The development of high-performance electron sources requires theoretical frameworks that accurately link the microscopic electronic properties of cathode materials to their macroscopic photoemission observables. Here, we present a many-body extension of the three-step photoemission model for semiconducting photocathodes, directly integrating the GW approximation and the solution of the Bethe-Salpeter equation on top of density functional theory (DFT). This approach overcomes the intrinsic limitations of standard DFT by explicitly accounting for quasiparticle and excitonic effects in the photoexcitation process. The quantum efficiency (QE) is evaluated by combining the ab initio absorption with an emission probability derived as an exciton-weighted average. We validate this model on representative alkali antimonides and demonstrate that a qualitative many-body description successfully captures complex spectral features that empirical models fail to reproduce. Furthermore, by incorporating macroscopic optical effects such as thin-film interference and polarization via Fresnel post-processing, we achieve quantitative agreement with experimental QE values without any adjustment. Minor discrepancies near the photoemission threshold are attributed to the idealized surface barrier adopted in the model and impurity effects in the samples, highlighting specific directions for future refinements. This work establishes a robust, parameter-free ab initio tool that bridges microscopic electronic correlation with macroscopic observables, providing a critical pathway for the rational design of next-generation electron sources.

The aviation sector is currently transitioning towards hybrid electric aircraft, driven by sustainability imperatives and technological progress. This article examines fuel cells’ potential to meet aeronautical power demands, analyzing the state of the art to identify key performance indicators (KPIs) and research challenges in advancing hydrogen-based aviation. Several technologies, including proton exchange membrane and solid oxide fuel cells, are evaluated as candidates for on-board installation. Then, the following research areas are identified and discussed: design, control, thermal management, and degradation. These interconnected tasks are essential for advancing the state of the art, a goal achievable through effective modeling approaches at the individual component and system levels. One of the KPIs requiring substantial improvement is the system’s mass-to-power ratio. This metric largely relies on the integration of advanced materials and manufacturing techniques at the stack level, aimed at reducing the bipolar plates mass and optimizing membrane electrode assembly performance to increase the operating temperature, thus leading to lighter and more compact thermal management systems. At system level, enhancing the hydrogen storage tank’s gravimetric capacity is a priority in keeping the aircraft’s maximal take-off mass (MTOM) within acceptable limits. Moreover, the integrated sizing of the fuel cell system alongside energy storage (e.g., batteries) and the development of multi-level control strategies can help mitigate MTOM increase, optimize performance, and enhance the durability of hydrogen-based devices. Finally, the original equipment manufacturers of fuel cell systems for the transportation sector, particularly aviation, are identified to offer insights into ongoing efforts towards achieving net-zero aviation.

Interface chemistry and defect formation in MoS2 thin films grown on single crystal substrates critically determine the electronic structure of MoS2 and thus can strongly modify material functionality relevant for many applications, including electronics, optoelectronics, and energy related catalysis. We investigate MoS2 grown on three technologically relevant substrates, namely SrTiO3(111), c-axis Al2O3(0001) and 6H-SiC(0001). Experimental investigations by temperature dependent resistivity, photoemission spectroscopy and scanning transmission electron microscopy with coupled energy dispersive spectroscopy, with the support of theoretical calculation by Density Functional Theory, allow the identification of the substrate induced specific defects and their correlation with the electronic properties. Ti interdiffusion in SrTiO3/MoS2 generates donor like states near the Fermi level, leading to metallic transport. Al2O3/MoS2 exhibits a high density of sulfur related defects that introduce localized states and yield nearly temperature independent conductivity. SiC/MoS2 exhibits significant interface disorder resulting in a semiconducting temperature dependent resistivity, yet deviating from the ideal bulk like behavior. These results demonstrate how substrate choice governs defect formation and ultimately dominates the electronic behavior of MoS2 thin films, making the control of film substrate interactions essential for the engineering of new functional devices.

We experimentally demonstrate a method for reconstructing the transverse 4D phase space of an electron beam at the time of emission from downstream diagnostics of the 4D phase space. This method does not rely on detailed knowledge of the beamline transport, besides assuming that linearity and symplecticity are satisfied. We apply this method to measure the transverse position and momentum phase space of electrons emitted from a spatially-structured alkali-antimonide cathode. This method can uncover local correlations between emission location and momentum spread. We formulate this method analytically and investigate resolution limits.

Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO2 and SrCuO2 that enables fair comparison of their interaction strengths. For both compounds, we study the orbital and magnetic excitations and extract their dispersions along high-symmetry directions. Using a single-band Hubbard model and including physically plausible assumptions about higher-order exchange interactions, we estimate the correlation factor U/t for both compounds. A key finding is that despite the prediction of a smaller Coulomb repulsion U, PrNiO2 exhibits a correlation strength U/t that is 20% stronger than that of its isostructural cuprate counterpart SrCuO2. This indicates that moderation of the correlation strength may further optimize superconductivity in nickelates.

Molybdenum disulfide (MoS2) has emerged as a promising, cost-effective catalyst for hydrogen production via water splitting. We investigate the structural and electrocatalytic properties of MoS2 thin films deposited on different substrates (Al2O3, SiC, STO) to study their hydrogen evolution reaction (HER) activity. In particular, in order to study the substrate influence on the stabilization of different polymorphic MoS2 phases, the films are synthesised using pulsed laser deposition on substrates with different crystal symmetries and lattice parameters. All the deposited samples are characterized by X-Ray Diffraction, Raman Spectroscopy, Linear Sweep Voltammetry and Electrochemical Impedance Spectroscopy analyses. The films grown on Al2O3 substrates exhibit the best HER performance, likely due to the stabilization of the metastable 1T phase through the interfacial interactions between film and substrate. Presence of the 1T phase in the samples grown on Al2O3 improves the charge transfer efficiency and the electrochemically active surface with a better response to the applied potential, demonstrating their enhanced catalytic behaviour for hydrogen evolution.

The development of high-performance photocathodes is essential for generating high-brightness electron beams required by existing and future accelerators. This work introduces a state-of-the-art triple evaporation growth system designed for bialkali antimonide photocathodes. By enabling the simultaneous deposition of all three materials, this system significantly enhances vacuum stability and the reproducibility of photocathode fabrication. Complementing this, the novel characterization system PhoTEx allows spatially and spectrally resolved measurements of key photocathode parameters, such as quantum efficiency (QE), mean transverse energy (MTE), reflectance and lifetime. Crucially, all measurements are performed within a single compact setup, without moving the sample, preserving ultra-high vacuum conditions. The spectral resolved measurement of the reflectance allows the investigation of the color. Photocathode colorimetry may provide valuable insights into material homogeneity and aging. A Na-K-Sb photocathode was grown using the triple evaporation method, achieving an initial QE of 5.5 % at 520nm. The photocathode was characterized at PhoTEx over two months, demonstrating consistent MTE measurements and a dataset with spectral response, reflectance and colorimetry data. Together, the triple evaporation growth system and PhoTEx mark a significant advancement in optimizing photocathodes with exceptional performance, paving the way for brighter and more stable electron sources for next-generation accelerator facilities.

Understanding the intricate electrochemical processes in solid oxide fuel cell (SOFC) components, especially air electrodes, is crucial for enhancing device performance and durability. In this work, following recent results showing important improvements in the electrochemical performance of SOFCs with room-temperature sputtered gadolinium-doped ceria (GDC) barrier layers, we investigate by standard Distribution of Relaxation Time (DRT) analysis the influence of the GDC thickness and uniformity on the cathodic reaction kinetics. The outcomes highlight the role played by the GDC thickness in the oxygen reactions at the electrolyte/cathode interface and the GDC thickness uniformity in the anode electrochemical charge transfer reactions involving oxygen ions from the cathode. The results of our work, obtained for industrial-scale SOFCs, are particularly interesting in view of enhancing both their performance and stability.

Alkali antimonides are well established as high efficiency, low intrinsic emittance photocathodes for accelerators and photon detectors. However, conventionally grown alkali antimonide films are polycrystalline with surface disorder and roughness that can limit achievable beam brightness. Ordering the crystalline structure of alkali antimonides has the potential to deliver higher brightness electron beams by reducing surface disorder and enabling the engineering of material properties at the level of atomic layers. In this report, we demonstrate the growth of ordered Cs3Sb films on single crystal substrates 3C-SiC and graphene-coated 4H-SiC using pulsed laser deposition and conventional thermal evaporation growth techniques. The crystalline structures of the Cs3Sb films were examined using reflection high energy electron diffraction and x-ray diffraction diagnostics, while film thickness and roughness estimates were made using x-ray reflectivity. With these tools, we observed ordered domains in less than 10 nm thick films with quantum efficiencies greater than 1% at 530 nm. Moreover, we identify structural features such as Laue oscillations indicative of highly ordered films. We found that Cs3Sb films grew with flat, fiber-textured surfaces on 3C-SiC and with multiple ordered domains and sub-nanometer surface roughness on graphene-coated 4H-SiC under our growth conditions. We identify the crystallographic orientations of Cs3Sb grown on graphene-coated 4H-SiC substrates and discuss the significance of examining the crystal structure of these films for growing epitaxial heterostructures in future experiments.

