the inelastic-scattering processes. These lead to additional structure inside the bands, in particular asymmetric resonances or dips that become more pronounced in the strong-coupling regime close to the antiferromagnet-paramagnetic Kondo insulator quantum phase transition. These features, which we name "spin resonances", appear generically in all models where the $f$-orbital electrons are itinerant (large Fermi surface) and there is N\'eel antiferromagnetic order (staggered magnetization), such as periodic Anderson model and Kondo lattice model with antiferromagneitc Kondo coupling, but are absent in antiferromagnetic phases with localized $f$-orbital electrons (small Fermi surface), such as the Kondo lattice model with ferromagnetic Kondo coupling. We show that with increasing temperature and external magnetic-field the spin resonances become suppressed at the same time as the staggered magnetization is reduced. Optical conductivity $\sigma(\Omega)$ has a threshold associated with the indirect gap, followed by a plateau of low conductivity and the main peak associated with the direct gap, while the spin resonances are reflected as a secondary peak or a hump close to the main optical peak. This work demonstrates the utility of high-spectral-resolution impurity solvers to study the dynamical properties of strongly correlated fermion systems.