Search

Polarization-dependent photoemission electron microscopy (PD-PEEM) exploits spatial variation in the optical selection rules of materials to image domain formation and material organization on the nanoscale. In this Perspective, we discuss the mechanism of PD-PEEM that results in the observed image contrast in experiments and provide examples of a wide range of material domain structures that PD-PEEM has been able to elucidate, including molecular and polymer domains, local electronic structure and defect symmetry, (anti)ferroelectricity, and ferromagnetism. In the end, we discuss challenges and new directions that are possible with this tool for probing domain structure in materials, including investigating the formation of transient ordered states, multiferroics, and the influence of molecular and polymer order and disorder on excited state dynamics and charge transport. [ABSTRACT FROM AUTHOR]

Electron–phonon coupling is an important energy transfer mechanism in solids after ultrafast laser excitation. In this study, we present an extreme ultraviolet (EUV) and infrared (IR) pump-probe photoemission experiment to investigate the electron–phonon coupling in nonequilibrium gold. The energy of IR-laser-emitted photoelectrons is shifted due to the EUV photoemission and oscillates with a ∼ 4 T H z frequency. Such oscillation is considered as the effective excitation of the longitudinal acoustic phonon mode in gold through the spectral-dependent electron–phonon coupling. Our study showcases the capability of time-resolved photoemission electron microscopy to monitor the non-equilibrium lattice vibrations with ultrahigh spatial and temporal resolution. [ABSTRACT FROM AUTHOR]

We tested and compared the stability and usability of three different cathode materials and configurations in a thermionic-based ultrafast electron microscope: (1) on-axis thermionic and photoemission from a custom 100 μm diameter LaB6 source with a graphite guard ring, (2) off-axis photoemission from the Ni aperture surface of the Wehnelt electrode, and (3) on-axis thermionic and photoemission from a custom 200 μm diameter polycrystalline Ta source. For each cathode type and configuration, including the Ni Wehnelt aperture, we illustrate how the photoelectron beam-current stability is deleteriously impacted by simultaneous cooling of the source following thermionic heating. Furthermore, we demonstrate usability via collection of parallel- and convergent-beam electron diffraction patterns and by formation of the optimum probe size. We find that usability of the off-axis Ni Wehnelt-aperture photoemission is at least comparable to on-axis LaB6 thermionic emission, as well as to on-axis photoemission [the heretofore conventional approach to ultrafast electron microscopy (UEM) in thermionic-based instruments]. However, the stability and achievable beam currents for off-axis photoemission from the Wehnelt aperture were superior to that of the other cathode types and configurations, regardless of the electron-emission mechanism. Beam-current stability for this configuration was found to be ±1% (one standard deviation from the mean) for 70 min (longest duration tested), and steady-state beam current was reached within the sampling-time resolution used here (∼1 s) for 15 pA beam currents (i.e., 460 electrons per packet for a 200 kHz repetition rate). Repeatability and robustness of the steady-state condition were also found to be within ±1% of the mean. We discuss the implications of these findings for UEM imaging and diffraction experiments, for pulsed-beam damage measurements, and for practical switching between optimum conventional TEM and UEM operation within the same instrument. [ABSTRACT FROM AUTHOR]

Field emission is an important process with a variety of applications. Quantitative predictions of such electron emission need to include details of the internal potentials that shape the electronic wavefunctions (and hence the tunneling probability), predictive analysis of the work function barrier (ΦB), and knowledge of the electron distribution at the surface that constitutes the supply function. Here, these various factors were all collectively considered based on a combined Monte Carlo-density functional theory approach. Results were obtained for both the field-dependent cold electron emission current density as well as photoemission from a short laser pulse. The method also allows for calculations of field-dependent emittance. The technique is general and could be extended to include plasmon–polariton modes, different thicknesses of coatings, and role of surface adsorbates and defects. [ABSTRACT FROM AUTHOR]

In the present study, we have investigated the effect of magnetic Cr doping on the transport properties of a sputtered Bi2Se3 thin film on the SrTiO3 (110) substrate. The high-resolution x-ray diffraction and Raman spectroscopy measurements revealed the growth of rhombohedral Bi2Se3 thin films. Further electronic and compositional analysis was done by x-ray photoemission spectroscopy and Rutherford backscattering spectroscopy, and the x-value was estimated to be 0.18 in the Bi2−xCrxSe3 thin film. The variation in the resistivity with temperature (2–300 K) revealed the metallic nature in undoped Bi2Se3 up to 30 K and upturn resistivity below 30 K. The Cr-doped Bi2Se3 resistivity data show a traditional semiconducting nature up to 25 K and take an abrupt upturn resistivity below 25 K. The resistivity behavior of both samples was explained by adopting a model that consists of the total resistance, a combination of bulk and surface resistance in parallel. The bulk bandgap value determined by this method is obtained to be 256 meV in an undoped Bi2Se3 thin film. Magnetoconductance data of the undoped thin film revealed a weak anti-localization (WAL) effect, while the Cr-doped thin film showed a weak localization (WL) effect at low temperatures (<50 K). At low magnetic field and low temperature, a competing nature of WAL and WL effects was prominent in the Cr-doped film. A drastic increase in the electrical resistance suggests that Cr doping can significantly modify the electrical properties of Bi2Se3 thin films, which could have potential applications in futuristic devices. [ABSTRACT FROM AUTHOR]

Magnetic nanometric skyrmions are small complex vortex-like topological defects, mainly found in non-centrosymmetric crystals such as MnSi. They have potential applications for future spintronic devices. In this article, the structural, electronic, and magnetic states of the Mn atoms in a polycrystalline MnSi thin film facing a c-sapphire substrate were studied using x-ray diffraction, x-ray photo-emission spectroscopy, resonance photoemission spectroscopy (RPES), and extended x-ray absorption fine structure (EXAFS). The valence band spectra indicate the metallic nature of the film. The RPES study reveals the presence of major itinerant Mn 3d states near EF and also the mixed Mn 3d and Si 3s–3p states from 5.3 to 11.3 eV. The EXAFS spectrum does not show the existence of oxygen vacancies in the system, and the obtained magnetic moment in the non-stoichiometric MnSi thin film is a combination of the partially itinerant and partially localized Mn 3d states. [ABSTRACT FROM AUTHOR]

