Seminarium Fizyki Materii Skondensowanej

I present a comprehensive analysis of the magnetic excitations and electronic properties of fully quantum double-exchange ferromagnets, i.e., systems where ferromagnetic ordering emerges from the competition between spin, charge, and orbital degrees of freedom, but without the canonical approximation of using classical localized spins. Specifically, I investigate spin excitations within the Kondo lattice-like model, as well as a two-orbital Hubbard Hamiltonian in the orbital-selective Mott phase. Computational analysis of the magnon dispersion, damping, and spectral weight of these models reveal unexpected phenomena, such as magnon mode softening and the anomalous decoherence of magnetic excitations as observed in earlier experimental efforts, but without the need to use phononic degrees of freedom. I show that these effects are intrinsically linked to incoherent spectral features near the Fermi level, which arise due to the quantum nature of the local (on-site) triplet. This incoherent spectrum leads to a Stoner-like continuum on which spin excitations scatter, governing magnon lifetime and strongly influencing the dynamical spin structure factor. By varying the electron density, our study explores the transition from coherent to incoherent magnon spectra. Finally, I will show that this behavior is also present in multi-orbital models with partially filled orbitals, namely in systems without localized spin moments, provided that the model is in a large coupling strength regime.