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 ${\mathrm{Cs}}_3\mathrm{Sb}$ 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.

The magnetic and electronic modifications induced at the interfaces in ${\left({\text{SrMnO}}_3\right)}_n/{\left({\text{LaMnO}}_3\right)}_{2n}$ superlattices have been investigated by linear and circular magnetic dichroism in the $\text{Mn}{L}_{2,3}$ x-ray absorption spectra. Together with theoretical calculations, our data demonstrate that the charge redistribution across interfaces favors in-plane ferromagnetic (FM) order and $e_g\left(x^2-y^2\right)$ orbital occupation, in agreement with the average strain. Far from interfaces, inside ${\text{LaMnO}}_3$, electron localization and local strain favor antiferromagnetism (AFM) and $e_g\left(3z^2-r^2\right)$ orbital occupation. For $n=1$ the high density of interfacial planes ultimately leads to dominant FM order forcing the residual AFM phase to be in-plane too, while for $n\ge 5$ the FM layers are separated by AFM regions having out-of-plane spin orientation.

The low-temperature $(<60\mathrm{K})$ transport properties of as-grown ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ ultrathin films, deposited by the molecular beam epitaxy, have been investigated as a function of the sample thickness (from $40\text{to}3.5\mathrm{nm}$) and in the presence of an external magnetic field. With decreasing thickness, a clear low-temperature resistivity minimum slightly affected by the application of the magnetic field has been observed, and its presence has been possibly interpreted in terms of quantum interference effects. As a function of the thickness, a crossover from a three-dimensional (3D) to a two-dimensional (2D) behavior of the system takes place below $20\mathrm{nm}$. A re-entrant 3D behavior is induced in ultrathin films by the application of large $(>20\mathrm{kOe})$ magnetic fields. Negative values of the magnetoresistance have been observed in all of the investigated samples for all of the measured magnetic fields.

We report on epitaxial growth of Bi₂Se₃ topological insulator thin films by Pulsed Laser Deposition (PLD). X-ray diffraction investigation confirms that Bi₂Se₃ with a single (001)-orientation can be obtained on several substrates in a narrow (i.e., 20 °C) range of deposition temperatures and at high deposition pressure (i.e., 0.1 mbar). However, only films grown on (001)-Al₂O₃ substrates show an almost-unique in-plane orientation. In-situ spin-resolved angular resolved photoemission spectroscopy experiments, performed at the NFFA-APE facility of IOM-CNR and Elettra (Trieste), show a single Dirac cone with the Dirac point at $E_B\sim 0.38$ eV located in the center of the Brillouin zone and the spin polarization of the topological surface states. These results demonstrate that the topological surface state can be obtained in PLD-grown Bi₂Se₃ thin films.

Double-exchange (DE) mechanisms in RE1$-${}xAExMnO3 manganites (where RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion) relies on the strong exchange interaction between two Mn3+ and Mn4+ ions through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions has ever been considered ``silent'' with respect to the DE conducting mechanisms. Here we show that a new path for DE mechanism is indeed possible by partially replacing the RE-AE elements by Mn2+ ions, in La-deficient LaxMnO3$-${}$\delta${} thin films. X-ray absorption spectroscopy demonstrated the relevant presence of Mn2+ ions, which is unambiguously proved to be substituted at La site by resonant inelastic x-ray scattering. Mn2+ is proved to be directly correlated with the enhanced magnetotransport properties because of an additional hopping mechanism trough interfiling Mn2+ ions. Such a scenario has been theoretically confirmed by calculations within the effective single-band model. The use of Mn2+ both as a doping element and an ion electronically involved in the conduction mechanism reveals an unconventional phenomenon in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials.

We use resonant inelastic x-ray scattering to probe the propagation of plasmons in the electron-doped cuprate superconductor ${\mathrm{Sr}}_{0.9}{\mathrm{La}}_{0.1}{\mathrm{CuO}}_2$. We detect a plasmon gap of $\sim 120\mathrm{meV}$ at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in ${\mathrm{Sr}}_{0.9}{\mathrm{La}}_{0.1}{\mathrm{CuO}}_2$ are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered $t\text{−}J\text{−}V$ model calculations. A similar analysis performed on recent resonant inelastic x-ray scattering data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping $t_z$. Our work signifies the three dimensionality of the charge dynamics in layered cuprates and provides a new method to determine $t_z$.