Magnon dressing by orbital excitations in ferromagnetic planes of K2CuF4 and LaMnO3

Abstract

We show that even when spins and orbitals disentangle in the ground state, spin excitations are renormalized by the local tuning of e g orbitals in ferromagnetic planes of K2CuF4 and LaMnO3. As a result, dressed spin excitations (magnons) obtained within the electronic model propagate as quasiparticles and their energy renormalization depends on momentum . Therefore magnons in spin-orbital systems go beyond the paradigm of the effective Heisenberg model with nearest neighbor spin exchange derived from the ground state - spin-orbital entanglement in excited states predicts large magnon softening at the Brillouin zone boundary, and in case of LaMnO3 the magnon energy at the M = (π, π) point may be reduced by ∼45%. In contrast, simultaneously the stiffness constant near the Goldstone mode is almost unaffected. We elucidate physics behind magnon renormalization in spin-orbital systems and explain why long wavelength magnons are unrenormalized while simultaneously energies of short wavelength magnons are reduced by orbital fluctuations. In fact, the -dependence of the magnon energy is modified mainly by dispersion which originates from spin exchange between second neighbors along the cubic axes a and b.