Effect of the RE ionic size on the REBa2Cu3O7−δ ceramics oxygenated at 250 bar

REBa₂Cu₃O_7−δ (RE=Ho, Y, Dy, Gd, Sm, Nd) ceramics have been oxygenated at 1 bar pressure (LP) and, subsequently, at 250 bar (HP). Despite the noticed slight uptake of oxygen (up to the value of 0.07), after the HP processing, the electrical resistivity (ρ) of all samples increased, what was attributed to the deterioration of the grain boundaries. The increase of ρ was much more pronounced and also accompanied by a change of ρ(T) characteristics into a semiconducting-like one in the case of 123 compounds based on REs which ionic size is large enough to form solid solutions of RE_1+xBa_2−xCu₃O_y type (i.e., RE=Gd, Sm, Nd). As shown in the literature, such 123s usually contain more structural defects. Thus, the observed effect may be attributed to the migration of the defects induced by the elevated pressure oxygenation. The defects could be trapped near the grain boundaries resulting in the deterioration of their electrical properties. The possible role of the oxygen-pressure-induced modifications of the impurity phases has been also discussed. The materials obtained in the HP process may be regarded as 3D arrays of superconducting grains coupled by the Josephson junction or weak links only, as was shown in a tunneling experiment.     

Co2+:ZnAl2O4晶体电子光谱及其2E态双重谱线结构的Jahn-Teller效应研究

在中间场耦合图象中,利用Racak不可约张量算符法和Wigner－Eckart定理,建立了3d⁷(C_3v)电子组态的完全能量矩阵;利用该矩阵研究了Co²⁺:ZnAl₂O₄晶体的电子光谱,理论与观测十分吻合．首次定量地解释了2E态的双重谱线结构,并从理论上预言了⁴A₂基态2.93cm^-1的零场分裂(ZFS)．研究结果定量地表明,Jahn－Teller效应所致Co²⁺:ZnAl₂O₄晶体Co²⁺中心局域结构的畸变假设是合理的．     

Optical properties and electronic transitions of SnO2 thin films by reflection electron energy loss spectroscopy

Optical properties of SnO₂ thin films in the 4–60 eV energy range are determined by reflection electron energy loss spectroscopy. Bulk and surface electron loss functions, real and imaginary parts of the dielectric function, refraction index, extinction and absorption coefficients are obtained from the analysis of the electron energy loss spectra. Electronic transitions are identified through the interpretation of the optical data. The samples (250–500 nm thick) were produced by ion beam-induced chemical vapor deposition. It is found that the compacity of the SnO₂ thin films affects their optical properties and therefore the relative intensity of the observed electronic transitions. The advantages of this method to determine optical properties of thin films are discussed. Inelastic mean free paths (6.2, 17 and 41 Å for electrons traveling in SnO₂ with kinetic energies of 300, 800 and 2000 eV, respectively) are obtained from the corresponding inelastic electron scattering cross-sections.     

Ge quantum dots on a large band gap semiconductor: the first growth stages on 4H–SiC(0 0 0 1)

The Ge growth on SiC(0 0 0 1) follows a Stranski–Krastanov mode for Si-rich (3×3) and reconstructed surfaces. For Ge deposit in particular temperature conditions, a new (4×4) superstructure takes place and the reflection high energy electron diffraction (RHEED) specular spot intensity presents one oscillation proving a wetting layer formation. An island nucleation is then ascertained by the oscillation vanishing and by the appearance of a k-modulated RHEED pattern. On the other hand, on a C-rich surface, a direct Ge island nucleation is observed from the first growth stage. Indeed, for 1 ML Ge, the RHEED diagram consists in spots and rings, and the atomic force microscopy analysis indicates a high density (8×10¹⁰ cm⁻²) of small islands (30 nm, h3 nm). The RHEED spot analysis shows a preferential epitaxial relationship with the substrate Ge(1 1 1)//SiC(0 0 0 1). The Ge–C bonding being energetically unfavourable, Ge tends to form islands immediately rather than wetting the graphite-terminated surface. The Ge growth mode on C-rich surface is thus of Volmer–Weber type.     

Microwave electric field induced spin transitions of AlAs 2D electrons

We present the first direct electron spin resonance (ESR) on a 2D electron gas in a III–V semiconductor. ESR on a high mobility 2D electron gas in a single AlAs quantum well reveals an electronic g-factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimal linewidth of 7 Gauss. Both the signal amplitude and its dependence on the position and orientation of the sample in the cavity unambiguously demonstrate that the spin transitions in our experiment are caused by the microwave electric field. We present a model that ascribes the spin transitions to the effective magnetic field acting on the electron spins that arises from (Bychkov–Rashba) spin-orbit interaction and the modulation of the electron wavevector around k_F induced by the microwave electric field.     

Cross-section and rate coefficient calculation for electron impact excitation, ionisation and dissociation of H2 and OH molecules

The weighted total cross-section (WTCS) theory is used to calculate electron impact excitation, ionisation and dissociation cross-sections and rate coefficients of OH, H₂, OH⁺, H₂ ⁺, OH^- and H₂ ^- diatomic molecules in the temperature range 1500–15000 K. Calculations are performed for H₂(X, B, C), OH(X, A, B), H₂ ⁺(X), OH⁺(X, a, A, b, c), H₂ ^-(X) and OH^-(X) electronic states for which Dunham coefficients are available. Rate coefficients are calculated from WTCS assuming Maxwellian energy distribution functions for electrons and heavy particles. One and two temperature (θ_e and θ_g respectively for electron and heavy particles kinetic temperatures) results are presented and fitting parameters (a, b and c) are given for each reaction rate coefficient: k(θ) = a (θ^b)exp (-c/θ).     

Frenkel exciton model of electron energy loss spectroscopy in -PTCDA

Recent experimental findings concerning electron energy loss spectroscopy in -Perylene-tetracarboxylic-dianhydride are analysed in terms of a Frenkel exciton model. Taking into account the energy dispersion of excitations with finite momentum transfer, the k-dependence of the dielectric tensor and the corresponding electron energy loss functions can be calculated. The exciton dispersion with a minimum at k≠0 yields a red shift of the lineshape of loss functions at large k, as observed experimentally.     

电子回旋共振等离子分解CH19Si2N制备SICNB膜XPS分析

电子回旋共振等离子分解ＣＨ１９Ｓｉ２Ｎ－Ｎ２混合气体，采用溅射共混、气相液相共混沉积技术，制备四元素ＳｉＣＮＢ膜，由ＸＰＳ分析了膜的成份及生成的键结构，发现溅射共混、气液共混沉积过程中聚合成了Ｃ－Ｎ、Ｃ－Ｂ、Ｎ－Ｂ、Ｓｉ－Ｃ化合物。     

Reaction of 1,3-cyclohexadiene with the Ge(100) surface

The adsorption and reaction of 1,3-cyclohexadiene with the Ge(100)-(2×1) surface are investigated. The possibility of a surface Diels–Alder reaction ([4+2] cycloaddition), as well as a [2+2] cycloaddition reaction are explored. The surface reactions are followed by ultraviolet photoelectron and high-resolution electron energy loss spectroscopies. The vibrational spectroscopy results are compared with theoretically predicted frequencies for each of the possible surface reactions.