The present study focuses on the oxygen (O)-ion implantation-induced structural and electronic modifications in the single crystals of strontium-titanate (s-STO) using synchrotron-based x-ray diffraction (XRD), x-ray photoemission spectroscopy (XPS), and resonant x-ray photoemission spectroscopy. The crystallinity of the epitaxially aligned phases of s-STO is confirmed through XRD. This direct evidence of heavy ion implantation is supported by the Monte Carlo-based simulation of stopping and range of ions in matter/transport of ions in matter. XPS at different core levels is performed to detect the exact oxidation state of Ti ions in s-STO. The dominance of Ti3+ over Ti4+ upon oxygen implantation suggests the disorder in the perovskite material, primarily in the form of oxygen vacancies (VO). The confirmation of VO is explicitly shown by the enhancement in the spectral area of the assigned peak in the O 1s XPS. Resonant photoemission spectroscopy measurements were performed by varying photon energy from 32 to 46 eV to understand the nature of the valence band electronic structure of s-STO. The resonance in the different hybridized states of s-STO is confirmed by the spectral features of constant initial state plots. There is a correlation between the defective state of Ti and the oxygen-deficient state. The transformation from SrTiO3 to SrTiO2.5, partially or completely, is essentially required to underline any modification in the electronic properties of s-STO. s-STO is in a mixed state of an ionic conductor and an electronic conductor. This study outlines the creation of VO due to O-ion implantation and investigates the changes in the electronic structures of s-STO. [ABSTRACT FROM AUTHOR]

Illumination stress (IS) and negative bias under illumination stress (NBIS) cause considerable device instability in thin-film transistors based on amorphous In–Ga–Zn–O (a-IGZO). Models using in-gap states are suggested to explain device instability. Therefore, to provide reliably their density of states (DOS), this study investigated the valence band, conduction band, and in-gap states of an a-IGZO thin film. The DOS of in-gap states was directly determined in a dynamic range of six orders of magnitude through constant final state yield spectroscopy (CFS-YS) using low-energy and low-flux photons. Furthermore, light irradiation irreversibly induced extra in-gap states near the Fermi level and shifted the Fermi level to the vacuum level side, which should be related to the device instability due to IS and NBIS. Hard x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy using synchrotron radiation observed the large DOS of in-gap states near the Fermi level as in previous works. Here, we reveal that they are not intrinsic electronic states of undamaged a-IGZO, but induced by the intense measurement light of synchrotron radiation. This study demonstrates that CFS-YS is useful for determining the reliable DOS of the in-gap states for samples that are sensitive to light irradiation. The absorption spectrum measured through photothermal deflection spectroscopy is interpreted based on DOS directly determined via photoemission spectroscopies. This indicates that the line shape in the energy region below the region assigned to the Urbach tail in previous works actually roughly reflects the DOS of occupied in-gap states. [ABSTRACT FROM AUTHOR]

A resonance-enhanced GaInAsSb nanopillar array negative electron affinity potential photocathode based on Mie resonance excitation at infrared wavelengths can significantly enhance the absorption and quantum efficiency of nanopillar arrays. Our study indicates the different periodic structures have very little effect on the QE for the nanopillar arrays. However, the smaller the period of the array, the higher the quantum efficiency is achieved, but the probability of being collected by the anode is reduced, so a compromise should be considered in the structure design. Moreover, the incident light inclination and the external electric field can significantly affect the quantum efficiency of the nanopillar arrays. The photocathodes studied here have a quantum efficiency of over 5.7% with an angle of incidence of 30∘ and an applied electric field intensity of 1.2 V/ μ m. This work provides an important reference and theoretical basis for the design and optimization of resonance-enhanced GaInAsSb nanopillar arrays photocathode. [ABSTRACT FROM AUTHOR]

Determining the electronic levels associated with polarons, the fundamental charge carriers in organic semiconductors, is key to understanding the charge transport properties of these materials. Recent findings challenge the traditional view of these electronic levels by highlighting the importance of intra‐molecular Coulomb interactions in polarons. Experimental evidence was previously presented for a revised model of the negative polaron in the case of the polymer semiconductor poly(NDI2OD‐T2); there, the addition of an excess electron was seen to lead to the emergence of a singly occupied state within the energy gap of the undoped material and an unoccupied state above the edge of the conduction states. Here, focus is on a small‐molecule semiconductor, C60, and spectral evidence is provided of a similar picture for the new states appearing upon polaron formation. Specifically, direct and inverse photoemission spectroscopy is used to investigate the density of states in C60 films n‐doped with two dimeric dopants. The Coulomb interaction energy (Hubbard U) of the C60 anion is experimentally determined to be ≈1.1 eV, a value that aligns closely with theoretical predictions. [ABSTRACT FROM AUTHOR]

The first experimental results obtained by the ion beam-induced luminescence technique from the ceramic bodies of ancient tiles are reported in this work. The photon emission from the ceramic bodies is related to the starting minerals and the manufacturing conditions, particularly the firing temperature and cooling processes. Moreover, the results indicate that this non-destructive technique, performed under a helium-rich atmosphere instead of an in-vacuum setup and with acquisition times of only a few seconds, presents a promising alternative to traditional, often destructive, compositional characterisation methods. Additionally, by adding other ion beam-based techniques such as PIXE (Particle-Induced X-ray Emission) and PIGE (Particle-Induced Gamma-ray Emission), compositional information from light elements such as Na can also be inferred, helping to also identify the raw materials used. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study of hydrogenated amorphous silicon (a-Si)-based detectors, utilizing electrical characterization, Raman spectroscopy, photoemission, and inverse photoemission techniques. The unique properties of a-Si have sparked interest in its application for radiation detection in both physics and medicine. Although amorphous silicon (a-Si) is inherently a highly defective material, hydrogenation significantly reduces defect density, enabling its use in radiation detector devices. Spectroscopic measurements provide insights into the intricate relationship between the structure and electronic properties of a-Si, enhancing our understanding of how specific configurations, such as the choice of substrate, can markedly influence detector performance. In this study, we compare the performance of a-Si detectors deposited on two different substrates: crystalline silicon (c-Si) and flexible Kapton. Our findings suggest that detectors deposited on Kapton exhibit reduced sensitivity, despite having comparable noise and leakage current levels to those on crystalline silicon. We hypothesize that this discrepancy may be attributed to the substrate material, differences in film morphology, and/or the alignment of energy levels. Further measurements are planned to substantiate these hypotheses. [ABSTRACT FROM AUTHOR]

