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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. However, 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 550 nm, low surface roughness (down to 600 pm on a 1 um scale), and a robustness against oxidation up to 15 times greater then Cs3Sb. These properties suggest that CsSb has the potential to operate as an alternative to Cs$_3$Sb in electron source applications where the demands of the vacuum environment might otherwise preclude the use of traditional alkali antimonides., Comment: 11 pages, 6 figures, 1 table

Superconducting nickelates are a new family of strongly correlated electron materials with a phase diagram closely resembling that of superconducting cuprates. While analogy with the cuprates is natural, very little is known about the metallic state of the nickelates, making these comparisons difficult. We probe the electronic dispersion of thin-film superconducting 5-layer ($n=5$) and metallic 3-layer ($n=3$) nickelates by measuring the Seebeck coefficient, $S$. We find a temperature-independent and negative $S/T$ for both $n=5$ and $n=3$ nickelates. These results are in stark contrast to the strongly temperature-dependent $S/T$ measured at similar electron filling in the cuprate La$_{1.36}$Nd$_{0.4}$Sr$_{0.24}$CuO$_4$. The electronic structure calculated from density functional theory can reproduce the temperature dependence, sign, and amplitude of $S/T$ in the nickelates using Boltzmann transport theory. This demonstrates that the electronic structure obtained from first-principles calculations provides a reliable description of the Fermiology of superconducting nickelates, and suggests that, despite indications of strong electronic correlations, there are well-defined quasiparticles in the metallic state. Finally, we explain the differences in the Seebeck coefficient between nickelates and cuprates as originating in strong dissimilarities in impurity concentrations. Our study demonstrates that the high elastic scattering limit of the Seebeck coefficient reflects only the underlying band structure of a metal, analogous to the high magnetic field limit of the Hall coefficient. This opens a new avenue for Seebeck measurements to probe the electronic band structures of relatively disordered quantum materials., Comment: 13 pages, 10 figures

The properties of photoemission electron sources determine the ultimate performance of a wide class of electron accelerators and photon detectors. To date, all high-efficiency visible-light photocathode materials are either polycrystalline or exhibit intrinsic surface disorder, both of which limit emitted electron beam brightness. In this letter we demonstrate the synthesis of epitaxial thin films of Cs$_3$Sb on 3C-SiC (001) using molecular-beam epitaxy. Films as thin as 4 nm have quantum efficiencies exceeding 2\% at 532 nm. We also find that epitaxial films have an order of magnitude larger quantum efficiency at 650 nm than comparable polycrystalline films on Si. Additionally, these films permit angle-resolved photoemission spectroscopy measurements of the electronic structure, which are found to be in good agreement with theory. Epitaxial films open the door to dramatic brightness enhancements via increased efficiency near threshold, reduced surface disorder, and the possibility of engineering new photoemission functionality at the level of single atomic layers., Comment: Submitted Manuscript: 8 pages, 4 figures

Ultrawide-band-gap (UWBG) semiconductors are promising for fast, compact, and energy-efficient power-electronics devices. Their wider band gaps result in higher breakdown electric fields that enable high-power switching with a lower energy loss. Yet, the leading UWBG semiconductors suffer from intrinsic materials limitations with regards to their doping asymmetry that impedes their adoption in CMOS technology. Improvements in the ambipolar doping of UWBG materials will enable a wider range of applications in power electronics as well as deep- UV optoelectronics. These advances can be accomplished through theoretical insights on the limitations of current UWBG materials coupled with the computational prediction and experimental demonstration of alternative UWBG semiconductor materials with improved doping and transport properties. As an example, we discuss the case of rutile GeO$_2$ (r-GeO$_2$), a water-insoluble GeO$_2$ polytype which is theoretically predicted to combine an ultra-wide gap with ambipolar dopability, high carrier mobilities, and a higher thermal conductivity than \b{eta}-Ga$_2$O$_3$. The subsequent realization of single-crystalline r-GeO$_2$ thin films by molecular beam epitaxy provides the opportunity to realize r-GeO$_2$ for electronic applications. Future efforts towards the predictive discovery and design of new UWBG semiconductors include advances in first-principles theory and high-performance computing software, as well as the demonstration of controlled doping in high-quality thin films with lower dislocation densities and optimized film properties., Comment: 30 pages, 5 figures

Large single crystals of the double-perovskite Ba$_2$ScNbO$_6$ were grown from the melt for the first time. With a lattice parameter at room temperature of 4.11672(1) \r{A}, this cubic double-perovskite has an excellent lattice match to BaSnO$_3$, PbZr$_{0.9}$Ti$_{0.1}$O$_3$, LaInO$_3$, BiScO$_3$, and other perovskites of contemporary interest. Differential thermal analysis showed that Ba$_2$ScNbO$_6$ melts at 2165$\pm$30{\deg}C in an inert atmosphere. Competitive grain growth was visualized by energy dispersive Laue mapping. X-ray diffraction rocking curve measurements revealed full width at half maximum values between 21 and 33 arcsec for 002 and 004 reflections. The crystals were sufficiently large to yield (100)-oriented single-crystal substrates with surface areas as large as 10 x 10 mm$^2$., Comment: 15 pages, 8 figures, 2 tables, submitted to J. Crystal Growth

