An Efficient Algorithm for Automatic Structure Optimization in X-ray Standing-Wave Experiments

X-ray standing-wave photoemission experiments involving multilayered samples are emerging as unique probes of the buried interfaces that are ubiquitous in current device and materials research. Such data require for their analysis a structure optimization process comparing experiment to theory that is not straightforward. In this work, we present a new computer program for optimizing the analysis of standing-wave data, called SWOPT, that automates this trial-and-error optimization process. The program includes an algorithm that has been developed for computationally expensive problems: so-called black-box simulation optimizations. It also includes a more efficient version of the Yang X-ray Optics Program (YXRO) [Yang, S.-H., Gray, A.X., Kaiser, A.M., Mun, B.S., Sell, B.C., Kortright, J.B., Fadley, C.S., J. Appl. Phys. 113, 1 (2013)] which is about an order of magnitude faster than the original version. Human interaction is not required during optimization. We tested our optimization algorithm on real and hypothetical problems and show that it finds better solutions significantly faster than a random search approach. The total optimization time ranges, depending on the sample structure, from minutes to a few hours on a modern laptop computer, and can be up to 100x faster than a corresponding manual optimization. These speeds make the SWOPT program a valuable tool for realtime analyses of data during synchrotron experiments

Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2_2 bulk crystals

Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS$_2$ bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES) we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method we find that these valence band states at the $\overline{\text{K}}$ point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS$_2$, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS$_2$ can be accessed without thinning it down to the monolayer limit

Direct observation of the band gap transition in atomically thin ReS2_2

ReS$_2$ is considered as a promising candidate for novel electronic and sensor applications. The low crystal symmetry of the van der Waals compound ReS$_2$ leads to a highly anisotropic optical, vibrational, and transport behavior. However, the details of the electronic band structure of this fascinating material are still largely unexplored. We present a momentum-resolved study of the electronic structure of monolayer, bilayer, and bulk ReS$_2$ using k-space photoemission microscopy in combination with first-principles calculations. We demonstrate that the valence electrons in bulk ReS$_2$ are - contrary to assumptions in recent literature - significantly delocalized across the van der Waals gap. Furthermore, we directly observe the evolution of the valence band dispersion as a function of the number of layers, revealing a significantly increased effective electron mass in single-layer crystals. We also find that only bilayer ReS$_2$ has a direct band gap. Our results establish bilayer ReS$_2$ as a advantageous building block for two-dimensional devices and van der Waals heterostructures

Does Exchange Splitting persist above TCT_C? A spin-resolved photoemission study of EuO

The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy and density functional theory (GGA+$U$). Our spin-resolved data reveals that, while the macroscopic magnetization of the sample vanishes at the Curie temperature, the exchange splitting of the O 2$p$ band persists up to $T_{C}$. Thus, we provide evidence for short-range magnetic order being present at the Curie temperature

Interface properties and built-in potential profile of a LaCrO3_3/SrTiO3_3 superlattice determined by standing-wave excited photoemission spectroscopy

LaCrO$_3$ (LCO) / SrTiO$_3$ (STO) heterojunctions are intriguing due to a polar discontinuity along (001), two distinct and controllable interface structures [(LaO)$^+$/(TiO$_2$)$^0$ and (SrO)$^0$/(CrO$_2$)$^-$], and interface-induced polarization. In this study, we have used soft- and hard x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to generate a quantitative determination of the elemental depth profiles and interface properties, band alignments, and the depth distribution of the interface-induced built-in potentials in the two constituent oxides. We observe an alternating charged interface configuration: a positively charged (LaO)$^+$/(TiO$_2$)$^0$ intermediate layer at the LCO$_\textbf{top}$/STO$_\textbf{bottom}$ interface and a negatively charged (SrO)$^0$/(CrO$_2$)$^-$ intermediate layer at the STO$_\textbf{top}$/LCO$_\textbf{bottom}$ interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS) mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO-contributed and STO- contributed matrix-element-weighted density of states (MEWDOSs) from the valence band (VB) spectra for the LCO/STO superlattice (SL). Monitoring the VB edges of the deconvoluted MEWDOS shifts with a change in probing profile, the alternating charge- induced built-in potentials are observed in both constituent oxides. Finally, using a two-step simulation approach involving first core-level binding energy shifts and then valence-band modeling, the built-in potential gradients across the SL are resolved in detail and represented by the depth distribution of VB edges.Comment: Main text: 29 pages, 5 figures; Supplementary Information: 20 pages, 10 figure

The electronic structure of transition metal dichalcogenides investigated by angle-resolved photoemission spectroscopy

