Dynamical corrections to density-functional theory for quasiparticles in ferromagnetic 4fsystems. I.T=0results for EuO

A theory for the electronic quasiparticle spectrum of ferromagnetic 4f systems is presented and applied to the semiconductor EuO. The starting point is a d-f exchange model, which we solve exactly for T=0. One of the results is a simple relationship between the spin-up quasiparticle energies and the ‘‘free’’ Bloch energies ${\varepsilon }_m$(k), which we use to fix the ${\varepsilon }_m$(k) in a highly realistic manner by performing a new self-consistent spin-polarized band-structure calculation based on density-functional theory. With the so-determined Bloch energies we investigate the spin-down quasiparticle spectrum, which exhibits even at T=0 strong many-body effects as a consequence of spin-exchange processes between localized magnetic 4f moments and itinerant conduction electrons. We discuss in detail the spin-down quasiparticle spectral density for the ΓL direction, which should be observable in an inverse photoemission experiment. The shape of this function is strongly k dependent, revealing different types of quasiparticles. The prominent quasiparticle peaks in the spin-down quasiparticle spectral density are used to construct a quasiparticle band structure, which shows some striking deviations from the one-particle solution of the density-functional theory. Furthermore, results for the electronic self-energy and the quasiparticle density of states are presented.