Brendan D. Faeth

Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (11̄02) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy and thermal laser epitaxy, demonstrating single-oriented, monodomain growth of anisotropically strained thin films. Microstructural investigation is performed using synchrotron x-ray diffraction and scanning transmission electron microscopy. The tetravalent oxidation state of tantalum is confirmed using x-ray absorption and photoemission spectroscopy as well as electron energy-loss spectroscopy. Optical properties are investigated via spectroscopic ellipsometry and reveal a 0.3 eV Mott gap of the tantalum 5d electrons. Density-functional theory and group theoretical arguments are used to evaluate the limited stability of the rutile phase and reveal the potential to unlock a hidden metal–insulator transition concomitant with a structural phase transition to a distorted rutile phase, akin to NbO2. Our study expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.

Common belief is that the large band shifts observed in incommensurate misfit compounds, e.g., (LaSe)_{1.14}(NbSe_{2})_{2}, are due to interlayer charge transfer. By contrast, our analysis, based on both angle-resolved photoemission spectroscopy (ARPES) measurements and a specialized ab initio framework employing only quantities well defined in incommensurate materials, demonstrates that the large band shifts instead reflect changes in valence band hybridization and interlayer bonding. The strong alignment of our ab initio predictions and ARPES measurements confirms our understanding of the incommensurate electronic structure and charge transfer.

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.

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Oxides have attracted enormous attention for both application-driven and fundamental solid-state research owing to their broad range of unusual and unique properties. Oxides play an important role in countless applications, e.g., as catalysts or functional materials in electronic devices. The ability to produce high-quality epitaxial films is often crucial for these purposes. Various approaches to oxide epitaxy have been evolving for many years, each of which has its own features and strengths. However, oxide epitaxy also poses numerous challenges, the main ones being (1) the difficulty of finding a universal, versatile, and clean way to transfer an element from a source onto a substrate and (2) the ability to control the phase formation in a growing film. For oxides, this is an especially relevant topic due to the high oxidization potentials needed to achieve many desired compounds, the high temperatures required for numerous oxide phases to form, and the high temperatures necessary to grow films in adsorption-controlled growth modes. We provide a non-exhaustive overview of the state-of-the-art of oxide epitaxy within the context of these challenges. We also examine exciting advances and recent trends to overcome those challenges. Concluding, we discuss the implications of ongoing developments and the future of oxide epitaxy. An emphasis is put on thermal laser epitaxy and CO2 laser heaters, which we deem especially promising.

Alkali antimonide semiconductor photocathodes provide a promising platform for the generation of high-brightness electron beams, which are necessary for the development of cutting-edge probes, including x-ray free electron lasers and ultrafast electron diffraction. Nonetheless, to harness the intrinsic brightness limits in these compounds, extrinsic degrading factors, including surface roughness and contamination, must be overcome. By exploring the growth of CsxSb thin films monitored by in situ electron diffraction, the conditions to reproducibly synthesize atomically smooth films of CsSb on 3C–SiC (100) and graphene-coated TiO2 (110) substrates are identified, and detailed structural, morphological, and electronic characterization is presented. These films combine high quantum efficiency in the visible (up to 1.2% at 400 nm), an easily accessible photoemission threshold of 566 nm, low surface roughness (down to 600 pm on a 1 μm scale), and a robustness against oxidation up to 15 times greater than Cs3Sb. These properties lead us to suggest that CsSb has the potential to operate as an alternative to Cs3Sb in electron source applications where the demands of the vacuum environment might otherwise preclude the use of traditional alkali antimonides.

Plasma discharge lamps are widely utilized in the practice of angle-resolved photoemission spectroscopy (ARPES) experiments as narrow-linewidth ultraviolet photon sources. However, many emission lines such as Ar-I, Ne-I, and Ne-II have closely spaced doublet emission lines, which result in superimposed replica on the measured ARPES spectra. Here, we present a simple method for subtracting the contribution of these doublet emission lines from photoemission spectra. Benchmarking against ARPES spectra of well-characterized 2D materials, we demonstrate that this algorithm manages to subtract the doublet signal and reproduce the key features of the monochromated He-I$α$ spectra in a physically sound manner that reliably reproduces quantifiable dispersion relations and quasiparticle lifetimes.

Moiré superlattices constructed from transition metal dichalcogenides have demonstrated a series of emergent phenomena, including moiré excitons, flat bands, and correlated insulating states. All of these phenomena depend crucially on the presence of strong moiré potentials, yet the properties of these moiré potentials, and the mechanisms by which they can be generated, remain largely open questions. Here, we use angle-resolved photoemission spectroscopy with submicron spatial resolution to investigate an aligned WS ₂ /WSe ₂ moiré superlattice and graphene/WS ₂ /WSe ₂ trilayer heterostructure. Our experiments reveal that the hybridization between moiré bands in WS ₂ /WSe ₂ exhibits an unusually large momentum dependence, with the splitting between moiré bands at the Γ point more than an order of magnitude larger than that at K point. In addition, we discover that the same WS ₂ /WSe ₂ superlattice can imprint an unexpectedly large moiré potential on a third, separate layer of graphene (g/WS ₂ /WSe ₂ ), suggesting new avenues for engineering two-dimensional moiré superlattices.