A combined approach of band bending is employed to enhance the near‐infrared (NIR) photovoltaic (PV) response of a Si/Au Schottky junction (SHJ) device. As a reference, the basic architecture of the SHJ device consists of textured n‐Si and gold nanoparticles (Au NPs). Its photoelectric conversion efficiency at 1319 nm wavelength is 0.0302%. A series of band‐bending approaches are then applied to the reference device in sequence, aiming to minimize the electric loss of the structure by strengthening built‐in electric field. These approaches include introductions of an Al2O3 layer for front field passivation, a p+‐Si layer to form a p+n‐like junction at the front of the device and a SiO2 layer for rear field passivation. With these modifications, under 1319 nm illumination, the open‐circuit voltage eventually increases by 14%, the short‐circuit current increases by 18% and the photoelectric conversion efficiency of the device increases by 66% to reach 0.0501%. The results of this work propose a strategy to minimize the electric loss in an NIR PV device. [ABSTRACT FROM AUTHOR]

The experimental efficiency has been a central concern for time-consuming experiments. Spin- and angle-resolved photoemission spectroscopy (spin-resolved ARPES) is renowned for its inefficiency in spin-detection, despite its outstanding capability to directly determine the spin-polarized electronic properties of materials. Here, we investigate the potential enhancement of the efficiency of spin-resolved ARPES experiments through the integration of measurement informatics. We focus on a representative topological insulator Bi 2 Te 3 , which has well-understood spin-polarized electronic states. We employ Gaussian process regression (GPR) to assess the accumulation of spin polarization information using an indicator known as the GPR score. Our analyses based on the GPR model suggest that the GPR score can serve as a stopping criterion for spin-resolved ARPES experiments. This criterion enables us to conduct efficient spin-resolved ARPES experiments, significantly reducing the time costs by 5-10 times, compared to empirical stopping criteria. [ABSTRACT FROM AUTHOR]

Emergent phases often appear when the electronic kinetic energy is comparable to the Coulomb interactions. One approach to seek material systems as hosts of such emergent phases is to realize localization of electronic wavefunctions due to the geometric frustration inherent in the crystal structure, resulting in flat electronic bands. Recently, such efforts have found a wide range of exotic phases in the two-dimensional kagome lattice, including magnetic order, time-reversal symmetry breaking charge order, nematicity, and superconductivity. However, the interlayer coupling of the kagome layers disrupts the destructive interference needed to completely quench the kinetic energy. Here we demonstrate that an interwoven kagome network—a pyrochlore lattice—can host a three dimensional (3D) localization of electron wavefunctions. Meanwhile, the nonsymmorphic symmetry of the pyrochlore lattice guarantees all band crossings at the Brillouin zone X point to be 3D gapless Dirac points, which was predicted theoretically but never yet observed experimentally. Through a combination of angle-resolved photoemission spectroscopy, fundamental lattice model and density functional theory calculations, we investigate the novel electronic structure of a Laves phase superconductor with a pyrochlore sublattice, CeRu2. We observe evidence of flat bands originating from the Ce 4f orbitals as well as flat bands from the 3D destructive interference of the Ru 4d orbitals. We further observe the nonsymmorphic symmetry-protected 3D gapless Dirac cone at the X point. Our work establishes the pyrochlore structure as a promising lattice platform to realize and tune novel emergent phases intertwining topology and many-body interactions. [ABSTRACT FROM AUTHOR]

Proximity effects allow for the adjustment of magnetic properties in a physically elegant way. If two thin ferromagnetic (FM) films are brought into contact, electronic coupling alters their magnetic exchange interaction at their interface. For a low- T C rare-earth FM coupled to a 3d transition metal FM, even room temperature magnetism is within reach. In addition, magnetic proximity coupling is particularly promising for increasing the magnetic order of metastable materials such as europium monoxide (EuO) beyond their bulk T C , since neither the stoichiometry nor the insulating properties are modified. We investigate the magnetic proximity effect at Fe/EuO and Co/EuO interfaces using hard X-ray photoelectron spectroscopy. By exciting the FM layers with circularly polarized light, magnetic dichroism is observed in angular dependence on the photoemission geometry. In this way, the depth-dependence of the magnetic signal is determined element-specifically for the EuO and 3d FM parts of the bilayers. In connection with atomistic spin dynamics simulations, the thickness of the EuO layer is found to be crucial, indicating that the observed antiferromagnetic proximity coupling is a short-ranged and genuine interface phenomenon. This fact turns the bilayer into a strong synthetic ferrimagnet. The increase in magnetic order in EuO occurs in a finite spatial range and is therefore particularly strong in the 2D limit—a counterintuitive but very useful phenomenon for spin-based device applications. [ABSTRACT FROM AUTHOR]

