Abstract: The magneto-Stark effect in a diluted magnetic semiconductor (DMS) coupled quantum well (CQW) induced by an in-plane magnetic field is investigate theoretically. Unlike the usual electro-Stark effects, in a DMS CQW the Lorenz force leads to a spatially separated exciton. The in-plane magnetic field can shift the ground state of the magnetoexciton from a zero in-plane center of mass (CM)/momentum to a finite CM momentum, and render the ground state of magnetoexciton stable against radiative recombination due to momentum conservation. (C) 2002 American Institute of Physics.

Abstract: We present a new method to measure structure factors from electron spot diffraction patterns recorded under almost parallel illumination in transmission electron microscopes. Bloch wave refinement routines have been developed to refine the crystal thickness, its orientation and structure factors by comparison of experimentally recorded and calculated intensities. Our method requires a modicum of computational effort, making it suitable for contemporary personal computers. Frozen lattice and Bloch wave simulations of GaAs diffraction patterns are used to derive optimised experimental conditions. Systematic errors are estimated from the application of the method to simulated diffraction patterns and rules for the recognition of physically reasonable initial refinement conditions are derived. The method is applied to the measurement of the 200 structure factor for GaAs. We found that the influence of inelastically scattered electrons is negligible. Additionally, we measured the 200 structure factor from zero loss filtered two-dimensional convergent beam electron diffraction patterns. The precision of both methods is found to be comparable and the results agree well with each other. A deviation of more than 20% from isolated atom scattering data is observed, whereas close agreement is found with structure factors obtained from density functional theory [A. Rosenauer, M. Schowalter, F. Glas, D. Lamoen, Phys. Rev. B 72 (2005), 085326-1], which account for the redistribution of electrons due to chemical bonding via modified atomic scattering amplitudes.

Abstract: Tunneling of holes through a trilayer structure made of two diluted magnetic semiconductors, (Ga,Mn)As, separated by a thin layer of nonmagnetic AlAs is investigated. The problem is treated within the 6x6 Luttinger-Kohn model for valence bands with the split-off band included. The influence of the spin-orbit coupling is pronounced as the spin-splitting Delta(ex) is comparable with the split-off Delta(SO) splitting. It is assumed that direct tunneling is the dominant mechanism due to the high quality of the tunnel junctions. Our theoretical results predict the correct order of magnitude for the tunneling magnetoresistance ratio, but various other effects, such as scattering on impurities and defects, should be included in order to realize a quantitative agreement with experiment.

Abstract: The structure and defect structure of Bi2-xPbxSr3.5Cu2 (CO3)O8-delta compounds with 0 less-than-or-equal-to x less-than-or-equal-to 0.75 are carefully investigated by electron diffraction and high-resolution electron microscopy. All compounds have an orthorhombic structure with a almost-equal-to b almost-equal-to 5.4 angstrom and c almost-equal-to 39.5 angstrom. The length of the b-axis decreases monotonically with increasing x. The space group for the basic structure is Abm2. The structure can be considered as an intergrowth of Bi2Sr2CuO6 lamellae with Sr2CuO2 (CO3) lamellae along the c-axis. CO3 groups behave as bridges connecting the CuO6 octahedra. In the x = 0 compound the carbon atoms are shifted away from their symmetry positions; the orientational ordering of the CO3 groups (or the carbon shift) in successive CO planes alternates along +b and -b. Typical Bi-type and Pb-type modulations are found along the b-axis up to a Pb content x = 0.5. Electron beam irradiation destroys the ordering of the CO3 groups and alters the modulated structure. (C) 1994 Academic press, inc.

Abstract: We report on the preparation of CuBa(2)(Sr(x)Ca(1-x))(n)Cu(n-1)O(y) compounds by fabrication of (Ba,Sr,Ca)CuO(2) superlattices with pulsed laser deposition (PLD). A technique called interval deposition is used to suppress multi-level or island growth resulting in high-quality superlattice structures. Both, the applicability of PLD to atomic engineering as well as the fabrication of artificial superconductors is demonstrated. The (Sr,Ca)CuO(2)-BaCuO(2) superlattices are characterized by X-ray diffraction, high-resolution electron microscopy (HREM) and selected area electron diffraction. The superlattice period has been deduced from electron diffraction patterns and XRD measurements. For Sr containing films, the best growth behavior is observed and films with the highest degree of crystallinity are obtained, whereas superconductivity is only found in less crystalline, Ca containing films. Under some deposition conditions and depending on the amount of Ba containing layers in the superlattice, it was observed that the BaCuO(2) material is converted to Ba(2)CuO(4-delta). Image simulations to interpret the HREM contrast are performed. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract: In this paper, we report on the electronic transport properties of mesoscopic, ultrathin graphite flakes with a thickness corresponding to a stack of 150 graphene layers. The graphite flakes show an unexpectedly strong positive magnetoresistance (PMR) already at room temperature, which scales in good approximation with the square of the magnetic field. Furthermore, we show that the resistivity is unaffected by magnetic fields oriented in plane with the graphene layers. Hall effect measurements indicate that the charge carriers are p-type and their concentration increases with increasing temperature while the mobility is decreasing. The Hall voltage is non-linear in higher magnetic fields. Possible origins of the observed effects are discussed. Ball and stick model of the two topmost carbon layers of the hexagonal graphite structure.

