Percolation and the electron–electron interaction in an array of antidots

A square lattice of microcontacts with a period of 1 μm in a dense low-mobility two-dimensional electron gas is studied experimentally and numerically. At the variation of the gate voltage V _g , the conductivity of the array varies by five orders of magnitude in the temperature range T from 1.4 to 77 K in good agreement with the formula σ(V _g ) = (V _g ?V _g ^* (T))^β with β = 4. The saturation of σ(T) at low temperatures is absent because of the electron–electron interaction. A random-lattice model with a phenomenological potential in microcontacts reproduces the dependence σ(T, V _g ) and makes it possible to determine the fraction of microcontacts x(V _g , T) with conductances higher than σ. It is found that the dependence x(V _g ) is nonlinear and the critical exponent in the formula σ ∝ ? (x - 1/2) ^t in the range 1.3 < t(T, V _g ) < β.     

Direct measurement of the interfacial barrier height of the manganite p–n heterojunction

A manganite p-n heterojunction composed of Lao.67Sro.33MnO3 film and 0.05 wt% Nb-doped SrTiO3 substrate is fabricated. Rectifying behavior of the junction well described by the Shockley equation is observed, and the transport properties of the interface are experimentally studied. A satisfactorily logarithmic linear dependence of resistance on temperature is observed in a temperature range of 150 K-380 K, and the linear relation between bias and activation energies deduced from the R - lIT curves is observed. According to activation energy, the interfacial barrier of the heterojunction is obtained, which is 0.91 eV.     

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al–Cu–Mg alloy

The hardening precipitation of an Al–Cu–Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier–Preston–Bagaryatskii (GPB) zones type or to the so-called S-Al₂CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces.     

Scanning transmission electron microscopy investigation of an Al–Mg–Si–Ge–Cu alloy

Precipitation in a Mg-rich Al–Mg–Si–Ge–Cu alloy was investigated using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The precipitates were needle or lath shaped with the longest dimension parallel to ?001?_Al. The precipitates had no repeating unit cell when viewed along this direction. However, the precipitate structure in projection consisted of a hexagonal network of mixed Si and Ge columns, with Mg, Al, and Cu columns occupying specific sites in between the network columns. The Cu columns appeared with the same local arrangement of atomic columns as in Al–Mg–Si–Cu precipitates, and the Cu-free regions consisted of structural units with Mg and Al at specific sites. These structural units were often arranged in a locally ordered fashion, and in some cases the projected structure possessed and overall point symmetry. The amount of strain on the surrounding matrix was found to vary depending on the width of the precipitate cross section.     

Synchrotron-radiation photoemission study of the ultrathin Ba/3C–SiC(111) interface

Electronic structure of the Ba/3C–SiC(111) interface has been detailed studied in situ in an ultrahigh vacuum using synchrotron radiation photoemission spectroscopy with photon energies in the range of 100–450 eV. The 3C–SiC(111) samples were grown by a new method of epitaxy of low-defect unstressed nanoscaled silicon carbide films on silicon substrates. Valence band photoemission and both the Si 2p, C 1s core level spectra have been investigated as a function of Ba submonolayer coverage. Under Ba adsorption two induced surface bands are found at binding energies of 2 eV and 6 eV. It is obtained that Ba/3C–SiC(111) interface can be characterized as metallic-like. Modification of both the Si 2p and C 1s surface-related components were ascertained and shown to be provided by redistribution effect of electron density between Ba adatoms and both the Si surface and C interface atoms.     

