Voordracht georganiseerd door EDF

Onderwerp: The Energy Dependence of the Underlying Event in Hadronic Collisions 

Spreker: Rick Field, University of Florida

Abstract:

 Charged particles production (pT > 0.5 GeV/c, |eta| < 0.8) is studied in proton-antiproton collisions at CDF at 300 GeV, 900 GeV, and 1.96 TeV and in proton-proton collisions at CMS at 7 TeV. The 300 GeV and 900 GeV data are a result of the Tevatron Energy Scan which was performed just before the Tevatron was shut down.  The direction of the leading charged particle in each event, PTmax, is used to define three regions of eta-phi space; toward, away, and transverse.  The transverse region is very sensitive to the underlying event and is separated into a MAX and MIN transverse region, which helps separate the hard component (initial and final-state radiation) from the beam-beam remnant and multiple parton interaction components of the scattering.  The data are corrected to the particle level and are compared with the PYTHIA 6.4 Tune Z1 at the particle level (i.e. generator level).  The goal is to study the energy dependence of the "underlying event" by examining the behavior of the transverse region as a function of PTmax at four center-of-mass energies.

Waar en wanneer : Dinsdag 28 mei 2013 om  14.30 u.   in lokaal U.203 , Campus Groenenborger

Voordracht georganiseerd door EDF

Onderwerp: Improved cross sections using re-summation for squark and gluino production at the LHC
Spreker: Chris Wever (Utrecht)

Abstract:

We start by considering the production of strongly interacting, heavy particle pairs at the LHC. When the centre of mass energy is close to the production threshold of the pair, their cross sections receive large higher-loop QCD corrections. These corrections are classified as the so-called soft logarithms and Coulomb singularities and they lead to a break down of the usual perturbation expansion. In this talk I review the origin of these large corrections and explain how they can be directly resummed by using Effective Field theories. Finally, I will present some resummed results for the pair production cross sections of heavy squarks and gluinos. Based on: arXiv:1202.2260 and 1211.3408.

Waar en wanneer : Maandag 13 mei 2013 om 14.00 u. in lokaal U.203 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Nanostructures and Supraparticles from Nanoparticles
Spreker: Marijn van Huis, Debye Institute for Nanomaterials Science, Utrecht University, The Netherlands

Abstract:

While many kinds of nanoparticles (NPs) can currently be synthesized, the assembly of these particles into functional three-dimensional nanomaterials is still at an early stage. For the full development of collective nanoparticle properties (catalytic activity, lasing, optical properties), it is required to arrive at binary and ternary superlattices that are in one phase and in well-controlled geometries. In the Soft Condensed Matter group, self-organisation of NPs is initiated in water-in-oil and oil-in-water emulsion droplets, leading to spherical superlattices having e.g., icosahedral and fcc packings. Concurrently, stabilization approaches are developed in order to ensure temporal and thermal stability of NPs and their superlattices. The characterization of the heterogeneous nanoparticles, and of the large assemblies in which they are contained, puts high demands on TEM techniques and methodologies. Several examples will be treated, in particular supraballs of silica-coated Au nanorods, supraballs of FeO/CoFe2O4 core/shell particles, and photocatalytic CdS/Au nanorods.

Waar en wanneer : Vrijdag 3 mei 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Recent Methods and Techniques in Optical Microscopy

Spreker: Emeric Mudry, EMAT, University of Antwerp

Abstract:

Optical microscopy is a centuries old technique, but has experienced a huge development since the introduction of laser, fluorescent markers and CCD cameras. This lecture aims at presenting some of the recently developed techniques that might be of interest for the electron microscopy community. It mainly focuses on phase measurement techniques, their applications and on different techniques for three-dimensional imaging. Finally some other interesting techniques are quickly reviewed.

