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[en] Raman scattering experiments together with detailed lattice dynamic calculations are performed to elucidate crystallographic and electronic peculiarities of SrCoO_3_-_δ films. We observe that the 85 cm"-"1 phonon mode involving the rotation of a CoO_4 tetrahedron undergoes a hardening by 21 cm"-"1 when the temperature is decreased. In addition, new phonon modes appear at 651.5 and 697.6 cm"-"1 . The latter modes are attributed to the Jahn-Teller activated modes. Upon cooling from room temperature, all phonons exhibit an exponential-like increase of intensity with a characteristic energy of about 103-107 K. We attribute this phenomenon to an instability of the CoO_4 tetrahedral chain structure, which constitutes a key ingredient to understand the electronic and structural properties of the brownmillerite SrCoO_2_._5
[en] Various groups have demonstrated that antineutrino monitoring can be successful in assessing the plutonium content in water-cooled nuclear reactors for nonproliferation applications. New reactor designs and concepts incorporate nontraditional fuel types and chemistry. Understanding how these properties affect the antineutrino emission from a reactor can extend the applicability of antineutrino monitoring. Thorium molten salt reactors breed "2"3"3U, that if diverted constitute a direct use material as defined by the International Atomic Energy Agency (IAEA). The antineutrino spectrum from the fission of "2"3"3U has been estimated for the first time, and the feasibility of detecting the diversion of 8 kg of "2"3"3U, within a 30 day timeliness goal has been evaluated. The antineutrino emission from a thorium reactor operating under normal conditions is compared to a diversion scenario by evaluating the daily antineutrino count rate and the energy spectrum of the detected antineutrinos at a 25 m standoff. It was found that the diversion of a significant quantity of "2"3"3U could not be detected within the current IAEA timeliness detection goal using either tests. In conclusion, a rate-time based analysis exceeded the timeliness goal by 23 days, while a spectral based analysis exceeds this goal by 31 days.
[en] Infrared-active optical phonons were studied in olivine LiFePO4 oriented single crystals by means of both rotating analyzer and rotating compensator spectroscopic ellipsometry in the spectral range between 50 and 1400 cm-1. The eigenfrequencies, oscillator strengths, and broadenings of the phonon modes were determined from fits of the anisotropic harmonic oscillator model to the data. Optical phonons in a heterosite FePO4 crystal were measured from the delithiated ab-surface of the LiFePO4 crystal and compared with the phonon modes of the latter. Good agreement was found between experimental data and the results of solid-state hybrid density functional theory calculations for the phonon modes in both LiFePO4 and FePO4.
[en] In this paper, vanadium thin films were deposited on sapphire substrates by DC magnetron sputtering and then oxidized in a tube furnace filled with oxygen under different temperatures and oxygen flow rates. The significant influence of the oxygen flow rate and oxidation temperature on the electrical and structural properties of the vanadium oxide thin films were investigated systematically. It shows the pure vanadium dioxide (VO2) state can only be obtained in a very narrow temperature and oxygen flow rate range. The resistivity change during the metal-insulator transition varies from 0.2 to 4 orders of magnitude depending on the oxidation condition. Large thermal hysteresis during the metal-insulator phase transition was observed during the transition compared to the results in literature. Proper oxidation conditions can significantly reduce the thermal hysteresis. Finally, the fabricated VO2 thin films showed the potential to be applied in the development of electrical sensors and other smart devices.