Abstract: The effects produced by swift heavy ions in the martensitic (18R) and austenitic phase (beta) of Cu based shape memory alloys were characterized. Single crystal samples with a surface normal close to [210](18R) and [001](beta) were irradiated with 200 MeV of Kr(15+), 230 MeV of Xe(15+), 350 and 600 MeV of Au(26+) and Au(29+). Changes in the microstructure were studied with transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). It was found that swift heavy ion irradiation induced nanometer sized defects in the 18R martensitic phase. In contrast, a hexagonal close-packed phase formed on the irradiated surface of beta phase samples. HRTEM images of the nanometer sized defects observed in the 18R martensitic phase were compared with computer simulated images in order to interpret the origin of the observed contrast. The best agreement was obtained when the defects were assumed to consist of local composition modulations.

Abstract: The effect of thermal treatment and mechanical stress on the structural and photocatalytic properties of eight different (synthetic and commercial) photocatalysts has been thoroughly investigated. Different mesoporous Ti-based materials were prepared via surfactant based synthesis routes (e.g. Pluronic 123, CTMABr = Cetyltrimethylammonium bromide) or via template-free synthesis routes (e.g. trititanate nanotubes). Also, the stabilizing effect of the NaOH/NH4OH post-treatment on the templated mesoporous materials and their photocatalytic activity was investigated. Furthermore, the thermal and mechanical properties of commercially available titanium dioxides such as P25 Evonik® and Millenium PC500® were studied. The various photocatalysts were analyzed with N2-sorption, X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) to obtain information concerning the specific surface area, pore volume, crystal structure, morphology, phase transitions, etc. In general, results show that the NaOH post-treatment leads to an increased control of the crystallization process during calcination resulting in a higher thermal stability, but at the same time diminishes the photocatalytic activity. Mesoporous materials in which pre-synthesized nanoparticles are used as titania source have the best mechanical stability whereas the mechanical stability of the nanotubes is the most limited. At increased temperatures and pressures, the tested commercial titanium dioxides lose their superior photocatalytic activity caused by a decreased accessibility of the active sites. The observed changes in adsorption capacities and photocatalytic activities cannot be assigned to one single phenomenon. In this respect, it shows the need to define a general/standard method to compare different photocatalysts. Furthermore, it is shown that the photocatalytic properties do not necessarily deteriorate under thermal stress, but can be improved due to crystallization, even though the initial material is (partially) destroyed. It is shown that the usefulness of a specific type of photocatalyst strongly depends on the application and the temperature/pressure to which it needs to resist.

Abstract: This paper focuses on the investigation of CF4 decomposition in a low-pressure inductively coupled plasma by means of a global model. The influence of O2 on the CF4 decomposition process is studied for conditions used in semiconductor manufacturing processes. The model is applied for different powers and O2 contents ranging between 2% and 98% in the CF4/O2 gas mixture. The model includes the reaction mechanisms in the gas phase coupled with the surface reactions and sticking probabilities of the species at the walls. The calculation results are first compared with experimental results from the literature (for the electron density, temperature and F atom density) at a specific power, in the entire range of CF4/O2 gas mixture ratios, and the obtained agreements indicate the validity of the model. The main products of the gas mixture, obtained from this model, include CO, CO2 and COF2 together with a low fraction of F2. The most effective reactions for the formation and loss of the various species in this process are also determined in detail. Decomposition of CF4 produces mostly CF3 and F radicals. These radicals also contribute to the backward reactions, forming again CF4. This study reveals that the maximum decomposition efficiency of CF4 is achieved at a CF4/O2 ratio equal to 1, at the applied power of 300 W.

Abstract: The low-temperature topotactic reduction of La0.33Sr0.67MnO3 with NaH results in the formation of La0.33Sr0.67MnO2.42. A combination of neutron powder and electron diffraction data show that La0.33Sr0.67MnO2.42 adopts a novel anion-vacancy ordered structure with a 6-layer OOTOOT' stacking sequence of the octahedral and tetrahedral layers (Pcmb, a=5.5804(1) Å, b=23.4104(7) Å, c=11.2441(3) Å). A significant concentration of anion vacancies at the anion site, which links neighbouring octahedral layers means that only 25% of the octahedral manganese coordination sites actually have 6-fold MnO6 coordination, the remainder being MnO5 square-based pyramidal sites. The chains of cooperatively twisted apex-linked MnO4 tetrahedra adopt an ordered -LRLR- arrangement within each tetrahedral layer. This is the first published example of a fully refined structure of this type which exhibits such intralayer ordering of the twisted tetrahedral chains. The rationale behind the contrasting structures of La0.33Sr0.67MnO2.42 and other previously reported reduced La1−xSrxMnO3−y phases is discussed.