Waar en wanneer : Vrijdag 26 april 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Quantitative characterization of microstructure evolution during the fabrication of an Al-Mg-Si-Cu 6111 Alloy

Spreker: Kai Li, EMAT, University of Antwerp

Abstract:

Aluminum alloys are important commercial alloys mainly due to their light weight and excellent corrosion resistance. The designing of aluminum alloys with the aid of computational simulations urgently needs exact quantitative experimental data of the microstructure evolution during fabrication as verifications. In this work, three methods such as scanning electron microscopy, convergent beam electron diffraction and 3-dimensional atom probe are applied to quantitatively characterize the microstructures of different stages of the fabrication, and the volume fractions of different types of second phase particles at different size scales have been measured. The advantages and drawbacks of the three methods will be discussed.
 

Waar en wanneer : Vrijdag 19 april 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door TQC

Onderwerp : Quantum Gases: Fluctuations & Dynamics

Spreker : Nick Proukakis, Newcastle University & Joint Quantum Centre Durham-Newcastle

Abstract:

Ultracold trapped bosonic gases have become a standard test-bed for the study of macroscopic quantum coherence and many-body dynamics under different conditions, due to their excellent experimental control. In addition to the usual Bose-Einstein phenomenon exhibited in three-dimensional geometries, such inhomogeneous systems also enable the controlled study of the enhancement of fluctuations in geometries of reduced dimensionalities. Although different models exist for describing such systems, an established approach that captures most of the relevant physics in the weakly-interacting limit is based on a nonlinear Langevin equation known as the stochastic Gross-Piteavskii equation, which does not rely on the usual picture of symmetry-breaking. In this talk I will show how this method can be used to describe a range of relevant phenomena in one, two and three dimensions, showing clearly its ab initio predictive power by direct comparison to numerous experiments, while also outlining some current limitations in its dynamical implementation.
 

Waar en wanneer : Maandag 15 april 2013 om 16.00 u. in lokaal U.408 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Producing electron vortex beams by (ab-)using the aberration corrector.

Spreker: Laura Clark, EMAT, University of Antwerp

Abstract:

The field of electron vortex beams is very new and developing rapidly.  Following the first experimental demonstrations in 2010, many developments have been made in the techniques for electron vortex production.  Methods used thus far employ techniques borrowed from the field of singular optics, such as spiral phase plates, holographic masks and distortion of Hermite-Gaussian modes. Here we present an alternative method for producing electron vortex beams, ideally suited for use in an aberration corrected TEM, avoiding some of the side effects of the other techniques.  The effectiveness of this method is compared to the published techniques and we discuss the relative merits of the different electron vortex production methods.
 

Waar en wanneer : Vrijdag 12 april 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT
 

Onderwerp : Microstructure study of the Coffin-Shaped ZSM-5 Zeolite Crystals Unraveled by FIB-Assisted HRTEM

Spreker : Jiangbo Lu, EMAT, University of Antwerp

Abstract:

Zeolite ZSM-5 crystals possessing the MFI (mordenite framework inverted)-type micro-porous structure have drawn much attention because of their industrial applications as a leading shape selective catalyst in the synthesis of dimethylbenzene. Because molecular diffusion barriers inside zeolites have a large impact on their catalytic behavior, and internal intergrowth boundaries might act as such diffusion barriers to the passing molecules, it is essential to know the details of the various intergrowth boundaries and the internal architecture inside the crystal. In this talk, I will show some results about microstructure study of the coffin-shaped ZSM-5 zeolite crystals unraveled by FIB-assisted HRTEM. This talk includes two major parts. In the first part, I will show some results about the intergrowth of 90° rotational components and of ramps. 90° rotational boundaries with local zigzag features between different intergrowth components are observed in the main part of crystal. A new kind of displacement boundary is described. The formation of this displacement boundary at unit cell scale is illustrated, and a multi-site nucleation model of 90° intergrowth components projected along [001] is proposed. Based on lamellas prepared with FIB from different positions of the ramps, the orientation relationships between ramp and main part of the crystal are studied, and the 3D morphology of the ramp is illustrated. In the second part, I will show some results about the internal architecture of the coffin-shaped ZSM-5 crystals in which the hourglass structure was observed by optical microscopy. The debating about the origin of the hourglass structure lasts for years. Our FIB-assisted HRTEM observation revealed the shapes of the interfaces inside the crystal, which suport the so-called 2-component model about the origin of the hourglass structure.