Spinel-type lithium manganese oxide (LiMn2O4) is considered one of the most promising cathode materials for rechargeable batteries due to its high operating voltage, reduced toxicity, and lower cost compared to cobalt-based cathodes. However, the stability of LiMn2O4 is a significant challenge, as it tends to degrade over time, with manganese ion segregation leading to a reduction in battery capacity.The crystal structure of LixMn2O4 is highly dependent on the lithium content (x), and it can exist either in a single-phase or a two-phase form. These structural variations significantly affect the material's electrochemical performance. In this work, we present the growth and structural characterization of LixMn2O4 thin films, with (x) values of 1 and 0.7, aimed at improving chemical stability and overall performance.The epitaxial LixMn2O4 films were deposited using Pulsed Laser Deposition on different single-crystal substrates, including water-soluble Sr3Al2O6-buffered SrTiO3(100). Films grown on MgO(001) for both x = 1 and x = 0.7 exhibited excellent crystallographic quality, while films deposited on SrTiO3(001) showed good quality for x = 1, albeit with a slightly higher mosaic spread compared to those on MgO. Notably, for x = 0.7, a two-phase region was observed on the SrTiO3 and MgO substrates, where both phases shared the same structure but differed slightly in lattice parameters.

Alkali antimonide photocathodes exhibit high efficacy as photoemissive materials in electron sources. This proceeding explores the fabrication of thin, ordered films of sodium potassium antimonide via molecular-beam epitaxy (MBE) at the PHotocathode Epitaxy Beam Experiments (PHOEBE) laboratory at Cornell University. Utilizing a sequential deposition technique, the photocathodes are characterized in terms of both quantum efficiency (QE) and crystal structure with the goal of reducing the chemical and physical roughness. A spectral response from 400 to 700 nm demonstrates oscillations resulting from optical interference within the (SiN) substrate. Reflection high-energy electron diffraction (RHEED) patterns confirmed the successful growth of ordered crystal structures for the first time in a sodium potassium antimonide photocathode. Additionally, we investigated the photocathodes' sensitivity to oxidation, revealing their relative robustness compared to CsSb or KSb photocathodes. Notably, the incorporation of higher partial pressures of oxygen during growth improved QE and extended the operational lifetime of the photocathodes.

Cornell University has been working on developing techniques to grow single crystal photocathodes for electron sources using the Molecular Beam Epitaxy (MBE) technique. As a result, the first single crystal Cs3Sb photocathode was produced, which has shown high quantum efficiency and is expected to have a low Mean Transverse Energy (MTE). Now, other alkali materials are being explored. In this work, we report the epitaxial growth of Na-Sb photocathodes at the PHOtocathode Epitaxy Beam Experiments (PHOEBE) laboratory at Cornell University, employing a sequence of shuttered growth steps to form distinct unit cells. The photocathodes were characterized by Quantum Efficiency (QE) measurements and Reflection High-Energy Electron Diffraction (RHEED) patterns collected during growth. The RHEED streaky pattern shows angle dependence, confirming their single crystal structure. Notably, these Na-Sb photocathodes exhibited a QE exceeding 1% at 400 nm, which is much higher than previous reports on this compound. The possible reasons for this discrepancy are discussed.

Renewable electricity-powered water-splitting offers a clean pathway to produce green hydrogen fuel, vital for energy security and emissions reduction. However, optimizing this process requires bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under different conditions. Transition metal-based composites, such as Co-doped MoS2, are potentially suitable for this application but important questions regarding their mechanisms of water-splitting reaction remain unanswered. In this work, we employ in situ soft-X-ray absorption spectroscopy, combined with ex situ X-ray photoelectron spectroscopy and X-ray diffraction, to probe Co and Mo electronic states upon different environmental conditions (water exposure and applied voltage bias) shedding light on the role of cobalt in the catalytic mechanisms and advancing sustainable energy solutions.

This study investigates the temperature-induced hysteretic behavior of resistivity and magnetoresistance in electrodeposited bismuth films, with a focus on their application as absorbers in transition-edge sensors (TESs) for X-ray detection. Through a series of resistivity versus temperature measurements from room temperature to a few Kelvin, we explore the change in the conductive behavior of bismuth electrodeposited on various substrates. Our findings show for the first time both hysteretic and irreversible changes in resistivity as a function of temperature. Further, magnetoresistance measurements reveal notable variations in resistance behavior under different magnetic fields, highlighting the impact of magnetic fields on these films’ electronic transport properties, with an indication of potential weak anti-localization effects at the lowest temperatures. This study not only provides a deeper understanding of bismuth’s conductivity characteristics at low temperatures but also sheds light on the practical implications for developing more effective TESs for synchrotron X-ray facilities.

