![]() An experimental access to the spectral function, containing all information on quasi-particle renormalization and broadening, is provided by photoelectron spectroscopy. In a band structure representation this has a particular consequence: the bands become smeared out in energy and momentum, as lifetime is shortened. As a quasi-particle spends only a limited time in one state before being scattered, it follows from Heisenberg’s uncertainty principle that also its energy is not sharply defined. Many-body interactions which are only partially captured in this independent-particle picture lead to a renormalization of quasi-particle energies and to a finite lifetime. The result is the well-known representation of electronic states in a diagram with infinitely sharp bands 4. This is actually not a trivial finding, as it requires the interactions between electrons to be screened in a way that each electron propagates in an effective one-particle potential. For many materials, in absence of strong correlations, the band features can be predicted successfully within the local density approximation (LDA) to density-functional theory (DFT) 3. Today, more refined models, accounting for the role of many-body interactions in solids, are just emerging.Ī fundamental concept in solid state physics is the description of electrons in a crystal by their energy E and their momentum k in a band structure of independent quasi-particles. The reason is that the microscopic origin of ferromagnetism is based on the quantum mechanical exchange interaction between the electrons-a fundamental many-particle effect. Nevertheless, the common description of electrons in solids in terms of a band structure of independent quasi-particles is only of limited use, even for the most simple elemental ferromagnets. Designing new magnetic materials requires a thorough understanding of their electronic structure. Today’s information technology vastly depends on ferromagnetic materials used not only for storage but also processing of information in prospective spintronic devices 1, 2. ![]() At the same time we observe regions of anomalously large “waterfall”-like band renormalization, previously only attributed to strong electron correlations in high- T C superconductors, making itinerant ferromagnets a paradigmatic test case for the interplay between band structure, magnetism, and many-body correlations. ![]() Together with one-step photoemission calculations, our experiments allow us to quantify the dispersive behaviour of the complex self-energy over the whole Brillouin zone. Here, we give evidence that in itinerant ferromagnets like cobalt these electron correlations are of nonlocal origin, manifested in a complex self-energy Σ σ( E, k) that disperses as function of spin σ, energy E, and momentum vector k. However, even for the most simple elemental ferromagnets, electron correlations are prevalent, requiring descriptions of their electronic structure beyond the simple picture of independent quasi-particles. Our understanding of the properties of ferromagnetic materials, widely used in spintronic devices, is fundamentally based on their electronic band structure. ![]()
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