Waar en wanneer : Vrijdag 22 maart 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT  

Onderwerp: Deposition of metal and metal oxide nanostructures using carbon as a template

Spreker: Mert Kurttepeli, EMAT, University of Antwerp

Abstract:

The different allotropes of carbon are ideal templates for the deposition of metal or metal oxide nanostructures such as nanotubes, thin films and nanoparticles. These nanostructures can be used at a wide variety of applications commonly found in everyday life. In this talk, the characteristics and the interactions of the nanostructures with their templates will be discussed,  which will enable a better understanding of the growth mechanisms involved. Nanostructured thin films of titanium dioxide and nanotubes of titanium dioxide and platinum processed by atomic layer deposition, as well as platinum/sucrose templated carbon will be analysed with respect to phase content, morphology and growth characteristics.

Waar en wanneer : Vrijdag 15 maart 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

  Voordracht georganiseerd door EDF  

Onderwerp : Analysis of the combined HERA data with the impact parameter dependent saturation model

Spreker: Merijn Van der Klundert, EDF, University of Antwerp

Waar en wanneer : Dinsdag 12 maart 2013 om 16.30 u.   in lokaal U.203 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp : Method and Applications of Model-Based Scanning Transmission Electron Microscopy

Spreker : Annick De Backer , EMAT, University of Antwerp

Abstract:

Quantitative electron microscopy allows one to obtain numbers for quantities hidden in experimental high resolution images, such as atomic positions and scattering intensities. In this talk, it will be explained how such quantities can be estimated. An optimized estimation procedure for application to larger nanostructures will be discussed. Furthermore, a methodology to count the number of atoms in monotype nanocrystals from high resolution high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images will be illustrated by means of different experimental examples. The validity of the method will be confirmed using image simulations and it will be shown that the high sensitivity of these measurements enables us to count atoms with trustworthy single atom sensitivity.

Waar en wanneer : Vrijdag 8 maart 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door TQC 

Onderwerp: Quantum Field Theory of Black Swan Events

Spreker: Prof. Hagen Kleinert, Frei Universität Berlin, Germany

Abstract:

Prof. Kleinert, a leading expert in path integration, will describe the link between extreme events, stochastic differential equations, fractional derivatives and quantum field theory.

Waar en wanneer : donderdag 7 maart 2013 om 16.00 u.  in lokaal U.408 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp : How valid are the Multislice simulations?

Spreker : Ivan Lobato , EMAT, University of Antwerp

Abstract:

Multislice image simulation of transmission electron microscopy is a standard tool in the microscopy field. It was originally proposed from physical-optical principles and it has shown to be a good numerical procedure for solving the high-energy Schrödinger equation that describes the electron-specimen interaction. The classical multislice method inherently uses different approximations like forward scattering, not sub-slicing the atomic potential, sharp boundaries, first order multislice expansion, all these approximations are used to speed up calculations. In this presentation, I will discuss these approximations and study they range of validity. We can void almost all these approximations by performing the calculation in real space of the n-moment of the potential distribution (by using the symmetry of the atomic potential, Legendre-Gauss Quadrature, etc.), using high order multislice expansion and proper inclusion of the surface boundary. These approximations might represent an important part of the total intensity for quantitative image simulations, but the proper inclusion of them are computational expensive. Nowadays, modern GPU computing can help us to reduce the computational cost. Therefore, it is important to write a new Multislice code that use GPU computing and the minimum number of approximation to solve the high-energy Schrödinger equation. The new code was written in cuda C++ and compile as a MEX file, in this way it can be called as a normal function from Matlab. In the second part of my talk, I will compare image simulations for TEM, PDE, and STEM carry out our code and the STEMSIM code.