Van der Waals (vdW) materials offer a perspective to revolutionize basically every facet of nowadays technology with a new generation of atomically thin devices. Transition metal dichalcogenides (TMDCs) are a family of vdW crystals, that includes several semiconducting materials with band gaps within the optical range. This makes them ideal for numerous applications such as transistors, optical sensors, solar cells, and LEDs. In this study we focuses on two members of the TMDC family: molybdenum disufide (MoS$_{2}$) and rhenium disulfide (ReS$_{2}$). Using a combination of angle-resolved photoemission spectroscopy (APRES) with density functional theory (DFT), we provide a thorough analysis of the electronic band structure of these two exceptional materials. In monolayers of MoS$_{2}$ the combination of broken inversion symmetry with the heavy element molybdenum leads to a large spin-splitting of distinct valleys within its electronic structure. Therefore, MoS$_{2}$ combines the essential ingredients for socalled $\textit{spintronics}$ and $\textit{valleytronics}$. It was generally believed that these fascinating features are forbidden in MoS$_{2}$ bulk crystals due to their centrosymmetric space group. This study demonstrates that the strong confinement of the valleys within the vdW layers leads to a recently discovered type of $\textit{hidden spin-polarization}$, which results in quasi two-dimensional, highly spin-polarized states in this centrosymmetricbulk crystal. Furthermore, we present the first ARPES study of ReS$_{2}$ bulk, monolayer, and bilayer crystals. Recent literature reported indications for a total confinement of the bulk electronic structure within the plains of the vdW layers. Our study comes to the opposite conclusion. Based on the observation of a considerable out-of-plane dispersion in the ARPES experiments, as well as in the band structure calculations, we show that valence electrons are significantly delocalized across the vdW gap. In addition, we identify the valence band maximum of bulk, monolayer, and bilayer ReS$_{2}$ experimentally. The combination of ARPES and band structure calculations shows that ReS$_{2}$ undergoes a transition from a direct band gap in the bulk and bilayer to an indirect gap in the monolayer

Spin- and angle-resolved photoemission studies at the VUV undulator beamline BL5 of the synchrotron radiation facility DELTA

The experimental endstation at the beamline BL5 of the synchrotron radiationfacility DELTA at the TU Dortmund is optimized for spin- and angle-resolvedphotoemission experiments in the VUV regime. The hemispherical electronanalyzer is equipped with a SPLEED detector for spin-resolved photoemissionas well as a two dimensional delay line detector for angle-resolvedmeasurements. The two detectors can be operated semi-simultaneously,therefore, a combination of spin- and angle-resolved measurements on thesame sample species becomes feasible. In addition, the undulator U250 and theplain grating monochromator beamline BL5 are optimized for high brilliance inthe VUV and low energy soft x-ray range, which is in particular suitable forspin-resolved experiments where obtaining ample countrates is usuallychallenging.At the moment, a time-resolved modus for the experiments is undercommissioning. In a close collaboration with the machine group of DELTA(AG Khan, TU Dortmund), a laser is used to produce ultra-short VUV pulses inthe fs-regime via the so-called coherent harmonic generation principle. A partof this seeding laser can be used as a pump laser for pump-probe experiments.During the last few years, the beamline was mainly used to investigatetopological insulators and thin magnetic films. For those sample systems, thecombination of spin- and angle-resolved photoemission experiments wasessential in order characterize the spin-texture of the electronic band structures.In our contribution, we will show those experimental results and give anoutlook on the new time-resolved modus that is currently undercommissioning

Localized segregation of gold in ultrathin Fe films on Au(001)

The growth of up to ten-monolayer-thick Fe films on a Au(001) surface was investigated during depositionat room temperature and during annealing, using low-energy electron diffraction and x-ray photoemissionspectroscopy, as well as locally with low-energy electron microscopy and photoemission electron microscopy.The growth proceeds with a submonolayer of Au segregating on the surface of Fe, which is in agreement withprevious studies. Annealing was found to be critical for the presence of Au on the Fe surface. Our study identifiesa spatially inhomogeneous Au segregation mechanism which proceeds by the formation of cracks in the Fe film,starting at the annealing temperature of 190 °C, through which Au diffuses towards the surface. As a result, asystem with a nonuniform surface electronic structure is obtained. This study shows the necessity to employspatially resolved techniques to fully understand the growth modes of the layered epitaxial systems

A short-pulse facility for time and angle-resolved photoe-mission experiments at the synchrotron light source DELTA

At the synchrotron light source DELTA (TU Dortmund), a short-pulsefacility is under commissioning. The U250 undulator of the storage ringis used to generate coherent sub-ps pulses at higher harmonics of aseeding laser. The VUV beamline BL5 operated by a group from PGI-6 (FZ Julich) ̈ guides those pulses into an end-station that is optimizedfor angle and spin-resolved photoemission spectroscopy experiments.A part of the seeding laser pulse is brought into the end-station via aseparate beamline and can be used as a pump beam for pump-probeexperiments. With those time and angle-resolved photoemission exper-iments that are now feasible with this unique setup we will, in the nearfuture, study magnetization dynamics of thin ferromagnetic films onmetal surfaces.The modifications of the photoemission setup that were necessaryto conduct time-resolved experiments and the current status of theshort-pulse facility will be described