We investigate the electronic structure of the epitaxial VO$_2$ films in the rutile phase using the density functional theory combined with the slave spin method (DFT+SS). In DFT-SS, the multiorbital Hubbard interactions are added to a DFT-fit tight-binding model, and we employ the slave-spin method to treat the electron correlation. We find that while stretching the system along the rutile $c$-axis results in a band structure favoring an anisotropic orbital fillings, the electron correlation favors an equal electron filling among $t_{2g}$ orbitals. These two distinct effects cooperatively induce interesting orbital-dependent redistributions of the electron occupations and the spectral weights, which pushes the strained VO$_2$ toward an orbital selective Mott transition (OSMT). The simulated single-particle spectral functions are directly compared to V L-edge resonant X-ray photoemission spectroscopy of epitaxial 10 nm VO$_2$/TiO$_2$ (001) and (100) strain orientations. Excellent agreement is observed between the simulations and experimental data regarding the strain-induced evolution of the lower Hubbard band. Simulations of rutile NbO$_2$ under similar strain conditions as VO$_2$ are performed, and we predict that OSMT will not occur in rutile NbO$_2$. Our results indicates that the electron correlation in VO$_2$ is important and can be modulated even in the rutile phase before the Peierls instability sets in., Comment: 8 pages, 9 figures, invited paper for Special Topic: Strain Engineering in Functional Materials by Journal of Applied Physics

We analyze the structural and magnetic characteristics of (111)-oriented lutetium iron garnet (Lu$_3$Fe$_5$O$_{12}$) films grown by molecular-beam epitaxy, for films as thin as 2.8 nm. Thickness-dependent measurements of the in- and out-of-plane ferromagnetic resonance allow us to quantify the effects of two-magnon scattering, along with the surface anisotropy and the saturation magnetization. We achieve effective damping coefficients of $11.1(9) \times 10^{-4}$ for 5.3 nm films and $32(3) \times 10^{-4}$ for 2.8 nm films, among the lowest values reported to date for any insulating ferrimagnetic sample of comparable thickness., Comment: 14 pages, 4 figures

We present evidence of strain-induced modulation of electron correlation effects and increased orbital anisotropy in the rutile phase of epitaxial VO$_2$/TiO$_2$ films from hard x-ray photoelectron spectroscopy and soft V L-edge x-ray absorption spectroscopy, respectively. By using the U(1) slave spin formalism, we further argue that the observed anisotropic correlation effects can be understood by a model of orbital selective Mott transition at a filling that is non-integer, but close to the half-filling. Because the overlaps of wave functions between $d$ orbitals are modified by the strain, orbitally-dependent renormalizations of the bandwidths and the crystal fields occur with the application of strain. These renormalizations generally result in different occupation numbers in different orbitals. We find that if the system has a non-integer filling number near the half-filling such as for VO$_2$, certain orbitals could reach an occupation number closer to half-filling under the strain, resulting in a strong reduction in the quasiparticle weight $Z_{\alpha}$ of that orbital. Moreover, an orbital selective Mott transition, defined as the case with $Z_{\alpha} = 0$ in some, but not all orbitals, could be accessed by epitaxial strain-engineering of correlated electron systems., Comment: 7 pages, 6 figures. First two authors contribute equally

We report the growth of ultrathin VO$_{2}$ films on rutile TiO$_{2}$ (001) substrates via reactive molecular-beam epitaxy. The films were formed by the cyclical deposition of amorphous vanadium and its subsequent oxidation and transformation to VO$_{2}$ via solid-phase epitaxy. Significant metal-insulator transitions were observed in films as thin as 2.3 nm, where a resistance change {\Delta}R/R of 25 was measured. Low angle annular dark field scanning transmission electron microscopy was used in conjunction with electron energy loss spectroscopy to study the film/substrate interface and revealed the vanadium to be tetravalent and the titanium interdiffusion to be limited to 1.6 nm., Comment: 25 pages, 6 figures

Space-based experiments today can uniquely address important questions related to the fundamental laws of Nature. In particular, high-accuracy physics experiments in space can test relativistic gravity and probe the physics beyond the Standard Model; they can perform direct detection of gravitational waves and are naturally suited for precision investigations in cosmology and astroparticle physics. In addition, atomic physics has recently shown substantial progress in the development of optical clocks and atom interferometers. If placed in space, these instruments could turn into powerful high-resolution quantum sensors greatly benefiting fundamental physics. We discuss the current status of space-based research in fundamental physics, its discovery potential, and its importance for modern science. We offer a set of recommendations to be considered by the upcoming National Academy of Sciences' Decadal Survey in Astronomy and Astrophysics. In our opinion, the Decadal Survey should include space-based research in fundamental physics as one of its focus areas. We recommend establishing an Astronomy and Astrophysics Advisory Committee's interagency ``Fundamental Physics Task Force'' to assess the status of both ground- and space-based efforts in the field, to identify the most important objectives, and to suggest the best ways to organize the work of several federal agencies involved. We also recommend establishing a new NASA-led interagency program in fundamental physics that will consolidate new technologies, prepare key instruments for future space missions, and build a strong scientific and engineering community. Our goal is to expand NASA's science objectives in space by including ``laboratory research in fundamental physics'' as an element in agency's ongoing space research efforts., Comment: a white paper, revtex, 27 pages, updated bibliography

We study the quantum mechanical behavior of a macroscopic, three-body, superconducting circuit. Microwave spectroscopy on our system, a resonator coupling two large Josephson junctions, produced complex energy spectra well explained by quantum theory over a large frequency range. By tuning each junction separately into resonance with the resonator, we first observe strong coupling between each junction and the resonator. Bringing both junctions together into resonance with the resonator, we find spectroscopic evidence for entanglement between all three degrees of freedom and suggest a new method for controllable coupling of distant qubits, a key step toward quantum computation., Comment: 4 pages, 3 figures