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Braun, J.; Ebert, H. (2018): Relativistic theory of two-photon photoemission from ferromagnetic materials. In: Physical Review B, Vol. 98, No. 24, 245142
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Abstract

A fully relativistic theory of angle-resolved two-photon photoemission for ferromagnetic materials is derived from the general theory of pump-probe photoemission that is based on the Keldysh formalism. Two-photon photoemission spectroscopy is a widely used analytical tool to study nonequilibrium phenomena in nonmagnetic as well as in magnetic solids, as for example ultrafast demagnetization processes. Our theoretical approach aims at a quantitative description of the time-dependent spectroscopic properties of specific solid systems under consideration, and it allows for the inclusion of static correlation effects via the local spin-density and dynamical mean-field theory electronic structure approach. To this end we follow Pendry's one-step theory of the photoemission process as closely as possible and make extensive use of concepts of relativistic multiple - scattering theory within the layer-Korringa-Kohn-Rostoker method in order to guarantee angular resolution in the spectroscopic calculations. As usual the final state of the two-photon photoemission process is represented by a time-reversed low-energy electron diffraction state. The formalism allows for a quantitative calculation of the time-dependent photocurrent for moderately correlated systems like simple ferromagnetic metals. A first application to the Fe(100) surface is discussed in detail.