Infrared photoemission of holes from ultrathin (3–20 nm) Pt/Ir-compound silicide films into silicon

The infrared responsivity is measured at low temperature on Schottky barrier detectors having ultrathin (3–20 nm) PtSi, IrSi, and compound silicide films as a metal electrode on p-type silicon. The total yield for internal hole photoemission is 1% per incident photon for PtSi and 0.1% for IrSi at a wavelength of =4 m. The cut-of wavelengths are =5.4 m and =8.2 m for PtSi and IrSi, respectively. The compound silicides fabricated by sequential evaporation of Pt and Ir and subsequent annealing at T=450° C show characteristics identical to that of PtSi.A Monte Carlo computer modelling is performed to simulate the scattering mechanisms in the thin silicide film leading to hole photoemission across the Schottky barrier into silicon. The optimum emission yield is observed for ultrathin films of the order of a few nanometers. The optimum film thickness is close to the escape depth d _esc2–3×L _el5 nm which scales with the mean free path L _el for quasi elastic scattering. The enhancement of the internal photoemission in ultrathin silicide films is predominantly due to the increase of the optical photoexcitation density rather than to an increase of the electrical emission yield in thin films.     

Influence of the channel–gate barrier height on the detection properties of a field-effect transistor in the microwave and terahertz ranges

The detection properties of a field-effect transistor with a low Schottky barrier gate in the microwave and terahertz ranges has been studied theoretically. Different detector circuits have been considered. The voltage and current distributions along the channel, the input impedance of the transistor, sensitivity, and noise equivalent power have been found. The influence of the Schottky barrier height on the above characteristics has been analyzed.     

Increase in corrosion resistance of Zn–22Al–2Cu alloy by depositing an Y2O3 film studied by Auger electron spectroscopy

Thin films of Y₂O₃ were deposited on the surface of a zinalco alloy (Zn–22Al–2Cu) in order to modify the surface and increase the corrosion resistance. By means of ion sputtering and surface analysis using Auger electron spectroscopy, in-depth relative elemental intensity profiles were obtained. The growth mechanism of the surface oxides layer is modified by the deposited yttrium oxide film. On samples without film, corrosion progresses mainly at the surface as indicated by the zinc excess, while on samples with film, the growth of the oxides layer occurs at inner points of the film where migrating anions and cations are allowed to find each other. The growth of the corrosion products layer is about nine times smaller in samples with a film of 1600 Å of Y₂O₃ with respect to samples without a film. Migration of aluminum particles is higher than that of zinc particles, producing a surface highly enriched in aluminum.     

The structure of an Al–Rh–Cu decagonal quasicrystal studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy

The structure of an Al–Rh–Cu decagonal quasicrystal formed with two quasiperiodic planes along the periodic axis in an Al₆₃Rh_18.5Cu_18.5 alloy has been studied by spherical aberration (Cs)-corrected high-angle annular detector dark-field (HAADF)- and annular bright-field (ABF)-scanning transmission electron microscopy (STEM). Heavy atoms of Rh and mixed sites (MSs) of Al and Cu atoms projected along the periodic axis can be clearly represented as separate bright dots in observed HAADF-STEM images, and consequently arrangements of Rh atoms and MSs on the two quasiperiodic planes can be directly determined from those of bright dots in the observed HAADF-STEM image. The Rh atoms are arranged in pentagonal tiling formed with pentagonal and star-shaped pentagonal tiles with an edge-length of 0.76 nm, and also MSs with a pentagonal arrangement are located in the pentagonal tiles with definite orientations. The star-shaped pentagonal tiles in the pentagonal tiling are arranged in τ²(τ: golden ratio)-inflated pentagonal tiling with a bond-length of 2 nm. From arrangements of Rh atoms placed in pentagonal tilings with a bond-length of 2 nm, which are generated by the projection of a five-dimensional hyper-cubic lattice, occupation domains in the perpendicular space are derived. Al atoms as well as Rh atoms and MSs are represented as dark dots in an observed ABF-STEM image, and arrangements of Al atoms in well-symmetric regions are discussed.     

