Imaging energy filters in photoelectron microscopes and momentum microscopes use spherical fields with deflection angles of 90°, 180° and even 2 × 180°. These instruments are optimized for high energy resolution, and exhibit image aberrations when operated in high transmission mode at medium energy resolution. Here, a new approach is presented for bandpass-filtered imaging in real or reciprocal space using an electrostatic dodecapole with an asymmetric electrode array. In addition to energy-dispersive beam deflection, this multipole allows aberration correction up to the third order. Here, its use is described as a bandpass prefilter in a time-of-flight momentum microscope at the hard X-ray beamline P22 of PETRA III. The entire instrument is housed in a straight vacuum tube because the deflection angle is only 4° and the beam displacement in the filter is only ∼8 mm. The multipole is framed by transfer lenses in the entrance and exit branches. Two sets of 16 different-sized entrance and exit apertures on piezomotor-driven mounts allow selection of the desired bandpass. For pass energies between 100 and 1400 eV and slit widths between 0.5 and 4 mm, the transmitted kinetic energy intervals are between 10 eV and a few hundred electronvolts (full width at half-maximum). The filter eliminates all higher or lower energy signals outside the selected bandpass, significantly improving the signal-to-background ratio in the time-of-flight analyzer.

We have experimentally demonstrated different operating modes for the front lenses of the momentum microscopes described in Part I. Measurements at energies from vacuum UV at a high-harmonic generation (HHG)-based source to the soft and hard X-ray range at a synchrotron facility validated the results of theoretical ray-tracing calculations. The key element is a ring electrode concentric with the extractor electrode, which can tailor the field in the gap. First, the gap-lens-assisted extractor mode reduces the field strength at the sample while mitigating image aberrations. This mode gave good results in all spectral ranges. Secondly, by compensating the field at the sample surface with a negative voltage at the ring electrode we can operate in zero-field mode, which is beneficial for operando experiments. Finally, higher negative voltages establish the repeller mode, which removes all slow electrons below a certain kinetic energy to eliminate the primary contribution to the space-charge interaction in pump-probe experiments. The switch from extractor to repeller mode is associated with a reduction in the k-field-of-view (10-20 % at hard-X-ray energies, increasing to ~50% at low energies). Real-space imaging also benefits from the new lens modes as confirmed by ToF-XPEEM imaging with 650 nm resolution.

High-order harmonics were generated from mono- and polycrystaline molybdenum disulfide (MoS2) monolayers with an infrared femtosecond pulse. We control the Orbital Angular Momentum (OAM) and spatial polarization distribution of the generation beam by using a liquid crystal Q-plate. We then measure the OAM and the full polarization map of the emitted harmonics. We observe that monocrystaline MoS2 behaves as a polarization converter, while polycrystaline MoS2 may be used as a phase mask.

We have determined the quasiparticle dispersion for semimetallic 1T-TiTe2 at 20 K using time-of-flight momentum microscopy with tunable soft-x-ray excitation and high-resolution momentum microscopy with 6.4 eV ultraviolet excitation. In particular, we have studied the quasiparticle interactions of the electronic states of the Te 5p-hole pockets with high Fermi velocity near the Gamma point. Kinks in the otherwise parabolic dispersions suggest the onset of many-body interactions at a binding energy of about 30 meV. We attribute these kinks to electron-phonon coupling. Our study complements previously published results on the Ti 3d electron pockets near the M and L points. The electron-phonon coupling parameters (lambda = 0.3-0.8) are in agreement with previously reported values. An apparently nonzero real part of the self-energy at the Fermi level for one of the two bands may be caused by residual charge-density-wave fluctuations.