Owing to their exceptional mechanical, electronic, and phononic transport properties, compositionally complex alloys, including high-entropy alloys, represent an important class of materials. However, the interplay between chemical disorder and electronic correlations, and its influence on electronic structure-derived properties, remains largely unexplored. This is addressed for the archetypal CrMnFeCoNi alloy using resonant and valence band photoemission spectroscopy, electrical resistivity, and optical conductivity measurements, complemented by linear response calculations based on density functional theory. Utilizing dynamical mean-field theory, correlation signatures and damping in the spectra are identified, highlighting the significance of many-body effects, particularly in states distant from the Fermi edge. Electronic transport remains dominated by disorder and potentially short-range order, especially at low temperatures, while visible-spectrum optical conductivity and high-temperature transport are influenced by short quasiparticle lifetimes. These findings improve our understanding of element-specific electronic correlations in compositionally complex alloys and facilitate the development of advanced materials with tailored electronic properties. Compositionally complex alloys have attracted significant attention recently, but the role of electronic correlations in these materials is unknown. Redka et al. study the CrMnFeCoNi alloy using a combination of experimental and theoretical techniques, revealing strong correlation effects far from the Fermi edge. [ABSTRACT FROM AUTHOR]

Modern catalysts are complex systems whose performance depends both on space and time domains and, most importantly, on the operational environment. As a direct consequence, understanding their functionalities requires sophisticated techniques and tools for measurement and simulation, addressing the proper spatial and temporal scale and being capable of mimicking the working conditions of every single component, such as catalyst supports, electrodes, electrolytes, as well as of the entire assembly, e.g., in the case of fuel cells or batteries. Scanning photoelectron spectro-microscopy (SPEM) is one of the approaches that allow combining X-ray photoelectron spectroscopy with sub-micron spatial resolution; in particular, the SPEM hosted at the ESCA Microscopy beamline at Elettra has been upgraded to conduct in situ and operando experiments. Three different case studies are presented to illustrate the capabilities of the SPEM in the investigation of catalytic materials in different conditions and processes. [ABSTRACT FROM AUTHOR]

Efforts to push the spatiotemporal imaging-resolution limits of femtosecond laser-driven ultrafast electron microscopes (UEMs) to the combined angstrom–fs range will benefit from stable sources capable of generating high bunch charges. Recent demonstrations of unconventional off-axis photoemitting geometries are promising, but connections to the observed onset of structural dynamics are yet to be established. Here we use the in-situ photoexcitation of coherent phonons to quantify the relative time-of-flight (r-TOF) of photoelectron packets generated from the Ni Wehnelt aperture and from a Ta cathode set-back from the aperture plane. We further support the UEM experiments with particle-tracing simulations of the precise electron-gun architecture and photoemitting geometries. In this way, we measure discernible shifts in electron-packet TOF of tens of picoseconds for the two photoemitting surfaces. These shifts arise from the impact that the Wehnelt-aperture off-axis orientation has on the electron-momentum distribution, which modifies both the collection efficiency and the temporal-packet distribution relative to on-axis emission. Future needs are identified; we expect this and other developments in UEM electron-gun configuration to expand the range of material phenomena that can be directly imaged on scales commensurate with fundamental structural dynamics. [ABSTRACT FROM AUTHOR]

One of the most challenging aspects of X‐ray research is the delivery of liquid sample flows into the soft X‐ray beam. Currently, cylindrical microjets are the most commonly used sample injection systems for soft X‐ray liquid spectroscopy. However, they suffer from several drawbacks, such as complicated geometry due to their curved surface. In this study, we propose a novel 3D‐printed nozzle design by introducing microscopic flat sheet jets that provide micrometre‐thick liquid sheets with high stability, intending to make this technology more widely available to users. Our research is a collaboration between the EuXFEL and MAX IV research facilities. This collaboration aims to develop and refine a 3D‐printed flat sheet nozzle design and a versatile jetting platform that is compatible with multiple endstations and measurement techniques. Our flat sheet jet platform improves the stability of the jet and increases its surface area, enabling more precise scanning and differential measurements in X‐ray absorption, scattering, and imaging applications. Here, we demonstrate the performance of this new arrangement for a flat sheet jet setup with X‐ray photoelectron spectroscopy, photoelectron angular distribution, and soft X‐ray absorption spectroscopy experiments performed at the photoemission endstation of the FlexPES beamline at MAX IV Laboratory in Lund, Sweden. [ABSTRACT FROM AUTHOR]

This research investigates the multifaceted effects of electromagnetic and optical radiation on physical and biological entities, spanning a spectrum from fundamental physical interactions to intricate biological responses. The study encompasses electromagnetic radiation, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. It aims to elucidate the diverse consequences of electromagnetic radiation on physical objects, considering phenomena such as heating, photoelectric emission, and ionisation. Concurrently, the research delves into the intricate interactions between electromagnetic radiation and biological entities, encompassing ionizing and non-ionizing radiation. Furthermore, the research seeks to understand the impact of artificial light sources, especially those rich in blue wavelengths, on circadian rhythms, photosynthesis in plants, and animal vision. It also illustrates the Artifact introduced by electromagnetic and optical radiation in medical imaging. We proposed the Model-Based Iterative Reconstruction (MBIR) method to address the Artifact Reduction. The findings are anticipated to contribute to developing strategies for mitigating potential risks and harnessing beneficial applications in various scientific, medical, and technological domains. [ABSTRACT FROM AUTHOR]

Photoelectron spectroscopy allows for the investigation of the electronic structure and chemical bonding of actinide elements and their compounds, providing insights into oxidation states, chemical environments, and electronic configurations. This knowledge can aid in comprehending reactivity, stability, and other properties of actinide materials, which is essential for ensuring safe handling, storage, and disposal in nuclear applications. We have reviewed a number of results in actinide core-level photoemission studies, with a particular focus on x-ray photoemission spectroscopy (XPS) techniques. Actinides, due to their inherent radioactivity, have not been as well studied with XPS as have other segments of the periodic table. Given the inherent safety concerns, equipment requirements, and short isotopic lifetimes associated with actinide research, we outline the strategies and precautions necessary for conducting successful and safe XPS experiments on these elements. Core-level photoemission can be a powerful proven tool for investigating the electronic structure, chemical bonding behaviors, and physical properties of actinides, providing valuable insights into an incredibly complex behavior of these systems. We highlight key findings from recent studies that demonstrate the potential of core-level photoemission in uncovering the unique properties of actinides and their compounds. Finally, we identify current knowledge gaps and future research directions that could enhance our understanding of actinide chemistry and physics. [ABSTRACT FROM AUTHOR]