Electrical resistivity of alkali metal doped manganites LaxAyMnwO3 (A = Na,K, Rb): Role of electron–phonon,electron–electron and electron–magnon interactions

The electrical resistivity behaviour of alkali metal (Na, K, Rb) substitutions at La site in La_xA_yMn_wO₃ (A = Na, K, Rb) manganites is studied caused by electron–phonon, electron–electron and electron–magnon scattering. Substitutions affect average mass and ionic radii of A-site and hence resulting lattice and optical phonon softening. Estimated resistivity compared with reported metallic resistivity, accordingly ρ_diff. = [ρ_exp ? {ρ₀ + ρ_e–ph (=ρ_ac + ρ_op)}], infers electron–electron and electron–magnon dependence over most of the temperature range. Electron–phonon contribution indicates that alkali metal K doping provoked larger lattice distortion, while electron–electron interaction is more dominating process for Na and Rb doped compound favouring motion of excess charge carrier. Semiconducting nature is discussed with variable range hopping and small polaron conduction model. The change in activation energies and the density of states at the Fermi-level is consistently explained by cationic disorder and Mn valence.     

Methanol-driven structuring of Au–Pt bimetallic nanoclusters on a thin film of Al2O3/NiAl(100)

The adsorption of methanol altered structures of Au–Pt bimetallic nanoclusters on a thin film of Al₂O₃/NiAl(100). Methanol adsorbed on the Au–Pt intermixed bimetallic clusters, of which the surfaces consist of both Au and Pt, induced a segregation of Au from Pt. This segregation state was unstable, as the clusters returned to the initial Au–Pt intermixed state upon desorption or decomposition of adsorbed methanol. Ethanol and cyclohexene were adsorbed on Au–Pt bimetallic clusters for comparisons, indicating that the interaction of the hydroxyl group of methanol with the clusters accounts for the structural modifications.     

Probe the accumulation modes of the Au–C22H14 dimer on the structure and NLO properties

In 2006, the Au–C₂₂H₁₄ with a covalent bond between an individual pentacene (C₂₂H₁₄) and a gold (Au) atom was synthesised and characterised, and its nonlinear optical (NLO) properties were explored. To further investigate the NLO properties from molecules to materials, three kinds of different dimers (Au–C₂₂H₁₄)₂ (2, 3 and 4) were designed to probe the monomer accumulation modes on the structures and NLO properties. The results indicate that Au atoms doping breaks the conjugate structures of the two pentacenes to different extent. On the other hand, their NLO properties investigated by three density functional theory methods Becke-Half-and-Half-LYP (BHandHLYP), Coulomb-attenuating method Becke-3-Lee–Yang–Parr (CAM-B3LYP) and Minnesota 2005 double the amount of nonlocal exchange (M05-2X) show the same order, and 2 has the largest first hyperpolarisability (β_tot) than the other molecules. At the same time, natural bond orbital analysis shows the Au atoms play a crucial role in pushing electron density. Meanwhile, the frontier molecular orbital analysis shows that charge transfer has occurred between the two pentacene molecules and Au atoms. As a result, the order of transition energy is opposite to the order of β_tot values. Because the pentacene is taken as a simplified fragment of the graphene, our present work may be beneficial to the development of high-performance NLO materials.     

Structural,electronic and optical properties of ultrathin thallium nanowires – an ab initio study

The energetics and structural, electronic and optical absorption properties of thallium nanowires, Tl _n with n?=?1–18, have been investigated by employing a first-principles density functional theory in the local density approximation. The spin–orbit (SO) interaction has also been considered. We study four types of stable structures: planar, caged, pyramidal and helical. In general, the binding energy increases with the coordination number except in a few cases where the nearest-neighbours lie at comparatively larger separations. The maximum stability is seen for the helical configurations containing pentagons, hexagons, heptagons and octagons. Nanowires containing a core linear chain of atoms on the tube axis are more stable than the corresponding nanotubes having no such chains. All the wires or tubes are found to be metallic with or without consideration of the SO interaction. The electronic structures of the pentagonal-, hexagonal- and octagonal-configuration wires provide a large number of channels, which may give rise to large quantum ballistic conduction. One finds large differences between the optical absorption calculated with and without the SO interaction. Consideration of the SO interaction enhances the number of absorption peaks by approximately a factor of two. A strong and multi-peaked optical absorption, extending up to 4.0?eV including the visible region, appears for wires containing pentagons and octagons. These wires may thus be useful as a source of white radiation.     