We investigated the temperature-dependent electronic structure of the antiferromagnetic fcc-monolayer Mn on Re(0001) using vacuum-ultraviolet momentum microscopy. At T = 25 K the collinear, row-wise antiferromagnetic phase of the Mn monolayer results in a spin splitting of states. Density-functional theory, being in good agreement with the experimental results, reveals the spin and orbital projection of the observed electronic bands. The exchange split bands shift in opposite directions with increasing temperature, decreasing the exchange splitting of a pair of itinerant bands from 280 +/- 10 meV at 25 K down to 185 +/- 10 meV at the Neel temperature T-N = 75 +/- 5 K. The exchange splitting remains constant for T > TN. The persisting exchange splitting is attributed to a remaining short-range, fluctuating antiferromagnetic order far above TN.

The influence of the addition of Bi to the dilute ferromagnetic semiconductor (Ga,Mn)As on its electronic structure as well as on its magnetic and structural properties has been studied. Epitaxial (Ga,Mn)(Bi,As) layers of high structural perfection have been grown using low-temperature molecular-beam epitaxy. Post-growth annealing of the samples improves their structural and magnetic properties and increases the hole concentration in the layers. Hard X-ray angle-resolved photoemission spectroscopy reveals a strongly dispersing band in the Mn-doped layers, which crosses the Fermi energy and is caused by the high concentration of Mn-induced itinerant holes located in the valence band. An increased density of states near the Fermi level is attributed to additional localized Mn states. In addition to a decrease in the chemical potential with increasing Mn doping, we find significant changes in the valence band caused by the incorporation of a small atomic fraction of Bi atoms. The spin-orbit split-off band is shifted to higher binding energies, which is inconsistent with the impurity band model of the band structure in (Ga,Mn)As. Spectroscopic ellipsometry and modulation photoreflectance spectroscopy results confirm the valence band modifications in the investigated layers.

Impact of Bi and In incorporation into (Ga,Mn)As layers on their electronic- and band-structures as well as their magnetic and structural properties has been studied. Homogenous (In,Ga,Mn)As, (Ga,Mn)(Bi,As) and (In,Ga,Mn)(Bi,As) layers of high structural perfection have been grown by the low-temperature molecular-beam epitaxy technique. Post-growth annealing treatment of the layers results in an improvement of their structural and magnetic properties and an increase in the hole concentration in the layers. Hard-x-ray angular-resolved photoemission spectroscopy (HARPES) reveals a strongly dispersed band, crossing the Fermi energy in the highly Mn-doped layers, appearing because of high concentration of Mn-induced itinerant holes residing in the valence band. Moreover, an increased density of states near the Fermi level has been revealed in these layers and attributed to additional localized Mn states. In addition to a downward shift of the chemical potential with increasing Mn-doping, we find pronounced valence-band modifications, particularly in the case of the spin-split-off band, what disagrees with the pure impurity band model. The modulation photoreflectance spectroscopy results confirm the modifications of the valence band in the investigated layers.

Hard X-ray angle-resolved photoemission spectroscopy reveals the momentum-resolved band structure in an epitaxial Mn2Au(001) film capped by a 2 nm thick ferromagnetic Permalloy layer. By magnetizing the Permalloy capping layer, the exceptionally strong exchange bias aligns the Neel vector in the Mn2Au(001) film accordingly. Uncompensated interface Mn magnetic moments in Mn2Au were identified as the origin of the exchange bias using X-ray magnetic circular dichroism in combination with photoelectron emission microscopy. Using time-of-flight momentum microscopy, we measure the asymmetry of the band structure in Mn2Au resulting from the homogeneous orientation of the Neel vector. Comparison with theory shows that the Neel vector, determined by the magnetic moment of the top Mn layer, is antiparallel to the Permalloy magnetization. The experimental results demonstrate that hard X-ray photoemission spectroscopy can measure the band structure of epitaxial layers beneath a metallic capping layer and corroborate the asymmetric band structure in Mn2Au that was previously inferred only indirectly.