In this work, we investigated the effect of hole transporting poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) interfacing with Mn-doped CdS/ZnS quantum dots (QDs) deposited on an indium tin oxide (ITO) substrate on the photoemission of upconverted hot electrons under weak continuous wave photoexcitation in a vacuum. Among the various factors that can influence the photoemission of the upconverted hot electrons, we studied the role of PEDOT:PSS in facilitating the hole transfer from QDs and altering the energy of photoemitted hot electrons. Compared to hot electrons emitted from QDs deposited directly on the ITO substrate, the addition of the PEDOT:PSS layer between the QD and ITO layers increased the energy of the photoemitted hot electrons. The increased energy of the photoemitted hot electrons is attributed in part to the reduced steady-state positive charge on the QDs under continuous photoexcitation, which reduces the energy required to eject the electron from the conduction band. [ABSTRACT FROM AUTHOR]

We investigate the electron emission from 3D chiral silver alloy nanohelices initiated by femtosecond laser pulses with a central photon energy of hν = 1.65 eV, well below the work function of the material. We find hot but thermally distributed electron spectra and a strong anisotropy in the electron yield with left- and right-circularly polarized light excitations, which invert in sign between left- and right-handed helices. We analyze the kinetic energy distribution and discuss the role of effective temperatures. Measurements of the reflectance and simulations of the absorbance of the helices based on retarded field calculations are compared to the anisotropy in photoemission. We find a significant enhancement of the anisotropy in the electron emission in comparison to the optical absorption. Neither simple thermionic nor a multiphoton photoemission can explain the experimentally observed asymmetries. Single photon deep-UV photoemission from these helices together with a change of the work function suggests a contribution of the chirally induced spin selectivity effect to the observed asymmetries. [ABSTRACT FROM AUTHOR]

It has been demonstrated previously that electrons interact differently with chiral molecules depending on their polarization. For enantiomeric pure monolayers of heptahelicene, opposite asymmetries in spin polarization were reported and attributed to the so-called chirality-induced spin selectivity effect. However, these promising proof-of-concept photoemission experiments lack the angular and energy resolution that could provide the necessary insights into the mechanism of this phenomenon. In order to fill in the missing gaps, we provide a detailed spin- and angle-resolved photoemission spectroscopy study of heptahelicene layers on a Cu(111) substrate. Throughout the large accessible energy and angle range, no chirality induced spin asymmetry in photoemission could be observed. Possible reasons for the absence of signatures of the spin-dependent electron transmission through the chiral molecular layer are briefly discussed. [ABSTRACT FROM AUTHOR]

We demonstrate liquid-jet photoelectron spectroscopy from a flatjet formed by the impingement of two micron-sized cylindrical jets of different aqueous solutions. Flatjets provide flexible experimental templates enabling unique liquid-phase experiments that would not be possible using single cylindrical liquid jets. One such possibility is to generate two co-flowing liquid-jet sheets with a common interface in vacuum, with each surface facing the vacuum being representative of one of the solutions, allowing face-sensitive detection by photoelectron spectroscopy. The impingement of two cylindrical jets also enables the application of different bias potentials to each jet with the principal possibility to generate a potential gradient between the two solution phases. This is shown for the case of a flatjet composed of a sodium iodide aqueous solution and neat liquid water. The implications of asymmetric biasing for flatjet photoelectron spectroscopy are discussed. The first photoemission spectra for a sandwich-type flatjet comprised of a water layer encapsulated by two outer layers of an organic solvent (toluene) are also shown. [ABSTRACT FROM AUTHOR]

In the period December 12, 2022, to February 28, 2023, we performed a series of UBVRI observations of 65 asteroids, including 50 primitive-type objects, which could be observed near perihelion at that time, and calculated their approximated reflectance spectra. The observations were aimed at searching for spectral signs of sublimation-driven dust activity in the asteroids and formation of a transient dust exosphere, which are caused by the presence of Н2О ice in their interior and the highest subsolar temperature. Based on the analysis of the accuracy and shape of approximated reflectance spectra of the primitive-type asteroids, a set of which was conventionally restricted by the geometric albedo value not exceeding 0.10, we found sublimation-driven dust activity in asteroids 164 Eva, 360 Carlova, and 750 Oskar by distinct short-term variations in the reflectance of these asteroids in the U band. During one of the nights, similar spectral changes and sublimation-driven dust activity related to them were detected in asteroid 629 Bernardina; the geometric albedo of this asteroid is somewhat higher (0.14 or 0.19), which may be indicative of a heterogeneous composition of its material and, apparently, a local outcropping of water ice on its surface. Moreover, in three asteroids of a high-temperature type, 757 Portlandia, 1121 Natascha, and 1687 Glarona, noticeable variations were detected in the entire spectral range; they may be connected with ejection of submicron dust particles from the surface by some other mechanisms. To substantiate and interpret the results of observations, we numerically modeled the reflectance spectra of conditional asteroids of taxonomic types C and S (with low- and high-temperature mineralogy, respectively), which are enveloped by an optically thin dust exosphere containing aggregate or homogeneous submicron particles of different composition. On the basis of this modeling, we introduce a quantitative criterion to identify a weak sublimation-driven dust activity in primitive-type asteroids in the visible range. [ABSTRACT FROM AUTHOR]

Vibrationally resolved photoelectron spectra of anthracene anions have been measured for photon energies between 1.13 and 4.96 eV. In this energy range, photoemission mostly occurs via autodetaching electronically excited states of the anion, which strongly modifies the vibrational excitation of the neutral molecule after electron emission. Based on the observed vibrational patterns, eight different excited states could be identified, seven of which are resonances known from absorption spectroscopy. Distinctly different photon energy dependencies of vibrational excitations have been obtained for different excited states, hinting at strongly different photoemission lifetimes. Unexpectedly, some resonances seem to exhibit bimodal distributions of emission lifetimes, possibly due to electronic relaxation processes induced by the excitation of specific vibrational modes. [ABSTRACT FROM AUTHOR]

NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using magnetometry and x-ray magnetic circular dichroism based on x-ray absorption spectroscopy and spin-polarized inverse photoemission spectroscopy with various thicknesses down to 1.6 nm. The magnetic behavior can be understood in terms of a layer of optimal NCO and an interfacial layer (1.2 ± 0.1 nm), with a small canting of magnetization at the surface. The thickness dependence of the optimal layer can be described by the finite-scaling theory with a critical exponent consistent with the high perpendicular magnetic anisotropy. The interfacial layer couples antiferromagnetically to the optimal layer, generating exchange-spring styled magnetic hysteresis in the thinnest films. The non-optimal and measurement-speed-dependent magnetic properties of the interfacial layer suggest substantial interfacial diffusion. [ABSTRACT FROM AUTHOR]

The analytic nature of the transmission coefficient for a δ -function barrier makes it a useful tool to examine a variety of technologically important applications, such as photoemission from semiconductors with an alkali coating, the examination of tunneling times for wave packets incident on a barrier, and for parameterizing tunneling through the narrow barrier of a normal-superconducting point contact. The analytic model of a δ -function barrier inside a confining well is extended to the finite height and width rectangular barrier (a delta-function sequence). Methods to exactly evaluate the eigenstates are given and their dependencies are examined. The time evolution of a superposition of the lowest eigenstates is considered for barriers having comparable Gamow tunneling factors so as to quantify the impact of barrier height and shape on time evolution in a simple and exact system and, therefore, serve as a proxy for tunneling time. Last, density profiles and associated quantum potentials are examined for coupled wells to show changes induced by weaker and wider barriers. [ABSTRACT FROM AUTHOR]

Though CsPbBr3 single crystals (SCs) possess intriguing photoelectronic properties for x/γ-ray detection, the serious ion migration and high thermally activated carrier concentration at room temperature (RT), typically associated with defect states in CsPbBr3 crystals, result in a high dark current and drift of baseline, hindering their potential applications. In this investigation, liquid nitrogen cooling is proposed to freeze deep-level defects in CsPbBr3 SCs, thereby suppressing the ion migrations and decreasing the thermally excited carrier concentration. Utilizing photoluminescence (PL) and time-resolved PL spectra, coupled with theoretical models for photoexcitation and photoemission processes, the freezing of deep-level defects at liquid nitrogen temperature (LNT) is confirmed, which is conducive to decreasing non-radiative recombination. At LNT, the CsPbBr3 SC exhibits a higher resistivity of 4.95 × 1011 Ω cm and a higher mobility–lifetime product of 9.54 × 10−3 cm2 V−1, in contrast to the RT values of 3.86 × 109 Ω cm and 3.67 × 10−3 cm2 V−1, respectively. Furthermore, the x-ray detector at LNT exhibits a high sensitivity of 9309 μC Gyair−1 cm−2 and an impressively low detection limit of 0.054 nGy s−1, which offers a route for obtaining highly sensitive x-ray detectors for applications including ultra-low dose radiation imaging. [ABSTRACT FROM AUTHOR]

The multi-tier GW+EDMFT scheme is an ab-initio method for calculating the electronic structure of correlated materials. While the approach is free from ad-hoc parameters, it requires a selection of appropriate energy windows for describing low-energy and strongly correlated physics. In this study, we test the consistency of the multi-tier description by considering different low-energy windows for a series of cubic SrXO3 (X = V, Cr, Mn) perovskites. Specifically, we compare the 3-orbital t2g model, the 5-orbital t2g + eg model, the 12-orbital t2g + Op model, and (in the case of SrVO3) the 14-orbital t2g + eg + Op model and compare the results to available photoemission and X-ray absorption measurements. The multi-tier method yields consistent results for the t2g and t2g + eg low-energy windows, while the models with Op states produce stronger correlation effects and mostly agree well with experiment, especially in the unoccupied part of the spectrum. We also discuss the consistency between the fermionic and bosonic spectral functions and the physical origin of satellite features, and present momentum-resolved charge susceptibilities. [ABSTRACT FROM AUTHOR]

The indirect exchange interaction between local magnetic moments via surface electrons has been long predicted to bolster the surface ferromagnetism in magnetic topological insulators (MTIs), which facilitates the quantum anomalous Hall effect. This unconventional effect is critical to determining the operating temperatures of future topotronic devices. However, the experimental confirmation of this mechanism remains elusive, especially in intrinsic MTIs. Here, we combine time-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements to elucidate the unique electromagnetism at the surface of an intrinsic MTI MnBi2Te4. Theoretical modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the initial quenching of a surface-rooted exchange gap within a factor of two but overestimates the bulk demagnetization by one order of magnitude. This mechanism directly explains the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi2Te4. Furthermore, it leads to efficient light-induced demagnetization comparable to state-of-the-art magnetophotonic crystals, promising an effective manipulation of magnetism and topological orders for future topotronics. [ABSTRACT FROM AUTHOR]

Dichroic techniques are highly relevant in the field of topological materials, layered systems, and spin-polarized electronic states. Dichroism in angle-resolved photoemission is per se a matrix element effect, which depends on the initial and final states as well as on the perturbation by the light field. Although matrix element effects in ARPES such as dichroism are important for addressing properties of the initial state wave functions, the results can strongly depend on experimental geometry or final state effects. Combining experimental data on bulk WSe2 taken at soft x-ray photon energies with state-of-the-art photoemission calculations, we demonstrate that a dichroic observable called time-reversal dichroism remains unaffected against variation of photon energy, light polarization, and the angle of incidence. We demonstrate a direct link of TRDAD obtained with both linearly and circularly polarized photons to the initial state properties indicating its broad applicability. The robustness of this matrix element effect indicates a considerable benefit over other dichroic techniques and encourages further experimental and theoretical investigations. Matrix element effects in ARPES are important for addressing properties of the initial state wave functions, but the results can strongly vary with experimental configuration or final state. The authors show that an observable called time-reversal dichroism can filter out such effects, providing a direct link to orbital angular momentum and pseudospin. [ABSTRACT FROM AUTHOR]