Photocathodes activated to negative electron affinity with a cesium-based activation layer, such as GaAs and GaN, can be used for generating spin-polarized electron beams, but their extreme sensitivity to chemical poisoning limits their operational lifetimes. This work demonstrates that applying and subsequently heating a cesium iodide (CsI) coating can produce a more durable activation layer lacking iodine, but rich in stable cesium suboxides (formal O oxidation state >−2), which significantly extend the dark lifetimes of both GaAs and GaN photocathodes. Through x-ray photoelectron spectroscopy, we examine the stability and formation of these Cs suboxides, which exhibit remarkable resistance to chemical poisoning. Additionally, we investigate the subsequent surface quality using atomic force microscopy. Our findings show that CsI-based surface treatments not only prolong photocathode lifetime but also maintain high spin polarization, positioning this method as a promising approach for enhancing photocathode durability in demanding applications.

We investigate the ionic mobility in room-temperature RF-sputtered gadolinium doped ceria (GDC) thin films grown on industrial solid oxide fuel cell substrates as a function of the air-annealing at 800 and 1000 degrees C. The combination of X-ray diffraction, X-ray photoelectron spectroscopy, operando X-ray absorption spectroscopy, and Raman spectroscopy allows us to study the different Ce3+/ Ce4+ ratios induced by the post growth annealing procedure, together with the Ce valence changes induced by different gas atmosphere exposure. Our results give evidence of different kinetics as a function of the annealing temperature, with the sample annealed at 800 degrees C showing marked changes of the Ce oxidation state when exposed to both reducing and oxidizing gas atmospheres at moderate temperature (300 degrees C), while the Ce valence is weakly affected for the 1000 degrees C annealed sample. Raman spectra measurements allow us to trace the responses of the investigated samples to different gas atmospheres on the basis of the presence of different Gd-O bond strengths inside the lattice. These findings provide insight into the microscopic origin of the best performances already observed in SOFCs with a sputtered GDC barrier layer annealed at 800 degrees C and are fundamental to further improve sputtered GDC thin film performance in energy devices.

Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO_2, single- and bi-magnon dispersions are derived. Using an effective Heisenberg Hamiltonian generated from the Hubbard model, we show that the single-magnon dispersion is only described satisfactorily when including significant quantum corrections stemming from magnon-magnon interactions. Comparative results on La_2CuO_4 indicate that quantum fluctuations are much stronger in SrCuO_2 suggesting closer proximity to a magnetic quantum critical point. Monte Carlo calculations reveal that other magnetic orders may compete with the antiferromagnetic Néel order as the ground state. Our results indicate that SrCuO_2 - due to strong quantum fluctuations - is a unique starting point for the exploration of novel magnetic ground states.

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.

We investigated the influence of a fuel change from pure hydrogen to a hydrogen–ammonia mixture at different percentages on the electrochemical behavior of 50 mm in diameter Solid Oxide Fuel Cells (SOFCs) with sputtered thin buffer layers of Gd-doped ceria, varying the working temperatures from 800 °C to 650 °C. The results show that the performances of the cells are not affected by the fuel change for high working temperatures (800 °C and 750 °C). As an example, a power density value of 802 mW∙cm−2 at 1 A∙cm−2 is found when directly feeding the cell with 8 NmL∙min−1cm−2 of ammonia and with an equivalent flowrate of 12 NmL∙min−1cm−2 of H2. These power density output values are higher than those obtained in industrial state-of-art (SoA) SOFCs with screen-printed buffer layers fed with equivalent hydrogen flowrates, thanks to the improved electrochemical performances obtained in the case of cells with sputtered thin buffer layers of Gd-doped ceria. At lower working temperatures (700 °C and 650 °C), slight changes in the electrochemical behavior of the cells are observed. Nevertheless, in this temperature range, we also obtain an output current density value of 0.54 A∙cm−2 in a pure ammonia flowrate of 12 NmL min−1cm−2 at 800 mV and 700 °C, equal to the value observed in SoA button cells with industrial screen-printed GDC barrier layer fueled with 16 NmL∙min−1cm−2 of H2. These results pave the way towards the use of innovative SOFC structures with sputtered thin buffer layers fueled by ammonia.