Thesis defense of David Janas

This thesis investigates how electron correlation shapes the interaction between atomic and molecular adsorbates and ferromagnetic transition metal surfaces, using Fe(100) as a model substrate. Ferromagnetic 3d metals such as iron are central to surface science and applied fields like spintronics, catalysis, and organic-inorganic hybrid systems. While existing models such as the Newns-Anderson framework and the d-band model provide useful concepts to understand molecule-surface interactions, they typically neglect electron correlation, a key feature of 3d ferromagnets, and often assume spatially uniform coupling, which breaks down for large organic adsorbates.

To address these limitations, a combination of spin- and momentum-resolved photoemission spectroscopy, scanning probe microscopy, and advanced electronic structure calculations is used to systematically probe molecule-metal interfaces. This includes both density functional theory with effective Hubbard corrections (DFT+U) and dynamical mean-field theory (DMFT), which allow for a more accurate description of electron correlation effects. Starting from the chemisorption of atomic oxygen, the work demonstrates that adsorbates can enhance electron correlation in the Fe surface layer, leading to an energetic narrowing of the Fe d-bands, reduced exchange splitting, and the appearance of spin-dependent lifetime effects in oxygen-related states. These modifications, signatures of many-body interactions, in turn affect how subsequent molecular adsorption proceeds.

Building on this, the thesis examines pentacene adsorption on an oxygen-passivated Fe surface (Fe-O). Here, the aforementioned correlation-induced changes in the substrate d-band structure drive a transition from weak to strong molecule-metal coupling. A computationally efficient approach is developed to emulate the d-band renormalization revealed by DMFT using the DFT+U formalism with a tailored value of Ueff. Expanding on these findings, an extended d-band model is introduced that emphasizes the pivotal role of Fe dz² orbitals in coupling to the extended π-systems of organic molecules.

Finally, the adsorption of metalated tetraphenylporphyrins (ZnTPP and NiTPP) on Fe-O is explored. These molecules remain decoupled electronically, but exhibit measurable conformational distortions. Using photoemission orbital tomography (POT), the thesis reveals how molecular geometry and orbital alignment evolve across mono- and multilayer films. POT is thereby established as a powerful momentum-resolved probe of both electronic structure and molecular conformation.

Altogether, this work advances a correlation-aware framework for understanding hybrid interfaces on ferromagnetic substrates. It shows that electron correlation and orbital hybridization are interconnected phenomena that jointly determine the electronic and geometric properties of molecule-metal systems. These findings carry important implications for future developments in catalysis, molecular electronics, and spin-based technologies.