Abstract: The scattering of a Gaussian wave packet in armchair and zigzag graphene edges is theoretically investigated by numerically solving the time-dependent Schrodinger equation for the tight-binding model Hamiltonian. Our theory allows us to investigate scattering in reciprocal space, and depending on the type of graphene edge we observe scattering within the same valley, or between different valleys. In the presence of an external magnetic field, the well-known skipping orbits are observed. However, our results demonstrate that in the case of a pseudomagnetic field, induced by nonuniform strain, the scattering by an armchair edge results in a nonpropagating edge state.

Abstract: Hydrogen incorporation in diluted nitride semiconductors dramatically modifies the electronic and structural properties of the crystal through the creation of nitrogen-hydrogen complexes. We report a convergent beam electron-diffraction characterization of diluted nitride semiconductor-heterostructures patterned at a sub-micron scale and selectively exposed to hydrogen. We present a method to determine separately perpendicular mismatch and static disorder in pristine and hydrogenated heterostructures. The roles of chemical composition and strain on static disorder have been separately assessed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752464]

Abstract: We derive the extended Ginzburg-Landau (GL) formalism for a clean s-wave two-band superconductor by employing a systematic expansion of the free-energy functional and the corresponding matrix gap equation in powers of the small deviation from the critical temperature tau = 1 – T/T-c. The two lowest orders of this expansion produce the equation for T-c and the standard GL theory. It is shown that in agreement with previous studies, this two-band GL theory maps onto the single-band GL model and thus fails to describe the difference in the spatial profiles of the two-band condensates. We prove that this difference appears already in the leading correction to the standard GL theory, which constitutes the extended GL formalism. We derive linear differential equations that determine the leading corrections to the band order parameters and magnetic field, discuss the validity of these equations, and consider examples of an important interplay between the band condensates. Finally, we present numerical results for the thermodynamic critical magnetic field and temperature-dependent band gaps for recent materials of interest, which are in very good agreement with those obtained from the full BCS approach in a wide temperature range. To this end, we emphasize the advantages of our extended GL theory in comparison with the often used two-component GL-like model based on an unreconstructed two-band generalization of the Gor'kov derivation.

Abstract: The electronic and optical properties of InAs/GaSb core-shell nanowires are investigated within the effective mass k . p approach. These systems have a broken band gap, which results in spatially separated confinement of electrons and holes. We investigated these structures for different sizes of the InAs and GaSb core and shell radius. We found that for certain configurations, the conduction band states penetrate into the valence band states resulting in a negative band gap (E-g < 0), which leads to a conduction band ground state that lies below the valence band ground state at the Gamma point. For certain core-shell wires, only one conduction band state penetrates into the valence band and in this case, a minigap Delta opens up away from the Gamma point and as a consequence the electronic properties of the nanowire now depend on both E-g and Delta values.

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: Thumbnail image of graphical abstract To investigate nanoassemblies in three dimensions, electron tomography is an important tool. For large nanoassemblies, it is not straightforward to obtain quantitative results in three dimensions. An optimized acquisition technique, incoherent bright field scanning transmission electron microscopy, is combined with an advanced 3D reconstruction algorithm. The approach is applied to quantitatively analyze large nanoassemblies in three dimensions.

Abstract: We describe investigations of the largely unexplored field of mesoscopic type-I superconductors. Micromagnetometry and 3D Ginzburg-Landau simulations of our single crystal β-tin samples in this regime reveal size- and temperature-dependent supercritical fields whose behavior is radically different from the bulk critical field HcB. We find that complete suppression of the intermediate state in medium-size samples can result in a surprising reduction of the critical field significantly below HcB. We also reveal an evolution of the superconducting-to-normal phase transition from the expected irreversible first order at low temperatures through the previously unobserved reversible first-order to a second-order transition close to Tc, where the critical field can be many times larger than HcB. Finally, we have identified striking correlations between the mesoscopic Hc3 for nucleation of surface superconductivity and the thermodynamic Hc near Tc. All these observations are entirely unexpected in the conventional type-I picture.

Abstract: The calculation of defect levels in semiconductors within a density functional theory approach suffers greatly from the band gap problem. We propose a band gap correction scheme that is based on the separation of energy differences in electron addition and relaxation energies. We show that it can predict defect levels with a reasonable accuracy, particularly in the case of defects with conduction band character, and yet is simple and computationally economical. We apply this method to ZnO doped with group III elements (Al, Ga, In). As expected from experiment, the results indicate that Zn substitutional doping is preferred over interstitial doping in Al, Ga, and In-doped ZnO, under both zinc-rich and oxygen-rich conditions. Further, all three dopants act as shallow donors, with the +1 charge state having the most advantageous formation energy. Also, doping effects on the electronic structure of ZnO are sufficiently mild so as to affect little the fundamental band gap and lowest conduction bands dispersion, which secures their n-type transparent conducting behavior. A comparison with the extrapolation method based on LDA+U calculations and with the HeydScuseriaErnzerhof hybrid functional (HSE) shows the reliability of the proposed scheme in predicting the thermodynamic transition levels in shallow donor systems.

Abstract: When studying photocatalysis it is important to consider, beside the chemical approach, the engineering part related to process optimisation. To achieve this a fixed bed photocatalytic set-up consisting of different catalyst placings, in order to vary catalyst distribution, is studied. The use of a fixed quantity of catalyst placed packed or randomly distributed in the reactor, results in an almost double degradation for the distributed catalyst. Applying this knowledge leads to an improved performance with limited use of catalyst. A reactor only half filled with catalyst leads to higher degradation performance compared to a completely filled reactor. Taking into account this simple process optimisation by better distributing the catalyst a more sustainable photocatalytic air purification process is achieved. (C) 2012 Elsevier B.V. All rights reserved.

