Fermionic representation for the ferromagnetic Kondo lattice model: Diagrammatic study of spin-charge coupling effects on magnon excitations

Abstract

A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model which allows conventional diagrammatic tools to be employed to study correlation effects. Quantum 1/S corrections to magnon excitations are investigated using a systematic inverse-degeneracy expansion scheme which incorporates correlation effects in the form of self-energy and vertex corrections, while explicitly preserving the continuous spin-rotation symmetry. Magnon self-energy is studied in the full range of interaction strength, and shown to result in strong magnon damping and anomalous softening for zone boundary modes, which accounts for several zoneboundary anomalies observed in recent spin-wave measurements of ferromagnetic manganites.

Spin-charge coupling in a band ferromagnet: Magnon-energy reduction, anomalous softening, and damping

“…͑9͒, brings out the similarity with the corresponding result for the ferromagnetic Kondo lattice model, 27 where the term 1 / 2⌬Ј͑q , ; Q , ⍀͒ is simply equal to 1 / ͑2⌬ + ͒. For the Hubbard model as well, the two terms in the kЈ summation in Eq.…”

Multi-orbital quantum antiferromagnetism in iron pnictides—effective spin couplings and quantum corrections to sublattice magnetization

“…In the ferromagnetic state of the Ferromagnetic Kondo Lattice Model (FKLM), which has been extensively studied for understanding metallic ferromagnetism in doped manganites, the calculated spin wave dispersion in the strong coupling (double exchange) limit (J H /t → ∞) corresponds exactly to that for the Quantum Heisenberg Ferromagnet with NN spin coupling given by J 2 H χ 0 (q) 34 .…”

Magnon self-energy in the correlated ferromagnetic Kondo lattice model: Spin-charge coupling effects on magnon excitations in manganites

“…On the other hand, while RPA magnon energy in the variational method shows nearly Heisenberg form, quantum corrections yield significant non-Heisenberg behavior, a feature also present in the uncorrelated FKLM. 7,31 In the variational method, a strong doping dependence was obtained for the calculated spin stiffness near optimal doping, which is inconsistent with experiments. Both the Holstein-Primakoff and variational studies considered only the one-band model in two dimensions, and therefore focussed on the range 0.5≤n≤0.8 of band fillings n=1-x, corresponding to the metallic ferromagnetic regime (0.2≤x≤0.5).…”

“…Magnetic excitations in the ferromagnetic Kondo lattice model (FKLM) have been investigated for various systems such as the colossal magnetoresistive manganites 1-7 heavy fermion materials, 8 ferromagnetic metals Gd and doped EuX, 9 ordered diluted ferromagnets, 10 and diluted magnetic semiconductors. 11,12 Magnons in the FKLM yield important information about the emergent effective spin couplings generated by the exchange of the particle-hole propagator between the localized magnetic moments, the finite-temperature spin dynamics, and the zero-temperature magnon damping arising from the purely quantum spin-charge coupling effect in a band ferromagnet, 7,13,14 which is totally absent in a localized spin model.…”

“…The physically transparent purely fermionic representation of the FKLM will be employed which allows for a conventional many-body diagrammatic approach. 7 Guided by the inverse-degeneracy-expansion scheme, correlation effects in the form of self-energy and vertex corrections are systematically incorporated so as to explicitly preserve the continuous spin rotation symmetry and hence the Goldstone mode. 32 By treating both Hund's coupling (J) and the correlation term (U) on an equal footing, the diagrammatic approach is extended to the correlated FKLM, and quantum corrections to magnon excitations beyond the classical (RPA) level are obtained in two and three dimen-sions for different band fillings and interaction strengths.…”