arXiv subjects

Patrick Lömker

Publications and source records attributed to Patrick Lömker.

4 recordsLinked to original sources

arXiv2025

Poleval: A Python package for HAXPES analysis

POLEVAL provides a software toolbox for collaborative, persistent and reproducible analysis of XPS experiments. It allows to treat, analyse and visualise the results of an extended experimental campaign in a single python notebook in a consistent manner. Managing experimental data in adequate objects enables experimentalists to process and analyse measurements in very few lines of code, so as to provide decision aids through online data analysis during e.g. beamtime experiments. The persistent and self-documentary style of the notebook-based analysis allows for easy communication of intermediate results and enables progressive refinements into publishable figures or exporting the results to other programs. The toolbox facilitates various routines for data treatment (normalization, cropping, etc.) and aggregation of spectra into groups to analyse trends. It also enables quantitative analysis with three major functions: First, normalization to the photoionization cross-section and probability of emission into the analyser cone allows for quantitative comparisons between intensities from different core levels. The integrated haxquantpy package allows easy retrieval of literature values for this purpose. Second, an extensive fitting functionality is implemented to treat groups of spectra together, rather than spectrum-by-spectrum. This grouping allows reinforcing the fit algorithm with prior knowledge, such as the equivalence of peak widths or positions between spectra, which enables for more consistent, and importantly, more confident fit results for sets of potentially noisy spectra. Third, a simple formalism to estimate the thickness of adsorbate layers based on the ratio between the substrate's and adsorbate's XPS signal is implemented.

cond-mat.mtrl-sci↗

arXiv2022

Temperature-dependent spin-resolved electronic structure of EuO thin films

The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy (spin-ARPES) and density functional theory. EuO exhibits unique properties of hosting both weakly-dispersive nearly fully polarized Eu $4f$ bands, as well as O $2p$ levels indirectly exchange-split by the interaction with Eu nearest neighbors. Our temperature-dependent spin-ARPES data directly demonstrates the exchange splitting in O $2p$ and its vanishing at the Curie temperature. Our calculations with a Hubbard $U$ term reveal a complex nature of the local exchange splitting on the oxygen site and in conduction bands. We discuss the mechanisms of the indirect exchange in the O 2p levels by analyzing orbital-resolved band characters in ferromagnetic and antiferromagnetic phases. The directional effects due to spin-orbit coupling are predicted theoretically to be significant in particular in the Eu 4f band manifold. The analysis of the shape of spin-resolved spectra in the Eu $4f$ spectral region reveals signatures of hybridization with O $2p$, in agreement with the theoretical predictions. We also analyze spectral changes in the spin-integrated spectra throughout the Curie temperature and demonstrate they derive from both the magnetic phase transition and effects due to sample aging, unavoidable for this highly reactive material.

cond-mat.mtrl-sci↗

arXiv2019

Redox-controlled epitaxy and magnetism of oxide heterointerfaces: EuO/SrTiO$_3$

We demonstrate a novel route to prepare thin films of the ferromagnetic insulator Europium monoxide. Key is a redox-controlled interface reaction between metallic Eu and the substrate SrTiO$_3$ as the supplier of oxygen. The process allows tuning the electronic, magnetic and structural properties of the EuO films. Furthermore, we apply this technique to various oxidic substrates and demonstrate the universality and limits of a redox-controlled EuO film synthesis.

cond-mat.mtrl-sci↗

arXiv2017

Two-dimensional electron system at the magnetically tunable EuO/SrTiO$_3$ interface

We create a two-dimensional electron system (2DES) at the interface between EuO, a ferromagnetic insulator, and SrTiO3, a transparent non-magnetic insulator considered the bedrock of oxide-based electronics. This is achieved by a controlled in-situ redox reaction between pure metallic Eu deposited at room temperature on the surface of SrTiO3, an innovative bottom-up approach that can be easily generalized to other functional oxides and scaled to applications. Additionally, we find that the resulting EuO capping layer can be tuned from paramagnetic to ferromagnetic, depending on the layer thickness. These results demonstrate that the simple, novel technique of creating 2DESs in oxides by deposition of elementary reducing agents [T. C. Rödel et al., Adv. Mater. 28, 1976 (2016)] can be extended to simultaneously produce an active, e.g. magnetic, capping layer enabling the realization and control of additional functionalities in such oxide-based 2DESs.

cond-mat.mtrl-sci↗