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

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 Néel 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 σ(Ω) 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-spectralresolution impurity solvers to study the dynamical properties of strongly correlated fermion systems.

Fine structure of the spectra of the Kondo lattice model: Two-site cellular dynamical mean-field theory study

“…The non-local correlations do not destroy the approximate validity of the hybridization picture, as we are able to reproduce the self-energies for all parameters at least qualitatively. The fit is best in the intermediate J ≈ 0.3 regime and somewhat worse for small J, similarly to what is found in the DMFT 18 .…”

“…In our previous single-site DMFT study in Ref. 18, we have found additional fine structure of spectra in the antiferromagnetic phase. In the momentum-resolved spectral functions, we have observed that the hybridized bands are not truly degenerate at the band center and that the local (momentum-integrated) spectral function exhibits narrow features, "spin resonances", inside the bands.…”

Chebyshev expansion for impurity models using matrix product states

“…7, we show initial results. They were obtained with a linear discretization of the bath-DOS with N = 120 sites We see a narrowing of the quasiparticle peak at ω = 0 with increasing interaction, and the formation of Hubbard satellites at ω ≈ ±U/2 [15,30,37,[44][45][46]. For U/D = 2.5, an additional peak in the Hubbard band can be clearly identified.…”

Competition of striped magnetic order and partial Kondo screened state in the Kondo lattice model

“…Both mechanisms usually compete with each other, as best seen in the antiferromagnetic phase, where the magnetic phase vanishes via a second order phase transition when increasing the coupling strength between localized moments and the conduction electrons. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] The common interpretation according to the Doniach phase diagram 2 is that the Kondo effect, whose energy scale depends exponentially on the coupling, gains more energy at strong coupling than the RKKY interaction. However, at intermediate coupling strengths in the antiferromagnetic phase, the Kondo effect and the magnetic order created by the RKKY interaction cooperate with each other to gain maximal energy in the ground state.…”