X-ray photoelectron diffraction (XPD) is a powerful technique that yields detailed structural information of solids and thin films that complements electronic structure measurements. Among the strongholds of XPD we can identify dopant sites, track structural phase transitions, and perform holographic reconstruction. High-resolution imaging of kll-distributions (momentum microscopy) presents a new approach to core-level photoemission. It yields full-field kx-ky XPD patterns with unprecedented acquisition speed and richness in details. Here, we show that beyond the pure diffraction information, XPD patterns exhibit pronounced circular dichroism in the angular distribution (CDAD) with asymmetries up to 80%, alongside with rapid variations on a small kll-scale (0.1 angstrom- 1). Measurements with circularly-polarized hard X-rays (h nu = 6 keV) for a number of core levels, including Si, Ge, Mo and W, prove that core-level CDAD is a general phenomenon that is independent of atomic number. The fine structure in CDAD is more pronounced compared to the corresponding intensity patterns. Additionally, they obey the same symmetry rules as found for atomic and molecular species, and valence bands. The CD is antisymmetric with respect to the mirror planes of the crystal, whose signatures are sharp zero lines. Calculations using both the Bloch-wave approach and one-step photoemission reveal the origin of the fine structure that represents the signature of Kikuchi diffraction. To disentangle the roles of photoexcitation and diffraction, XPD has been implemented into the Munich SPRKKR package to unify the one-step model of photoemission and multiple scattering theory.

Multiple dopant configurations of Te impurities in close vicinity in silicon are investigated using photoelectron spectroscopy, photoelectron diffraction, and Bloch wave calculations. The samples are prepared by ion implantation followed by pulsed laser annealing. The dopant concentration is variable and high above the solubility limit of Te in silicon. The configurations in question are distinguished from isolated Te impurities by a strong chemical core level shift. While Te clusters are found to form only in very small concentrations, multi-Te configurations of type dimer or up to four Te ions surrounding a vacancy are clearly identified. For these configurations, a substitutional site location of Te is found to match the data best in all cases. For isolated Te ions, this matches the expectations. For multi-Te configurations, the results contribute to understanding the exceptional activation of free charge carriers in hyperdoping of chalcogens in silicon.

The structural and chemical composition of the surface layer (100–140 nm) of niobium radiofrequency cavities operating at cryogenic temperature has enormous impact on their superconducting characteristics. During the last years, cavities treated with a new thermal processing recipe, so-called nitrogen infusion, have demonstrated an increased efficiency and high accelerating gradients. The role and importance of nitrogen gas has been a topic of many debates. In the present work we employ variable-energy synchrotron x-ray photoelectron spectroscopy (XPS), to study the niobium surface subjected to the following treatments: vacuum annealing at 800 °C, nitrogen infusion, and vacuum heat treatment as for the infusion process but without nitrogen supply. Careful analysis of XPS energy-distribution curves revealed a slightly increased thickness of the native oxide Nb 2 O 5 for the infused samples (∼3.8 nm) as compared to the annealed one (∼3.5 nm) which indicates insignificant oxygen incorporation into niobium during 120 °C baking and no effect of nitrogen on the formation of oxides or other niobium phases. By conducting an additional in-situ annealing experiment and analyzing the niobium after the failed infusion process, we conclude that the vacuum furnace hygiene particularly during the high-temperature stage is the prerequisite for success of any treatment recipe.

The successful implementation of the baseline instruments at the European XFEL has triggered a second phase of instrument developments aiming to extend the portfolio of available techniques. At the soft X-ray undulator (SASE 3), the Soft X-ray Port (SXP) instrument is currently under construction. Conceived as an open port, it focuses primarily on femtosecond time-resolved X-ray photoelectron spectroscopy (TR-XPES), which has proven to be a powerful tool to understand the properties of materials and the interaction between their internal degrees of freedom. The extension of this technique to the soft X-ray energy range is only possible at MHz free electron lasers (FELs) due to space-charge effects which limit the maximum photon flux per pulse on the sample. In this contribution, the SXP instrument at the European XFEL and the implementation of TR-XPES using a momentum microscope are presented. The photon energy range available at SASE 3, 0.25 keV to 3.5 keV, and the variable polarization will allow for the simultaneous characterization of the electronic, magnetic, chemical and structural properties of materials with femtosecond time resolution. To this end, a wide range of laser excitation wavelengths, ranging from the XUV to the THz region, will be available.

