In this dissertation, the work is organized into three main parts: (i) sample preparation, (ii) characterization of the samples by scanning tunneling microscopy (STM) including scanning tunneling spectroscopy (STS), and (iii) the determination of the electronic band structure using µ-Angular Resolved Photoelectron Spectroscopy (µ-ARPES). The first section details the fabrication of magnetic 2D van der Waals materials, exemplified by the compounds FePS3, NiPS3, MnPS3, and CrPS4. Here, methodological insights from the sample preparation of other two-dimensional materials, such as graphene, were systematically adapted and optimized for the present systems. This includes the development of specialized procedures for cleaving and exfoliation as well as the selection and pre-treatment of substrates. In the present case, a 5 nm layer of gold is evaporated atop a 1 nm titanium film, which is deposited on a preheated 90 nm SiO2/Si substrate. Immediately before exfoliation at 60 ◦C, the substrate is treated with oxygen plasma to ensure optimal surface conditions. The second part presents the results of STM measurements performed on CrBr3, CrPS4, MnPS3, and FePS3. STM imaging was successfully achieved on all investigated materials for the first time, in some cases with atomic resolution, complemented by corresponding STS measurements. Notably, a superstructure was observed on FePS3 that provides evidence of the influence of the antiferromagnetic zigzag configuration on the sulfur atoms. The third section focuses on the band structures of FePS3 and NiPS3 as determined by µ-ARPES measurements. For NiPS3, photon-energy-dependent measurements were conducted for the first time, covering both temperatures above (T = 222 K) and below (T = 45 K) the Néel temperature. These measurements exhibit a pronounced dependence of the photoemission cross section on the photon energy, as well as a clear influence of the measured kz component. For the temperature comparison, a measurement at hν = 60 eV was employed, which distinctly reveals differences in the band structure between the paramagnetic and antiferromagnetic states. Analogous experiments on FePS3 were also performed for the first time; measurements below the Néel temperature were carried out at T = 90 K using hν = 58 eV, while measurements above the Néel temperature were conducted at room temperature. A systematic study, varying the photon energy in one-eV steps, enabled the identification of the Fe 3p – Fe 3d resonance, which was then utilized to enhance and so characterize bands with Fe 3d character. Furthermore, comparison of the experimental data with DFT+U calculations yielded an optimal effective Coulomb parameter of Ueff = 1.2 eV. It was found that the transition into the antiferromagnetic state is accompanied by three significant modifications in the band structure, each associated with bands of Fe 3d, S 3p, and P 3p character. This indicates that all three atomic species - iron, sulfur, and phosphorus - play an active role in the magnetic phase transition. This structured presentation provides a comprehensive foundation for understanding both the experimental methodologies and the underlying physical phenomena in magnetic 2Dvan der Waals systems.
Benjamin Jan Pestka (Thu,) studied this question.