Abstract: Two different kinds of experimental approaches yielding three-dimensional structural information on metastable semi-coherent precipitates are demonstrated. By combining high-resolution images from two independent viewing directions a full description of the strain field surrounding a nano-sized Ni4Ti3 precipitate in Ni-Ti can be obtained. The principal axes and strains correlate well with the transformation strain of the observed R-phase transformation close to the precipitate. Using a slice-and-view procedure in a FIB/SEM dual-beam instrument, a three-dimensional voxel dataset is produced from which morphological and distributional information on the same precipitates can be obtained yielding new insight into the particular transformation paths of these alloys, relevant for their functional behaviour.

Abstract: The Pb2Fe2O5 compound with a layered intergrowth structure has been prepared by a solid-state reaction at 700 °C. The incommensurate compound crystallizes in a tetragonal system with a = 3.9037(2) Å, c = 3.9996(4) Å, and q = 0.1186(4)c*, or when treated as a commensurate approximant, a = 3.9047(2) Å, c = 36.000(3) Å, space group I4/mmm. The crystal structure of Pb2Fe2O5 was resolved from transmission electron microscopy data. Atomic coordinates and occupancies of the cation positions were estimated from high-angle annular dark-field scanning transmission electron microscopy data. Direct visualization of the positions of the oxygen atoms was possible using annular bright-field scanning transmission electron microscopy. The structure can be represented as an intergrowth of perovskite blocks and partially disordered blocks with a structure similar to that of the Bi2O2 blocks in Aurivillius-type phases. The A-cation positions at the border of the perovskite block and the cation positions in the Aurivillius-type blocks are jointly occupied by Pb2+ and Fe3+ cations, resulting in a layer sequence along the c axis: PbOFeO2PbOFeO2Pb7/8Fe1/8O1xFe5/8Pb3/8O2Fe5/8Pb3/8. Upon heating, the layered Pb2Fe2O5 structure transforms into an anion-deficient perovskite modulated by periodically spaced crystallographic shear (CS) planes. Considering the layered Pb2Fe2O5 structure as a parent matrix for the nucleation and growth of CS planes allows an explanation of the specific microstructure observed for the CS structures in the PbFeO system.

Abstract: It is well known that the formation of discrete electron levels strongly influences the pairing in metallic nanograins. Here, we focus on another effect of quantum confinement in superconducting grains that was not studied previously, i.e., spatially nonuniform pairing. This effect is very significant when single-electron levels form bunches and/or a kind of shell structure. We find that, in highly symmetric grains, the order parameter can exhibit variations with position by an order of magnitude. Nonuniform pairing is closely related to a quantum-confinement-induced modification of the pairing-interaction matrix elements and size-dependent pinning of the chemical potential to groups of degenerate or nearly degenerate levels. For illustrative purposes, we consider spherical metallic nanograins and also rectangular shapes. We show that the relevant matrix elements are, as a rule, enhanced in the presence of quantum confinement, which favors spatial variations of the order parameter, compensating the corresponding energy cost. The size-dependent pinning of the chemical potential further increases the spatial variation of the pair condensate. The role of nonuniform pairing is smaller in less symmetric confining geometries and/or in the presence of disorder. However, it always remains of importance when the energy spacing between discrete electron levels δ is approaching the scale of the bulk gap ΔB, i.e., δ>0.10.2 ΔB.

Abstract: We demonstrate controllable dual-bath electrodeposition of nickel on architecture-tunable three-dimensional (3D) silver microcrystals. Magnetic hysteresis loops of individual highly faceted Ag-Ni core-shell elements reveal magnetization reversal that comprises multiple sharp steps corresponding to different stable magnetic states. Finite-element micromagnetic simulations on smaller systems show several jumps during magnetization reversal which correspond to transitions between different magnetic vortex states. Structures of this type could be realizations of an advanced magnetic data storage architecture whereby each element represents one multibit, storing a combination of several conventional bits depending on the overall number of possible magnetic states associated with the 3D core-shell shape.