Density fluctuation potential induced by a screening of the photohole scatters the photoelectron and generally causes its emission delay from the scattering matrix in the photoemission spectroscopy, where the photoemission delay usually quantifies the extrinsic loss of the photoelectron depending on the atomic orbital. Without the potential scattering, however, the photoemission from the coherent two-state mixture created by the laser driving is found to undergo the unexpected photoemission delay, which originates from the mixed photoemission matrix. Using the Haldane model, we analytically calculate such coherent mixing induced photoemission delay in an angle-resolved mode, which is found to reveal the local Berry curvature structure as long as the coherent mixing is sustained. This finding is confirmed through the streaking computation for the photoemission delay by solving the time-dependent Schrödinger equation and suggests that the photoemission delay be a new spectroscopic diagnosis of the material topology of two-dimensional semiconductors. [ABSTRACT FROM AUTHOR]

High-temperature superconductivity (HTSC) remains one of the most challenging and fascinating mysteries in condensed matter physics. Recently, superconductivity with transition temperature exceeding liquid-nitrogen temperature is discovered in La3Ni2O7 at high pressure, which provides a new platform to explore the unconventional HTSC. In this work, using high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structures of La3Ni2O7 at ambient pressure. Our experiments are in nice agreement with ab initio calculations after considering an orbital-dependent band renormalization effect. The strong electron correlation effect pushes a flat band of d z 2 orbital component below the Fermi level (E F), which is predicted to locate right at E F under high pressure. Moreover, the d x 2– y 2 band shows pseudogap-like behavior with suppressed spectral weight and diminished quasiparticle peak near E F. Our findings provide important insights into the electronic structure of La3Ni2O7, which will shed light on understanding of the unconventional superconductivity in nickelates. [ABSTRACT FROM AUTHOR]

High-temperature superconductivity (HTSC) remains one of the most challenging and fascinating mysteries in condensed matter physics. Recently, superconductivity with transition temperature exceeding liquid-nitrogen temperature is discovered in La3Ni2O7 at high pressure, which provides a new platform to explore the unconventional HTSC. In this work, using high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structures of La3Ni2O7 at ambient pressure. Our experiments are in nice agreement with ab initio calculations after considering an orbital-dependent band renormalization effect. The strong electron correlation effect pushes a flat band of d z 2 orbital component below the Fermi level (E F), which is predicted to locate right at E F under high pressure. Moreover, the d x 2– y 2 band shows pseudogap-like behavior with suppressed spectral weight and diminished quasiparticle peak near E F. Our findings provide important insights into the electronic structure of La3Ni2O7, which will shed light on understanding of the unconventional superconductivity in nickelates. [ABSTRACT FROM AUTHOR]

The growing interest in the growth and study of thin films of low-dimensional metallic delafossites, with the general formula ABO2, is driven by their potential to exhibit electronic and magnetic characteristics that are not accessible in bulk systems. The layered structure of these compounds introduces unique surface states as well as electronic and structural reconstructions, making the investigation of their surface behavior pivotal to understanding their intrinsic electronic structure. In this work, we study the surface phenomena of epitaxially grown PtCoO2, PdCoO2, and PdCrO2 films, utilizing a combination of molecular-beam epitaxy and angle-resolved photoemission spectroscopy. Through precise control of surface termination and treatment, we discover a pronounced 3 × 3 surface reconstruction in PtCoO2 films and PdCoO2 films, alongside a 2 × 2 surface reconstruction observed in PdCrO2 films. These reconstructions have not been reported in prior studies of delafossites. Furthermore, our computational investigations demonstrate the BO2 surface's relative stability compared to the A-terminated surface and the significant reduction in surface energy facilitated by the reconstruction of the A-terminated surface. These experimental and theoretical insights illuminate the complex surface dynamics in metallic delafossites, paving the way for future explorations of their distinctive properties in low-dimensional studies. [ABSTRACT FROM AUTHOR]

Spin-polarized photoemission from wurtzite and zinc blende gallium nitride (GaN) photocathodes has been observed and measured. The p-doped GaN photocathodes were epitaxially grown and activated to negative electron affinity with a cesium monolayer deposited on their surfaces. A field-retarding Mott polarimeter was used to measure the spin polarization of electrons photoemitted from the top of the valence band. A spectral scan with a tunable optical parametric amplifier constructed to provide low-bandwidth light revealed peak spin polarizations of 17% and 29% in the wurtzite and zinc blende photocathodes, respectively. Zinc blende GaN results are analyzed with a spin polarization model accounting for experimental parameters used in the measurements, while possible mechanisms influencing the obtained spin polarization values of wurtzite GaN are discussed. [ABSTRACT FROM AUTHOR]

A two-dimensional (2D) Weyl semimetal, akin to a spinful variant of graphene, represents a topological matter characterized by Weyl fermion-like quasiparticles in low dimensions. The spinful linear band structure in two dimensions gives rise to distinctive topological properties, accompanied by the emergence of Fermi string edge states. We report the experimental realization of a 2D Weyl semimetal, bismuthene monolayer grown on SnS(Se) substrates. Using spin and angle-resolved photoemission and scanning tunneling spectroscopies, we directly observe spin-polarized Weyl cones, Weyl nodes, and Fermi strings, providing consistent evidence of their inherent topological characteristics. Our work opens the door for the experimental study of Weyl fermions in low-dimensional materials. 2D Weyl semimetals are spin-polarized analogues of graphene that promise access to various topological properties of matter. Here, the authors evidence spin-polarized Weyl cones, Weyl nodes, and Fermi strings in monolayer bismuthene. [ABSTRACT FROM AUTHOR]

X‐ray photoelectron spectroscopy (XPS) provides quantitative information from photoemission peaks and shapes observed within the background due to the inelastic scattering of photoelectrons. To quantify the signal, both photoemission peaks and background in spectra must be adjusted for instrumental transmission variations that are a consequence of changes in efficiency when recording electrons with different kinetic energy. While it is generally assumed that correcting spectroscopic data for transmission is a necessary part of quantification by XPS, there are consequences for the quantification of spectra measured using an instrument for which transmission has significant curvature. In this Insight, the implications of curvature in transmission characteristics are discussed and a method based on XPS microscopy is proposed that ensures the transmission response of an instrument is free from significant curvature. An example of an instrument for which a flat transmission response is presented is achieved through collecting spectra using lens modes designed to measure stigmatic images. [ABSTRACT FROM AUTHOR]

