[en] Solution techniques allow the preparation of La_1-yMn_1-yO₃ at low temperatures. However, the as-prepared compounds show thermodynamically induced vacancies on both cationic sites. The transport and magnetic properties strongly depend on both bulk and surface defects. In order to separate these effects, we have studied La_1-yMn_1-yO₃ compositions by varying the vacancy content y and the grain size. The electronic structure of these phases has been investigated by means of the X-ray absorption spectroscopy at the Mn K-edge. XAS experiments have been carried out on the La_1-yMn_1-yO₃ system as compared with the La_1-xCa_xMnO₃ reference series. For both series, the absorption edge and the unit cell volume vary linearly with the formal Mn(IV) content, resulting from a direct correlation between the hole count in Mn 3d states and the concentration of doping or of cationic vacancies in the perovskite phase. However, in the La_1-yMn_1-yO₃ system, a deviation from this linearity occurs for vacancy contents above 30% of Mn(IV). This corresponds to a limit of solubility of the cationic vacancy in the bulk. Larger hole contents (up to 40%) may still be measured, but XANES spectra indicate that the excess holes are then trapped onto Mn sites, probably located at the surface. Despite this localization, transport measurements indicate a transition from an insulating to a metallic behavior in the low-temperature ferromagnetic regime beyond the critical concentration of 30% of Mn(IV)

[en] We report x-ray photoemission, resonant photoemission and Bremsstrahlung isochromat spectra of the 4f core levels, the valence band and the conduction band, respectively, of the heavy fermion compound U₂Zn₁₇ and the reference compound Th₂Zn₁₇, and we assess current efforts to interpret such spectra. 19 refs., 4 figs

[en] At low energy, a longitudinally spin-polarized electron beam impinges on an ultrathin, self-supported ferromagnetic target, consisting of a 1 nm-thick cobalt film sandwiched between 21 and 2 nm-thick gold layers, and which is magnetized perpendicularly to the surface. The current transmitted by the target depends on the spin of the electrons. Cesium deposition on both sides of the target increases the transmission ratio from about 1 x 10^-5 up to 3 x 10^-4 and also increases the transmission spin-asymmetry from 15 to about 40%. Such a structure is well suited to the construction of convenient and compact spin-detectors. (authors). 4 figs., 9 refs

[en] Recently a new theory has been presented for describing band gaps electronic structure of transition metal compounds. This theory takes as a model the Anderson Hamiltonian. This theory then differs from both traditional approaches, i.e., the Mott-Hubbard theory which neglects Δ, and the band theory which neglects U. The Anderson Hamiltonian approach provides a good description of resonant photoemission and bremsstrahlung isochromat spectra for NiO, a material which has served historically as a focus of controversy concerning the importance of U in causing it to be an insulator. The spectra show that although U is very large, approx. =7-9 eV, there are other states d⁸L which fall inside this correlation gap and cause of the charge-transport gap to be much smaller, approx. =4.3 eV

[en] We present a study of the formation of surface relief gratings in thin solid films containing azobenzene derivatives upon illumination with an interference pattern. This study is based on near-field microscopy techniques that provide real-time imaging of both the photomechanical response of the material and light excitation profile. We demonstrate that the material deformation follows two distinct regimes characterized by different kinetics, a different phase relative to the light intensity pattern, and a specific dependence on light polarization

[en] We present an original technique to investigate spin-dependent electron interactions in ferromagnetic metals, and place it in the context of previous studies. Our technique is based on spin-polarized electron transmission through ultrathin, free-standing, metal foils. A longitudinally spin-polarized, quasimonoenergetic, free-electron beam impinges onto a ferromagnetic target consisting of a few atomic layers of cobalt sandwiched between gold layers, for an overall thickness of the order of 25 nm. It is remanently magnetized perpendicular to the film plane. The current transmitted through the foil is energy analyzed and its dependence on the relative orientation between the spin polarization of the primary beam and the magnetization direction of the cobalt layer is measured. The experiments are performed over a wide primary energy range, starting from the vacuum level of the target; the work function of the target can be lowered down to 2 eV by cesium deposition. We demonstrate a spin-filter effect, favoring the transmission of majority electrons. It is very large at low primary energy, when the electrons travel close to the 3d bands. Perspectives for compact and highly discriminative spin detectors are discussed. copyright 1996 American Institute of Physics

[en] A spin-dependent transport experiment in which hot electrons pass through a ferromagnetic metal / semiconductor Schottky diode has been performed. A spin-polarized free-electron beam, emitted in vacuum from a GaAs photocathode, is injected into the thin metal layer with an energy between 5 and 1000 eV above to the Fermi level. The transmitted current collected in the semiconductor substrate increases with injection energy because of secondary - electron multiplication. The spin-dependent part of the transmitted current is first constant up to about 100 eV and then increases by 4 orders of magnitude. As an immediate application, the solid-state hybrid structure studied here leads to a very efficient and compact device for spin polarization detection

[en] We have monitored the cathodoluminescence (CL) emitted upon injection of free electrons into a hybrid structure consisting of a thin magnetic Fe layer deposited on a p-GaAs substrate, in which InGaAs quantum wells are embedded. Electrons transmitted through the unbiased metal/semiconductor junction recombine radiatively in the quantum wells. Because of the electron spin-filtering across the Fe/GaAs structure, the CL intensity, collected from the backside, is found to depend on the relative orientation between the injected electronic spin polarization and the Fe layer magnetization. The spin asymmetry of the CL intensity in such junction provides a compact optical method for measuring spin polarization of free electrons beams or of hot electrons in solid-state devices

[en] Why a chapter on photoelectronic processes in semiconductors studied by negative electron affinity photoemission, in a volume on hot electrons in semiconductors? It is well-known that standard photoemission uses excitations with energies larger than the work function (≥5 eV) for a clean semi-conductor surface to overcome the surface energy barrier. This corresponds to UV light, so that such an excitation is absorbed in the first few angstroems next to the semiconductor-vacuum interface. Standard photoemission then essentially probes surface properties and is not suited for the analysis of energy relaxation. (author). 115 refs., 38 figs

[en] We report X-ray photoemission, resonant photoemission and Bremsstrahlung isochromat spectra of the 4f core levels, the valence band and the conduction band, respectively, of the heavy fermion compound U₂Zn₁₇ and the reference compound Th₂Zn₁₇, and we assess current efforts to interpret such spectra. (orig.)