[en] The surface electronic structure of 1500 ?A thick Fe3O4(100) films has been investigated by spinresolved photoemission spectroscopy with vacuum ultra-violet (VUV) synchrotron radiation. The films, epitaxially grown on MgO(100) substrates using plasma assisted molecular beam deposition, are briefly exposed to air during transfer to the photoemission chamber. It is shown that clean surfaces of Fe3O4 films after exposure to air can be recovered through mild annealing in an oxygen atmosphere. The presence of the characteristic Verwey transition, as detected in the valence band photoemission spectra, provides evidence for the excellent electronic quality at the surface of these films. The top of the valence band is found to be negatively spin-polarized, with a value of ∼ ?50%; this result strongly points towards the localized nature of the 3d states in this system
[en] Here, we report the effect of multivalent Cr-ion doping on the structural, optical, magnetic, and resistive switching properties of BiFe1-xCrxO3 (BFCO) thin films (where, 0 ≤ x ≤ 0.15). BFCO thin films were deposited on Pt/TiO2/SiO2/Si (100) substrate using pulsed laser deposition technique. X-ray diffraction and micro-Raman analysis revealed the presence of a secondary phase in BFCO thin films, above 5% Cr doping concentrations. Enhanced magnetization was observed in BFCO films owing to ferromagnetic superexchange interaction between Fe and Cr-ions. X-ray photoelectron spectroscopy measurements revealed the multivalent states of Cr and Fe-ions, where suppression of oxygen vacancies due to Cr-ion doping in BFCO films was discussed based on the defect chemistry viewpoint. Moreover, current conduction and resistive switching properties were studied and the dominant switching mechanism was explained in the light of oxygen vacancies assisted filamentary conduction model.
[en] K2CsSb is a promising photocathode candidate to serve as an electron source in next-generation light sources such as Free Electron Lasers (FEL) and Energy Recovery Linacs (ERL). As the traditional recipe for creation of K2CsSb photocathodes typically results in a rough surface that deteriorates electron beam quality, significant effort has been made to explore novel growth methods for K2CsSb photocathodes. In this work, a method of ternary co-evaporation of K, Cs, and Sb is described. By using in-situ synchrotron X-ray techniques, the quality of the photocathode is characterized during and after the growth. K2CsSb photocathodes grown by this method on Si (100) and MgO (001) substrates show strong (222) texture, and the two photocathodes exhibit 1.7% and 3.4% quantum efficiencies at a wavelength of 530 nm, with a rms surface roughness of about 2–4 nm. Finally, this represents an order of magnitude reduction in roughness compared to typical sequential deposition and should result in a significant improvement in the brightness of the generated electron beam.
[en] Polymethylpentene, commonly referred to by its trade name TPX (Mitsui Chemicals, Inc.), is a thermoplastic polymer that has the potential to be a useful window material for dynamic compression experiments. For such experiments, an optically transparent or a low x-ray absorptive window is often used to maintain stress within the sample during compression. TPX can be used as a low-impedance optical and x-ray window due to its good transmittance in most parts of the electromagnetic spectrum, very low density (0.83 g/cm3), and low x-ray absorption. In dynamic compression experiments, interferometry can be used to determine the particle velocity at the interface between the sample and window. However, velocimetry measures the rate of change of the optical path length, commonly referred to as the apparent particle velocity. An experimentally determined window correction factor is needed to ascertain the actual particle velocity from the measured apparent velocity. Here, we present the results of a series of dynamic compression experiments from 1 to 31 GPa designed to characterize the mechanical and optical response of TPX, determine the range of stresses over which TPX is transparent, and determine the window correction factor. Finally, the index of refraction was found to be essentially linear in density, resulting in a simple constant correction factor. TPX was found to remain largely transparent over the entire stress range examined.
[en] Carrier transport and recombination are modeled for a heterojunction diode containing defect traps. Here, particular attention is given to the role of band-to-trap tunneling and how it is affected by band offsets at the junction. Tunneled states are characterized by numerical solution of the Schrodinger equation, and the interaction with traps is treated assuming capture and emission by the multi-phonon mechanism. It is shown that tunneling can increase carrier recombination at defects by orders magnitude in the presence of large band offsets. This explains why InGaP/GaAs/GaAs Npn HBTs with displacement damage from energetic particle irradiation have higher carrier recombination in the emitter-base depletion region.