Abstract: In 1956 Cooper demonstrated (1956 Phys. Rev. 104 1189) that, no matter how weak the attraction is, two electrons in three-dimensional (3D) space just above the Fermi sea could be bound. In this work we investigate the influence of confinement on the binding energy of a Cooper pair. We show that confinement-induced modification of the Fermi sea results in a significant increase of the binding energy, when the bottom of an energy subband is very close to the Fermi surface.

Abstract: Recent advances in near-edge X-ray-absorption fine-structure spectroscopy coupled with transmission X-ray microscopy (NEXAFS-TXM) allow large-area mapping investigations of individual nano-objects with spectral resolution up to E/Delta E = 104 and spatial resolution approaching 10 nm. While the state-of-the-art spatial resolution of X-ray microscopy is limited by nanostructuring process constrains of the objective zone plate, we show here that it is possible to overcome this through close coupling with high-level theoretical modelling. Taking the example of isolated bundles of hydrothermally prepared sodium titanate nanotubes ((Na,H)TiNTs) we are able to unravel the complex nanoscale structure from the NEXAFS-TXM data using multichannel multiple-scattering calculations, to the extent of being able to associate specific spectral features in the O K-edge and Ti L-edge with oxygen atoms in distinct sites within the lattice. These can even be distinguished from the contribution of different hydroxyl groups to the electronic structure of the (Na,H)TiNTs.

Abstract: Over the past years a number of studies have described the instability of the pigment cadmium yellow (CdS). In a previous paper we have shown how cadmium sulfide on paintings by James Ensor oxidizes to CdSO4 center dot H2O. The degradation process gives rise to the fading of the bright yellow color and the formation of disfiguring white crystals that are present on the paint surface in approximately 50 mu m sized globular agglomerations. Here, we study cadmium yellow in the painting “Flowers in a blue vase” by Vincent van Gogh. This painting differs from the Ensor case in the fact that (a) a varnish was superimposed onto the degraded paint surface and (b) the CdS paint area is entirely covered with an opaque crust. The latter obscures the yellow color completely and thus presents a seemingly more advanced state of degradation. Analysis of a cross-sectioned and a crushed sample by combining scanning microscopic X-ray diffraction (mu-XRD), microscopic X-ray absorption near-edge spectroscopy (mu-XANES), microscopic X-ray fluorescence (mu-XRF) based chemical state mapping and scanning microscopic Fourier transform infrared (mu-FT-IR) spectrometry allowed unravelling the complex alteration pathway. Although no crystalline CdSO4 compounds were identified on the Van Gogh paint samples, we conclude that the observed degradation was initially caused by oxidation of the original CdS pigment, similar as for the previous Ensor case. However, due to the presence of an overlying varnish containing lead-based driers and oxalate ions, secondary reactions took place. In particular, it appears that upon the photoinduced oxidation of its sulfidic counterion, the Cd2+ ions reprecipitated at the paint/varnish interface after having formed a complex with oxalate ions that themselves are considered to be degradation products of the resin and/or oil in the varnish. The SO42- anions, for their part, found a suitable reaction partner in Pb2+ ions stemming from a dissolved lead-based siccative that was added to the varnish to promote its drying. The resulting opaque anglesite compound in the varnish, in combination with the underlying CdC2O4 layer at the paint/varnish interface, account for the orange-gray crust that is disfiguring the painting on a macroscopic level. In this way, the results presented in this paper demonstrate how, through a judicious combined use of several microanalytical methods with speciation capabilities, many new insights can be obtained from two minute, but highly complex and heterogeneous paint samples.

Abstract: The interaction of an injected electron towards a quantum dot (QD) containing a single confined electron is investigated using a flexible time-dependent quantum mechanics formalism, which allows both electrons to move and undergo quantum transitions. Different scenarios combining quantum dot dimensions, dielectric constant, injected wave packet energy, and width were explored, and our main results are: (i) due to the large characteristic transitions times between the confined state in the quantum dot and the delocalized state in the continuum, it is relatively difficult to ionize the occupied QD by Coulomb interaction solely and (ii) the charging state of the quantum dot can be sensed by direct injection of charges. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759292]

Abstract: Within the time-dependent Ginzburg-Landau theory we study the dynamic properties of current-carrying superconducting strips in the presence of a perpendicular magnetic field. We found pronounced voltage peaks as a function of the magnetic field, the amplitude of which depends both on sample dimensions and external parameters. These voltage oscillations are a consequence of moving vortices, which undergo alternating static and dynamic phases. At higher fields or for high currents, the continuous motion of vortices is responsible for the monotonic background on which the resistance oscillations due to the entry of additional vortices are superimposed. Mechanisms for such vortex-assisted resistance oscillations are discussed. Qualitative changes in the magnetoresistance curves are observed in the presence of random defects, which affect the dynamics of vortices in the system.

Abstract: In this paper, an overview is given of modeling investigations carried out in our research group for a better understanding of plasmas used for medical, environmental and nano applications. The focus is both on modeling the plasma chemistry and the plasma-surface interactions. The plasma chemistry provides the densities and fluxes of the important plasma species. This information can be used as input when modeling the plasma-surface interactions. The combination of plasma simulations and plasma – surface interaction simulations provides a more comprehensive understanding of the underlying processes for these applications.