Probing the onset of strong localization and electron–electron interactions with the presence of a direct insulator–quantum Hall transition

We have performed low-temperature transport measurements on a disordered two-dimensional electron system (2DES). Features of the strong localization leading to the quantum Hall effect are observed after the 2DES undergoes a direct insulator–quantum Hall transition on increasing the perpendicular magnetic field. However, such a transition does not correspond to the onset of strong localization. The temperature dependences of the Hall resistivity and Hall conductivity reveal the importance of the electron–electron interaction effects for the observed transition in our study.     

Pulsed photoconductivity in diamond upon quasi-continuous laser excitation at 222 nm at the formation of an electron–hole liquid

An order-of-magnitude enhancement of the pulsed photocurrent in a polycrystalline diamond sample synthesized by chemical vapor deposition is observed under the conditions of formation of an electron–hole liquid. Nonequilibrium charge carriers are excited by laser pulses at a wavelength of 222 nm with FWHM pulse duration of 18 ns and peak intensity above 2.5 MW/cm² upon cooling the sample to 90 K. For peak intensities of laser excitation lower than 1 MW/cm², sample cooling from 300 to 90 K leads to a decrease in pulsed photocurrent by about a factor of 5. The observed increase in pulsed photocurrent is attributed to the formation of the electron–hole liquid.     

Quantitative description of the T1 formation kinetics in an Al–Cu–Li alloy using differential scanning calorimetry,small-angle X-ray scattering and transmission electron microscopy

The object of the present study is to design a methodology to follow the kinetics of T₁ precipitation, in an AA2198 alloy, in terms of precipitate size, morphology (thickness, diameter) and volume fraction, during a two-temperature isothermal heat treatment. We used in situ small-angle X-ray scattering (SAXS) as a way to measure the evolution of the T₁ mean thickness and diameter during the heat treatment. Transmission electron microscopy (TEM) was then performed in order to calibrate these evolutions. Furthermore, we demonstrate that the volume fraction evolution can be described successfully using a simple analysis of the differential scanning calorimetry (DSC) thermograms. The latter was calibrated by selected observations in high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). Microstructure evolution during DSC heating ramps was analysed using in situ SAXS: the T₁ phase transformation is found to consist in a two-step thickening process explained by two consecutive diffusion stages. The enthalpy of formation of the T₁ phase is deduced from the DSC measurements.     

Adsorption and diffusion of an Au atom and dimer on a θ-Al2O3 (001) surface

With density-functional calculations we have investigated adsorption and diffusion of an Au atom and an Au₂ dimer on a θ-Al₂O₃(001) surface. The surface structure of θ-Al₂O₃(001) has an armchair-like configuration containing flat and trench areas and the Au_n (n = 1 or 2) cluster prefers to adsorb on the flat area. A single Au atom adsorbs on an O–Al bridge site with adsorption energy 0.35 eV, whereas an Au₂ dimer bonds to the oxide with adsorption energy 0.78 eV, with one Au coordinated singly to a surface O. Formation of Au₂ from Au₁ is favored, with a negligible energy barrier. The calculated energy barriers for diffusion indicate that an Au atom diffuses more rapidly than an Au₂ dimer but both prefer to diffuse anisotropically, along the flat area of the θ-Al₂O₃(001) surface.     

Microstructural characterisation of a commercial Al–Cu–Mg alloy combining transmission electron microscopy and positron annihilation spectroscopy

Transmission electron microscopy and positron annihilation spectroscopy techniques were used to characterise the microstructure of the 2024 Al–Cu–Mg commercial alloy artificially aged with and without pre-deformation. In non-deformed samples, a very dense dispersion of small, needle-shaped particles, with mean size in the order of the nanometres, was observed homogeneously distributed in the matrix, together with a coarse distribution of S-phase precipitates. In pre-deformed samples, the needle-shape particles were not seen, only a high density of S-phase precipitates nucleated on dislocations. The needle-shaped particles were identified as solute-rich Guinier–Preston–Bagaryatski (GPB) zones by combining coincidence Doppler broadening positron annihilation measurements with TEM imaging. The relationship between the microstructure and measurements of hardness and positron lifetime evolution during artificial ageing is also discussed.