SFB Extra Seminar

A fundamental concept in solid state physics describes the degrees of freedom of electrons in a solid by the relation of the energy E vs. the crystal momentum k in a band structure of independent quasi particles. In a real electron system, exchangeand correlation interaction are collective phenomena that lead, for instance, to effects like ferromagnetism. Consequently, for the 3d ferromagnets Fe, Ni, and Co, a description of the band structure in the widely used local density approximation (LDA) is of limited use, as seen by the fact that predicted well defined electronic bands are not observed experimentally. Only recently, experimental access to the spin resolved band structure at every point in the Brillouin zone became feasible by spin-resolved momentum microscopy [1]. This novel concept combines high resolution imaging of photoelectrons in two-dimensional (kx, ky) maps with a highly efficient imaging spin filter [2].
Our comprehensive measurements of the spectral-function by spin-resolved momentum microscopy give evidence that in itinerant ferromagnets like cobalt electron correlations are of nonlocal origin. This manifests in a complex self-energy that disperses as function of spin, energy, and momentum. Together with one-step photoemission calculations, we quantify the dispersion of the self-energy over the whole Brillouin zone [3]. The observation of nonlocal electron correlations in cobalt
substantially affects our understanding of electron interactions, and makes itinerant ferromagnets a paradigmatic test case for the interplay between band structure, magnetism, and correlations. Despite a pronounced lifetime broadening, direct optical inter-band excitations can be highly spin selective. This leads to the creation of nearly 100% polarized hot carriers in ferromagnetic cobalt, and might serve as a source of spin-polarized electroncurrents in spintronics applications [4].

References
[1] C. Tusche, A. Krasyuk, J. Kirschner, Ultramicroscopy 159, p. 520 (2015)
[2] C. Tusche, et al., Appl. Phys. Lett. 99, 9, 032505 (2011)
[3] C. Tusche et al., Nat. Commun. 9, 3727 (2018)
[4] M. Ellguth, C. Tusche, J. Kirschner, Phys. Rev. Lett., 115, 266801 (2015)