Abstract: The YbRuSn ternary system was investigated and a new material, Yb0.24Sn0.76Ru, with a simple cubic crystal structure, was discovered. Yb0.24Sn0.76Ru has a smaller lattice parameter a=3.217(4) Å, than its isostructural YbRu analogue (a=3.360 Å). Both X-ray diffraction and electron microscopy techniques were used to refine the crystal structure of Yb0.24Sn0.76Ru. It was found that a new compound forms in the CsCl structure, with Ru on the 1a site and a (Yb, Sn) mixture on site 1b. The XRD Rietveld analysis provides the occupation of Yb equal to 0.24, in agreement with the single crystal nano-electron diffraction refinement, which gives the occupation 0.21.

Abstract: We show that a superconducting quantum nanowire undergoes a new type of BCS-BEC crossover each time when an electron subband approaches the Fermi surface. In this case the longitudinal Cooper-pair size drops by two-three orders of magnitude down to a few nanometers. This unconventional BCS-BEC crossover is driven by quantum-size effects rather than by tuning the fermion-fermion interaction.

Abstract: Magnetically coupled superconductorferromagnet hybrids offer advanced routes for nanoscale control of superconductivity. Magnetotransport characteristics and scanning tunneling microscopy images of vortex structures in superconductorferromagnet hybrids reveal rich superconducting phase diagrams. Focusing on a particular combination of a ferromagnet with a well-ordered periodic magnetic domain structure with alternating out-of-plane component of magnetization, and a small coherence length superconductor, we find directed nucleation of superconductivity above the domain wall boundaries. We show that near the superconductor-normal state phase boundary the superconductivity is localized in narrow mesoscopic channels. In order to explore the Abrikosov flux line ordering in F/S hybrids, we use a combination of scanning tunneling microscopy and GinzburgLandau simulations. The magnetic stripe domain structure induces periodic local magnetic induction in the superconductor, creating a series of pinninganti-pinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes nonlinear due to a change in both the number of vortices and the confining potential. In F/S/F hybrids we demonstrate the evolution of the anisotropic conductivity in the superconductor that is magnetically coupled with two adjacent ferromagnetic layers. Stripe magnetic domain structures in both F-layers are aligned under each other, resulting in a directional superconducting order parameter in the superconducting layer. The conductance anisotropy strongly depends on the period of the magnetic domains and the strength of the local magnetization. The anisotropic conductivity of up to three orders of magnitude can be achieved with a spatial critical temperature modulation of 5% of T c. Induced anisotropic properties in the F/S and F/S/F hybrids have a potential for future application in switching and nonvolatile memory elements operating at low temperatures.

Abstract: Using reactive molecular dynamics simulations, we have investigated the effect of single-impact, low-energy (thermal-100 eV) bombardment of a Si(100){2 × 1} surface by atomic and molecular oxygen. Penetration probability distributions, as well as defect formation distributions, are presented as a function of the impact energy for both species. It is found that at low impact energy, defects are created chemically due to the chemisorption process in the top layers of the surface, while at high impact energy, additional defects are created by a knock-on displacement of Si. These results are of particular importance for understanding device performances of silica-based microelectronic and photovoltaic devices.

Abstract: A combined Monte Carlo (MC)/analytical surface model is developed to study the plasma processes occurring during the reactive sputter deposition of TiOx thin films. This model describes the important plasma species with a MC approach (i.e. electrons, Ar+ ions, {\rm O}_2

Abstract: Using the tight-binding formalism we show that in the recently experimentally realized AA-stacked graphite in essence two types of massless relativistic Dirac particles are present with a different effective speed of light. We also investigate how the electronic structure evolves from a single graphene sheet into AA-stacked graphite. It is shown that in contrast to AB-stacked graphene layers, the spectrum of AA-stacked graphene layers can be considered as a superposition of single-layer spectra and only particles with a linear spectrum at the Fermi energy around the K point are present. From the evolution of the band overlap we show that 6 multilayers of AA-stacked graphene already behave as AA-stacked graphite. The evolution of the effective speeds of light of the Dirac particles to their bulk values shows exactly the same behavior. The tight-binding parameters we use to describe AA-stacked graphite and multilayers of graphene are obtained by ab initio calculations.