Novel two-dimensional electron systems at the interfaces and surfaces of transition-metal oxides recently have attracted much attention as they display tunable, intriguing properties that can be exploited in future electronic devices. Here we show that a high-mobility quasi-two-dimensional electron system with strong spin-orbit coupling can be induced at the surface of a KTaO_3 (001) crystal by pulsed laser deposition of a disordered LaAlO_3 film. The momentum-resolved electronic structure of the buried electron system is mapped out by hard x-ray angle-resolved photoelectron spectroscopy. From a comparison to calculations it is found that the band structure deviates from that of electron-doped bulk KTaO_3 due to the confinement to the interface. Nevertheless, the Fermi surface appears to be clearly three-dimensional. From the k broadening of the Fermi surface and core-level depth profiling we estimate the extension of the electron system to be at least 1 nm but not much larger than 2 nm, respectively.

Parity symmetric photoemission spectra are ubiquitous in solid state research, being prevalent in many highly active areas, such as unconventional superconductors, nonmagnetic and antiferromagnetic topological insulators, and weakly relativistic collinear magnets, among others. The direct observation of parity-violating metallic Kramers degenerate bands has remained hitherto experimentally elusive. Here we observe the antiferromagnetic parity violation (APV) in the bandstructure of Mn_2Au thin films by using momentum microscopy with sub-mum spatial resolution, allowing momentum resolved photoemission on single antiferromagnetic domains. The APV arises from breaking the P symmetry of the underlying crystal structure by the collinear antiferromagnetism, while preserving the joint space-time inverison PT -symmetry and in combination with large spin-orbit coupling. In addition, our work also demonstrates a novel tool to directly image the Neel vector direction by combining spatially resolved momentum microscopy with ab-initio calculations.

The occupied and unoccupied electronic states of MoS2 monolayer isolated flake were studied using laboratory based photoemission electron microscope (PEEM) nanoESCA equipped with He-I photon source. PEEM real-space imaging allowed selecting the high quality flake. Altogether, the data will allow accurately recovering the band structures of MoS2. The band structures will be used in future pump-probe experiments to explore the dynamics of electrons in the conduction band and photo-induced multitopological states using trefoil polarization.

The small time gap of synchrotron radiation in conventional multi-bunch mode (100-500MHz) is prohibitive for time-of-flight (ToF) based electron spectroscopy. Even the new generation of delay-line detectors with improved time resolution (<100ps) yields only 20-100 resolved time slices within a 210ns gap. Here we present two techniques of implementing efficient ToF recording at sources with high repetition rate. A fast electron-optical beam blanking unit with GHz bandwidth, integrated in a photoelectron momentum microscope, allows chopping the photon-pulse train to any desired repetition period. Aberration-free momentum distributions have been recorded at ‘chopped ’ pulse periods of 5MHz (at MAX II) and 1.25MHz (at BESSY II). The approach is benchmarked against the alternative way of implementing a dispersive element, e.g. a hemispherical analyzer, in the electron optics. Both approaches, chopping in the time domain as well as bandpass pre-selection in the energy domain, can enable efficient ToF spectroscopy, spectroscopic real-space imaging and momentum microscopy with few-meV resolution using 100-500MHz Synchrotron radiation, highly-repetitive lasers or cavity-enhanced high-harmonic sources. For comparison, we show results recorded at BESSY II with a ‘parasitic’ 4-bunch island-orbit pulse train, coexisting with the 500MHz filling pattern on the main orbit.

