Abstract: The effect of a parallel magnetic field on the orbital motion of electrons in high-quality superconducting nanowires resulting in a superconductor-to-normal transition which occurs through a cascade of jumps in the order parameter as a function of the magnetic field. Such cascades originate from the transverse size quantization that splits the conduction band into a series of subbands. Here, based on a numerical solution of the Bogoliubov-de Gennes equations for a hollow nanocylinder, we investigate how the quantum-size cascades depend on the confining geometry, i.e., by changing the cylinder radius R and its thickness d we cover the range from the nanowire-like to the nanofilm-like regime. The cascades are shown to become much less pronounced when increasing R/d, i.e., when the nanofilm-like regime is approached. When the temperature is non-zero they are thermally smoothed. This includes the spin-magnetic-field interaction which reduces the critical (depairing) parallel magnetic field H-c,H-parallel to but does not have any qualitative effect on the quantum cascades. From our calculations it is seen that the paramagnetic limiting field H-par significantly exceeds H-c,H-parallel to even in extremely narrow nanocylinders, i.e., when R, d are down to a few nanometers, and H-c,H-parallel to is only about 10% larger when switching-off the spin-magnetic-field interaction in this case. Both characteristic fields, H-c,H-parallel to and H-par, exhibit pronounced quantum-size oscillations. We demonstrate that the quantum cascades and the quantum-size oscillations survive in the presence of surface roughness.

Abstract: A new method for including higher-order Laue zones (HOLZs) effects in an efficient way in electron scattering simulations has been developed and tested by detail calculations. The calculated results by the conventional multislice (CMS) method and the improved conventional multislice (ICMS) method using a large dynamical aperture to avoid numerical errors are compared with accurate results. We have found that the zero-order Laue zones (ZOLZs) reflection cannot be properly described only using the projected potential in the whole unit cell; in general, we need to subslice the electrostatic potential inside the unit cell. It is shown that the ICMS method has higher accuracy than the CMS method for the calculation of the ZOLZ, HOLZ and Pseudo-HOLZ reflections. Hence, ICMS method allows to use a larger slice thickness than the CMS method and reduces the calculation time. (C) 2012 Elsevier B.V. All rights reserved.

Abstract: We report single-crystal X-ray diffuse scattering measurements on C-60.C8H8 fullerene-cubane showing that the C-60 molecules are orientationally disordered at 300 and 150K and get ordered at low temperatures. Monte Carlo simulations provide further insight in the orientational behavior of both C-60 and C8H8 molecules; low-temperature molecular orientations are predicted.

Abstract: This HREM investigation focuses on the influence of point defects on the final epitaxial relation and atomic interface configuration in a CoSi2/Si(1 0 0) heterostructure. A two-step SPE-MBE grown CoSi2/Si(1 0 0) system is used and, by altering the number of deposited Co monolayers in the template layer, the point defect behavior is studied. We propose a film growth model in which the knowledge about the reconstructed (2 x 1) : Si(1 0 0) surface, the point defect behavior in the presence of an interface, especially a silicide interface, the migration of point defects through a lattice by formation of [1 0 0]-split interstitial (dumbbell) atomic configurations, and a new type of extended defect configurations in diamond type materials will all amalgamate. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

Abstract: Two CoSi2/Si1-xGex/Si(100) heterostructures, with different Ge content, made by molecular beam epitaxy are characterized by high resolution electron microscopy. In general, the interface between the CoSi2 thin film and the Si1-xGex layer is of a high structural quality and the strained Si1-xGex layer exhibits few defects. For both samples, different interface structures are present, although the dominant interfacial configuration is similar to the unreconstructed interface present at the CoSi2/Si(100) interface. Only occasionally (2x1) reconstructed interface regions are found which are just a few nanometers in length. Phenomena such as Ge segregation and the introduction of defects are also observed in the Si1-xGex layer. We attribute the minimal presence of the reconstructed interface to both the (2x8):Si1-xGex(100) surface reconstruction and the Ge segregation that takes place. (C) 1999 American Institute of Physics. [S0021-8979(99)02104-0].