Abstract: We demonstrate theoretically how local strains in graphene can be tailored to generate a valley-polarized current. By suitable engineering of local strain profiles, we find that electrons in opposite valleys (K or K′) show different Brewster-like angles and Goos-Hänchen shifts, exhibiting a close analogy with light propagating behavior. In a strain-induced waveguide, electrons in K and K′ valleys have different group velocities, which can be used to construct a valley filter in graphene without the need for any external fields.

Abstract: In thispaperweinvestigatewhichprobesizemaximizesthethroughputwhenmeasuringtheradiusof nanoparticlesinhighangleannulardarkfieldscanningtransmissionelectronmicroscopy(HAADFSTEM). The sizeandthecorrespondingcurrentoftheelectronprobedeterminetheprecisionoftheestimateofa particlesradius.Maximizingthroughputmeansthatamaximumnumberofparticlesshouldbeimaged withinagiventimeframe,sothataprespecifiedprecisionisattained.WeshowthatBayesianstatistical experimentaldesignisaveryusefulapproachtodeterminetheoptimalprobesizeusingacertainamount of priorknowledgeaboutthesample.Thedependenceoftheoptimalprobesizeonthedetectorgeometry and thediameter,variabilityandatomicnumberoftheparticlesisinvestigated.Anexpressionforthe optimalprobesizeintheabsenceofanykindofpriorknowledgeaboutthespecimenisderivedaswell.

Abstract: A unified theory of phonon dispersions and piezoelectricity in bulk and multilayers of hexagonal boron nitride (h-BN) is derived. The dynamical matrix is calculated on the basis of an empirical force constant model of intralayer valence and interlayer van der Waals interactions. Coulomb interactions are calculated by Ewalds method, adapted for the three-dimensional (3D) and the multilayer case. The deformation of the ionic charge distribution with long-wave lattice displacements is taken into account. Special attention is devoted to the nonanalytic long-range Coulomb contribution to the dynamical matrix which is different for the 3D crystal and the multilayer case. Consequently there is a splitting of the transverse optical (TO) and longitudinal optical (LO) phonon branches of E1u symmetry and a discontinuity of the A2u branch at the Γ point in 3D h-BN. No such splitting and discontinuity at Γ are present in multilayer crystals with a finite number N of layers. There a diverging bundle of N overbending optical phonon branches emerges from Γ. Borns long-wave theory is applied and extended for the study of piezoelectricity in layered crystals. While 3D h-BN and h-BN multilayers with an even number of layers (symmetry D6h) are not piezoelectric, multilayers with an uneven number of Nu layers (symmetry D3h) are piezoelectric; the piezoelectric coefficient e1,11 is inversely proportional to Nu.

Abstract: It is known that film structure may change dramatically with the extent of surface diffusion during the film growth process. In the present work, surface diffusion, induced thermally or activated by energetic impacts, is investigated theoretically under conditions appropriate for magnetron sputter-deposition of MgAlO thin films with varying stoichiometry. The distribution of surface diffusion energy barriers available to the system was determined for each stoichiometry, which allowed assessing in a qualitative way how much surface diffusion will take place on the time scale available between deposition events. The activation energy barriers increase with the Al concentration in the film, and therefore, the surface diffusion rates in the time frame of typical deposition rates drop, which can explain the decrease in crystallinity in the film structure and the transition to amorphous structure. The deposition process and the immediate surface diffusion enhanced by the energetic adatoms are simulated by means of a molecular dynamics model. The longer-time thermal surface diffusion and the energy landscape are studied by the temperature accelerated dynamics method, applied in an approximate way. The surface diffusion enhanced by the energetic impacts appears to be very important for the film structure in the low-temperature deposition regime.