The strength of electronic correlation effects in the spin-dependent electronic structure of ferromagnetic bcc Fe(110) has been investigated by means of spin and angle-resolved photoemission spectroscopy. The experimental results are compared to theoretical calculations within the threebody scattering approximation and within the dynamical mean-field theory, together with one-step model calculations of the photoemission process. This comparison indicates that the present state of the art many-body calculations, although improving the description of correlation effects in Fe, give too small mass renormalizations and scattering rates thus demanding more refined many-body theories including non-local fluctuations.PACS numbers: 75.70. Rf, 79.60.Bm, 73.20.At, 71.15.Mb, 75.50.Bb Since more than half a century it is clear that the bandstructure together with exchange and correlation effects play an important role for the appearance of ferromagnetism in 3d transition metals and their alloys [1]. A first step toward an understanding of the electronic structure of these metals has been achieved by calculations of the single-particle band dispersion [E(k)] within the density functional theory (DFT) in the local spin-density approximation (LSDA) [2] which takes into account correlation effects only in a limited extent. It soon turned out that for the ferromagnetic 3d transition metals such as Fe, Co, and Ni, calculations beyond DFT-based theories have to be developed to take into account many-body interaction, i.e., correlation effects, which normally are described by the energy and momentum dependent complex self-energy function Σ(E, k). Here the real part Σ is related to the mass enhancement while the imaginary part Σ describes the scattering rate. One of the successful schemes for correlated electron systems is the dynamical mean-field theory (DMFT). It replaces the problem of describing correlation effects in a periodic lattice by a correlated impurity coupled to a self-consistent bath [3]. An alternative approach is the three-body scattering (3BS) approximation which takes into account the scattering of a hole into an Auger-like excitation in the valence band, formed by one hole plus an electron-hole excitation [4]. Such many-body calculations allowed the qualitative description of the quenching of majority-channel quasiparticle excitations in Co [5] or the narrowing of the Ni 3d band [6]. While the above mentioned many-body theories give an improved description of the electronic structure, the central question is, whether they also lead to a quantitative agreement with experiments.Angle-resolved photoemission spectroscopy (ARPES) is a powerful method to determine the spectral function and by comparison with the bare-particle band structure (usually approximated by DFT band structure calculations) to obtain the self-energy [7]. Moreover, the spinresolved version of this method is very useful to disentangle the complex electronic structure of ferromagnets, in particular for systems with a strong overlap between majo...

We report on a quantitative investigation of the spin-dependent quasiparticle lifetimes and electron correlation effects in ferromagnetic hcp Co͑0001͒ by means of spin-and angle-resolved photoemission spectroscopies. The experimental spectra are compared in detail to state-of-the-art many-body calculations within the dynamical mean-field theory and the three-body scattering approximation, including a full calculation of the one-step photoemission process. From this comparison we conclude that although strong local many-body Coulomb interactions are of major importance for the qualitative description of correlation effects in Co, more sophisticated many-body calculations are needed in order to improve the quantitative agreement between theory and experiment, in particular, concerning the linewidths. The quality of the overall agreement obtained for Co indicates that the effect of nonlocal correlations becomes weaker with increasing atomic number.

We have investigated the spin-dependent quasiparticle lifetimes and the strength of electron correlation effects in the ferromagnetic 3d transition metals Fe, Co, and Ni by means of spin-and angle-resolved photoemission spectroscopy. The experimental data are accompanied by state-of-the-art many-body calculations within the dynamical mean-field theory and the three-body scattering approximation, including fully relativistic calculations of the photoemission process within the one-step model. Our quantitative analysis reveals that inclusion of local many-body Coulomb interactions are of ultimate importance for a realistic description of correlation effects in ferromagnetic 3d transition metals. However, we found that more sophisticated many-body calculations with larger modifications in the case of Fe and Co are still needed to improve the quantitative agreement between experiment and theory. In general, it turned out that not only the dispersion behavior of energetic structures should be affected by nonlocal correlations but also the line widths of most of the photoemission peaks are underestimated by the current theoretical approaches. The increasing values of the on-site Coulomb interaction parameter U and the band narrowing of majority spin states obtained when moving from Fe to Ni indicate that the effect of nonlocal correlations becomes weaker with increasing atomic number, whereas correlation effects tend to be stronger.

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