Abstract: The crystal structure of BiSr8-xCa3+xO22 has been determined by single-crystal X-ray diffraction. This phase is the same as Bi9Sr11Ca5Oy that was previously studied by several authors as a secondary phase in the Bi-Sr-Ca-Cu-O system and coexists in thermodynamic equilibrium with the superconductors Bi2Sr2CuO6 and Bi2Sr2CaCu2O8 It crystallizes in the monoclinic space group P2(1)/c, with cell parameters a 11.037(3) Angstrom, b = 5.971(2) Angstrom, c = 19.703(7) Angstrom, beta = 101.46(3)degrees Z = 2. The structure was solved by direct methods and full-matrix least-squares refinement. It is built up by perovskite-related blocks of composition [Sr8-xBi2Ca3+xO16] that intergrow with double rows [Bi4O6] running along b. The perovskite blocks are formed by groups of five octahedra that are shifted from each other 3/2 root 2a(p) along [110](p) (a(p) being the parameter of the cubic perovskite subcell) in a zigzag configuration and are aligned with this direction parallel to the one forming an angle of 25" with the c axis. In turn, the perovskite blocks [Sr8-xBi2Ca3+xO16] are shifted from each other 1/2 of both a(p) and root 2a(p) along [100](p) and [110](p), respectively. In the double rows, two trivalent bismuth atoms are placed, forming dimeric anion complexes [Bi2O6].(6-).6- The oxygen atoms around bismuth in these dimers are placed in the vertexes of a distorted trigonal bipyramid, with one vacant position that would be occupied by the lone pairs characteristic for the electronic configuration of Bi(III). The B sites in the perovskite blocks are occupied by pentavalent bismuth atoms and calcium atoms; the remaining Sr and Ca ions occupy the A sites of the perovskite blocks with coordination numbers with oxygen ranging from 10 to 12. The mean valence for Bi is +3.67 [33.3% of Bi(V) and 66.7% of Bi(III)]. The oxygen vacancies are located in the boundaries between domains having the two possible configurations of the perovskite subcell as in the anionic superconductor Bi3BaO5.5. The oxidation of Bi6Sr8-xCa3+xO22 at 650 degrees C allows the complete filling of the oxygen vacancies to form the double perovskite (Sr2-xCax)Bi1.4Ca0.6O6 that shows 92.5% of bismuth in +5 oxidation state. The experimental high-resolution electon microscopy image and the electron diffraction pattern of powder samples along the [010]* zone axis are in good agreement with those calculated from the structural model obtained by single-crystal X-ray diffraction. The material is almost free of defects and the occurrence of planar defects is very exceptional.

Abstract: Structural phase transitions in two vertically or horizontally coupled channels of strongly interacting particles are investigated. The particles are free to move in the x direction but are confined by a parabolic potential in the y direction. They interact with each other through a screened power-law potential (r(-n)e(-r/lambda)). In vertically coupled systems, the channels are stacked above each other in the direction perpendicular to the (x, y) plane, while in horizontally coupled systems both channels are aligned in the confinement direction. Using Monte Carlo (MC) simulations we obtain the ground-state configurations and the structural transitions as a function of the linear particle density and the separation between the channels. At zero temperature, the vertically coupled system exhibits a rich phase diagram with continuous and discontinuous transitions. On the other hand, the horizontally coupled system exhibits only a very limited number of phase transitions due to its symmetry. Further, we calculated the normal modes for the Wigner crystals in both cases. From MC simulations, we found that in the case of vertically coupled systems, the zigzag transition is only possible for low densities. A Ginzburg-Landau theory for the zigzag transition is presented, which predicts correctly the behavior of this transition from which we interpret the structural phase transition of the Wigner crystal through the reduction of the Brillouin zone.

Abstract: The nuclear and magnetic structure of a novel oxychloride Pb4BiFe4O11Cl has been studied over the temperature range 1.5700 K using a combination of transmission electron microscopy and synchrotron and neutron powder diffraction [space group P4/mbm, a = 5.5311(1) Å, c = 19.586(1) Å, T = 300 K]. Pb4BiFe4O11Cl is built of truncated (Pb,Bi)3Fe4O11 quadruple perovskite blocks separated by CsCl-type (Pb,Bi)2Cl slabs. The perovskite blocks consist of two layers of FeO6 octahedra located between two layers of FeO5 tetragonal pyramids. The FeO6 octahedra rotate about the c axis, resulting in a √2ap × √2ap × c superstructure. Below TN = 595(17) K, Pb4BiFe4O11Cl adopts a G-type antiferromagnetic structure with the iron magnetic moments confined to the ab plane. The ordered magnetic moments at 1.5 K are 3.93(3) and 3.62(4) μB on the octahedral and square-pyramidal iron sites, respectively. Pb4BiFe4O11Cl can be considered a member of the perovskite-based An+1BnO3n1Cl homologous series (A = Pb/Bi; B = Fe) with n = 4. The formation of a subsequent member of the series with n = 5 is also demonstrated.

Abstract: After a short review of electron tomography techniques for materials science, this overview will cover some recent results on different shape memory and nanostructured metallic systems obtained by various three-dimensional (3D) electron imaging techniques. In binary NiTi, the 3D morphology and distribution of Ni4Ti3 precipitates are investigated by using FIB/SEM slice-and-view yielding 3D data stacks. Different quantification techniques will be presented including the principal ellipsoid for a given precipitate, shape classification following a Zingg scheme, particle distribution function, distance transform and water penetration. The latter is a novel approach to quantifying the expected matrix transformation in between the precipitates. The different samples investigated include a single crystal annealed with and without compression yielding layered and autocatalytic precipitation, respectively, and a polycrystal revealing different densities and sizes of the precipitates resulting in a multistage transformation process. Electron tomography was used to understand the interaction between focused ion beam-induced Frank loops and long dislocation structures in nanobeams of Al exhibiting special mechanical behaviour measured by on-chip deposition. Atomic resolution electron tomography is demonstrated on Ag nanoparticles in an Al matrix.

Abstract: Richardson approach provides an exact solution of the pairing Hamiltonian. This Hamiltonian is characterized by the electron-hole pairing symmetry, which is however hidden in Richardson equations. By analyzing this symmetry and using an additional conjecture, fulfilled in solvable limits, we suggest a simple expression of the ground state energy for an equally-spaced energy-level model, which is applicable along the whole crossover from the superconducting state to the pairing fluctuation regime. Solving Richardson equations numerically, we demonstrate a good accuracy of our expression.