GeSn materials with Sn contents ranging between 0% and 11% have been successfully grown by using Ge2H6 and SnCl4 in a hot-wall UHV-CVD system at low reaction pressures of 0.2–0.4 Torr. Both fully strained and partially relaxed materials have been produced. It was found that the materials were of high crystallinity as evidenced by few threading dislocations from cross-sectional TEM. Photoluminescence studies have shown that the PL spectra shift to longer wavelengths, and the PL intensity increases exponentially as the Sn contents increase. The photoemission cutoff for a 9.5% Sn sample is beyond 2600 nm, and low temperature PL measurements have confirmed that this sample is already in the direct gap material regime. Prototype photoconductor devices were fabricated with a partially strained 5% Sn sample, showing much smaller dark currents and similar spectral coverage as compared to a fully strained 7.8% Sn device reported in the literature. The reported technical route has provided a new path toward high quality, low cost GeSn epitaxy, and will contribute to the development of group IV optoelectronics. [ABSTRACT FROM AUTHOR]

We report on the energy dependence of the photoemission time delay from the single-element layered dielectric HOPG (highly oriented pyrolytic graphite). This system offers the unique opportunity to directly observe the Eisenbud-Wigner- Smith (EWS) time delays related to the bulk electronic band structure without being strongly perturbed by ubiquitous effects of transport, screening, and multiple scattering. We find the experimental streaking time shifts to be sensitive to the modulation of the density of states in the high-energy region (E ≈ 100 eV) of the band structure. The present attosecond chronoscopy experiments reveal an energy-dependent increase of the photoemission time delay when the final state energy of the excited electrons lies in the vicinity of the bandgap providing information difficult to access by conventional spectroscopy. Accompanying simulations further corroborate our interpretation. [ABSTRACT FROM AUTHOR]

The topology of the electronic band structure of solids can be described by its Berry curvature distribution across the Brillouin zone. We theoretically introduce and experimentally demonstrate a general methodology based on the measurement of energy-and momentum-resolved optical transition rates, allowing to reveal signatures of Berry curvature texture in reciprocal space. By performing time-and angle-resolved photoemission spectroscopy of atomically thin WSe2 using polarization-modulated excitations, we demonstrate that excitons become an asset in extracting the quantum geometrical properties of solids. We also investigate the resilience of our measurement protocol against ultrafast scattering processes following direct chiroptical transitions. [ABSTRACT FROM AUTHOR]

The integration of complex oxides with a wide range of functionalities on conventional semiconductor platforms is highly demanded for functional applications. Despite continuous efforts to integrate complex oxides on Si, it is still challenging to harvest epitaxial layers using standard deposition processes. Here, a novel method is demonstrated to create high‐quality complex heterostructures on Si integrated with SrTiO3 membranes as a universal platform. The STO membrane successfully bridges a broad spectrum of complex heterostructures such as SrNbO3, SrVO3, TiO2, and dichalcogenide 2D superconducting FeSe toward semiconducting wafers (Si). Through electronic structures measured by angle‐resolved photoemission spectroscopy, the high quality and functionality of the heterostructures are verified. This study demonstrated a new pathway toward realizing electronic devices with multifunctional physical properties incorporated into Si. [ABSTRACT FROM AUTHOR]

SrTiO3 (STO) is a well‐known perovskite oxide often used, among others, as a crystalline substrate for epitaxial deposition. However, its indirect bandgap of 3.25 eV is too high for solar applications. Tentative experiments to reduce its bandgap are very welcome. To this end, sulfurization of STO substrates using ion implantation is performed. The simulated profile is controlled by time‐of‐flight secondary‐ion mass spectrometry. S is at least partially inserted into the STO lattice, as evidenced by X‐ray diffraction and X‐ray photoelectron spectroscopy. UV–vis spectroscopy indicates a drastic reduction of the bandgap of S:STO from 3.25 eV (indirect) down to 2.14 eV for 10% S:STO, a trend also observed by spectroscopic ellipsometry. The results are confronted to ab initio calculations. Besides bandgap, the valence band level, that is the ionization energy, is determined by ambient‐pressure photoemission spectroscopy so that the complete energy diagrams could be plotted. This work paves the way for tuning the bandgap of perovskites in a highly controlled manner for solar energy applications. [ABSTRACT FROM AUTHOR]

Two-dimensional borophene nanosheets are sensitive to oxygen, which has hindered their development and use in scientific research and subsequent applications. Conventional methods of borophene synthesis are based on its epitaxial growth on single-crystal metal substrates by using molecular beam epitaxy (MBE). The electron deficiency of boron atoms in the borophene structure has made the use of metal substrates in the growth process inevitable. Hence, a method to stabilize the borophene sheets for further applications seems necessary. In this work, the oxidation arrangement of Al-activated chemical vapor deposition (CVD) of borophene sheets was investigated via scanning photoemission microscopy (SPEM). This method allows the investigation of oxidation ordering via its high spatial resolution and elemental sensitivity. The results indicate the persistence of the borophene χ3 (and some β12) phase after exposure to air. The formation of boron oxide is primarily limited to the Al aggregated islands and the edges while maintaining the borophene configuration intact. Our results show that the electronic exchange between the Al aggregates and the interface of boron atoms at the edges of the borophene sheet can prevent the oxidation process and preserve the borophene sheets for long periods. The lower oxidation tendency of the Al-saturated borophene was confirmed via density functional theory calculation, showing a lower oxygen binding energy on Al/borophene compared with that of the metal-free borophene. The oxidation-resistive borophene can be exploited in innovative high-efficiency catalytic, electronic, and sensing devices. [ABSTRACT FROM AUTHOR]