Half-metallic ferromagnetic Heusler compounds represent an important class of materials for spintronic applications. We experimentally observe the dispersion of electronic bands with spin resolution in three representative Heusler compounds (${\mathrm{Co}}_{2}\mathrm{MnGa}, {\mathrm{Co}}_{2}\mathrm{MnSi}$, and ${\mathrm{Co}}_{2}{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$). Bulk-sensitive measurements at very low photon energy are complemented by spin-integrated hard x-ray angular resolved photoemission spectroscopy. The dispersion of majority and minority electrons allows us to link exchange splitting and band filling to the number of valence electrons ${N}_{v}$. Photoexcitation at $h\ensuremath{\nu}=6.05\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ gives access to the spin-polarization texture $\mathbit{P}({E}_{B},{k}_{x},{k}_{y})$ of the bulk bands covering a $({k}_{x},{k}_{y})$ range of 60% of the Brillouin zone. We find that ${\mathrm{Co}}_{2}\mathrm{MnSi}$ and ${\mathrm{Co}}_{2}{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ exhibit minority band gaps of 0.5 and 0.35 eV, i.e., decreasing band gap with increasing ${N}_{\mathrm{V}}$ in contrast to the rigid-band expectation. For ${\mathrm{Co}}_{2}\mathrm{MnGa}$, the minority valence state maximum lies at approx. 0.2 eV above ${E}_{\mathrm{F}}$.

The small time gaps of synchrotron radiation in conventional multi-bunch mode (100-500 MHz) or laser-based sources with high pulse rate (∼80 MHz) are prohibitive for time-of-flight (ToF) based photoelectron spectroscopy. Detectors with time resolution in the 100 ps range yield only 20-100 resolved time slices within the small time gap. Here we present two techniques of implementing efficient ToF recording at sources with high repetition rate. A fast electron-optical beam blanking unit with GHz bandwidth, integrated in a photoelectron momentum microscope, allows electron-optical `pulse-picking' with any desired repetition period. Aberration-free momentum distributions have been recorded at reduced pulse periods of 5 MHz (at MAX II) and 1.25 MHz (at BESSY II). The approach is compared with two alternative solutions: a bandpass pre-filter (here a hemispherical analyzer) or a parasitic four-bunch island-orbit pulse train, coexisting with the multi-bunch pattern on the main orbit. Chopping in the time domain or bandpass pre-selection in the energy domain can both enable efficient ToF spectroscopy and photoelectron momentum microscopy at 100-500 MHz synchrotrons, highly repetitive lasers or cavity-enhanced high-harmonic sources. The high photon flux of a UV-laser (80 MHz, <1 meV bandwidth) facilitates momentum microscopy with an energy resolution of 4.2 meV and an analyzed region-of-interest (ROI) down to <800 nm. In this novel approach to `sub-µm-ARPES' the ROI is defined by a small field aperture in an intermediate Gaussian image, regardless of the size of the photon spot.

Diluted ferromagnetic semiconductors combining ferromagnetic and semiconducting properties in one material provide numerous new functionalities, attractive for basic studies and potentially useful for novel device applications. The tailoring of the electronic structure in analogy to conventional semiconductors has yet to be explored. Here, we demonstrate the conservation of broken inversion symmetry and band structure tailoring for high-quality molecular-beam-epitaxy-grown (In,Ga,Mn)As films with 3% In plus 2.5% or 5.6% Mn using hard-x-ray photoelectron diffraction (hXPD) and momentum microscopy. Photon energies of 3--5 keV ensure that the results are not corrupted by surface effects, which are known to be strong in semiconductors. The missing inversion center of the GaAs host lattice leads to fingerprint-like hXPD signatures of As and Ga sites. For both concentrations, Mn predominantly occupies Ga substitutional sites. Momentum microscopy reveals a shift of the chemical potential with increasing Mn doping and a highly dispersing band, crossing the Fermi level for high Mn concentration. The Mn doping induces a pronounced modification of the spin-orbit split-off band.