Waar en wanneer : Vrijdag 1 maart 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EDF

Onderwerp : Recent results in high energy factorisation: momentum space equations and matrix elements

Spreker : Dr. K. Kutak (Instytut Fizyki Jądrowej, Kraków)

Abstract:

 We overview results obtained within high energy factorisation focusing on recently obtained equations and framework for calculations of matrix elements. The recent developement was motivated by the regime of QCD explored nowadays at LHC, where both the total energy of collision and momenta transfers are high. In order to study such effects like parton saturation in final states one is inevitably led

to investigate how to combine physics of the Color Glass Condensate with physics of exclusive processes. We propose a method how to combine these two. Also a new framework for obtaining matrix elements with off shell initial state gluons is presented. Such framework is necessary for efficient method of calculation of cross sections in high energy factorisation.

Waar en wanneer : Dinsdag 26 februari 2013 om16.30 u.  in lokaal U.203 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Quantitative composition determination at the atomic level using model-based high-angle annular dark field scanning transmission electron microscopy

Spreker: Gerardo T Martinez , EMAT, University of Antwerp

Abstract:

Recently, quantitative HAADF-STEM (High Angle Annular Dark Field Scanning Transmission Electron Microscopy) has drawn attention because of its capability to extract sample structure parameters with high accuracy and precision, such as composition, number of atoms and atomic column positions. The quantification process relies on a comparison of experimental images with time consuming and computing demanding simulations. In this talk, I will explain an alternative quantification procedure based on statistical parameter estimation theory using an empirical physics-based model. This method allows one to obtain local structural and chemical information that can be used as an input for detailed numerical simulations. This will be illustrated using practical examples. Furthermore, the underlying assumptions of the method will be discussed as well as important considerations when simulating HAADF-STEM images.

Waar en wanneer : Vrijdag 22 februari 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp : Atomic scale investigation of a PbTiO₃/SrRuO₃/DyScO₃heterostructure
Spreker : Ricardo Juan Egoavil Escobar , EMAT, University of Antwerp

Abstract:

Ferroelectric thin films have become an important and exciting field of research within material science because of their many technological applications. An essential component to create a contact to these films is a metallic conductive SrRuO₃ oxide layer. Epitaxially grown SrRuO₃ thin films were used as an intermediate layer between a ferroelectric PbTiO₃ film and a DyScO₃ substrate, to pattern the ferroelectric domains walls in the PbTiO₃. The aim of this talk is to provide a thorough structural and chemical characterization at the atomic scale, leading to a comprehensive understanding of the detailed growth of SrRuO₃ in heterostructures. Direct determination of the exact stacking of the atomic planes at the SrRuO₃/DyScO₃ as well as PbTiO₃/DyScO₃ interfaces is provided. A detailed atomic model of the interfaces is proposed which is confirmed by the excellent match between experimental and simulated images based on the proposed structure. In addition to these findings, the atomic structure of the PbTiO3 film has been investigated.

Waar en wanneer : Vrijdag 15 februari 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp : Peptide self-assemblies: from the supramolecular structure and the mechanism of formation to the size control

Spreker: Maité Paternostre, CEA iBiTec-S / SB²SM & UMR CNRS, Gif-sur-Yvette, France

Abstract:

Supramolecular self-assembly is an attractive pathway for bottom-up synthesis of novel nanomaterials. In particular, this approach allows the spontaneous formation of structures of well defined shapes and monodisperse characteristic sizes. Since nanotechnology is mainly relying on size-dependent physical phenomena, the control of the monodispersity is required, but the possibility of tuning the size is also essential. For self-assembling systems, shape, size and monodispersity are mainly settled by the chemical structure of the building block. Tentative of changing noticeably the size by chemical modification usually end up with the loss of self-assembly. In this context, Lanreotide is a dicationic octapeptide spontaneously forming long nanotubes (few hundreds of mm) but monodispersed in diameter (24.4nm). Their molecular and supramolecular structure and their mechanism of formation have been recently solved. The most noteworthy feature of these nanotubes is that their walls are curved bi-dimentional crystals and the mechanism of formation of these nanotubes shows that the curvature radius of the nanotube is fixed at a very early stage of the assembly, upholding the idea that molecular determinants are controlling the curvature radius. For diameter tuning, we based our strategy on a structural approach. We modified the size of a precise aromatic amino acid involved in close contacts between peptide within the nanotube walls. We demonstrate that this approach indeed enable the accurate tuning of the diameter of the nanotubes from 9 to 35 nm while keeping a strict monodispersity. We finally build a geometrical model taking into account the close-contact that explains how a modification of a few Å of a single aromatic residue induces a 4-fold increase of nanotube diameter. We further demonstrate the application of such strategy by the formation of composite (silica-peptide) nanotubes of various diameters. We also explored the role of the counterions and repulsive forces in the self-assembly mechanism and by this study we evidence that counterions are tightly bound to the nanotubes and that specific interactions occur.

Waar en wanneer : Vrijdag 1 februari 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Electron Vortex Beams: Behavior in a magnetic field, and applications to plasmonics

Spreker: Giulio Guzzinati, EMAT, University of Antwerp

Abstract:

Electron vortex beams are a new and attractive field in electron microscopy, holding significant promise for several applications.

This presentation will be divided into three parts. The first part is a brief introduction on electron vortices, their main properties in contrast to the conventional beams employed in electron Microscopy, and how they are produced in a Transmission Electron Microscope. The second part focuses on the interaction of the magnetic properties of electron vortex beams with external magnetic fields such as those of the lenses of electron microscopes. This interaction determines additional phases in a vortex wavefront and manifests in rich rotational phenomena that have been experimentally studied. These results shed a new light on the common phenomenon of Larmor rotation in the TEM. The third part introduces chiral plasmons, which are developing as promising branch of plasmonics, with applications in biosensing and metamaterials. Our experiments with chiral plasmons and electron vortices will be presented.

Waar en wanneer : Vrijdag 25 januari 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EDF 

Onderwerp: An application of the theory of solvable groups to the physics of Higgs bosons

Spreker: Dr. Igor Ivanov (Université de Liège)

Abstract:

Symmetries play a pivotal role in physics, and in particular in construction of non-minimal Higgs models in particle physics. One rather challenging problem here is classify finite symmetry groups in these models. I will report our recent solution of this problem for the so-called three-Higgs-doublet model. This work can be viewed as an interesting physics application of the theory of solvable groups and, in particular, of the Burnside's theorem. 

Waar en wanneer : Vrijdag 18 januari 2013 om  15.00 u. in lokaal U.203 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Characterization of nanocrystalline Pd thin films using CTEM, ACOM-TEM and HRTEM

Spreker: Behnam Amin-Ahmadi, EMAT, University of Antwerp

Abstract:

The influence of the deposition rate on the formation of growth nanotwins in nanocrystalline Palladium films deposited by electron beam evaporation has been investigated. The statistics results reveal that twin boundary (TB) density, volume fraction of grains containing twins and dislocation density increase with increasing deposition rate. The coherency of the TBs decreased significantly at highest deposition rate. The fraction of high angle grain boundaries (in the range of 55-65˚) continuously increases with increasing the deposition rate. The effect of Hydrogen on the microstructure changes of Pd thin film was also investigated. HRTEM results show that coherency of the TBs decreased after H loading. Surprisingly, the ∑3‹112› incoherent TBs dissociate after hydriding into two phase boundaries bounding a 9R phase. This phase was not expected because of the high stacking fault energy of Pd. This observation is connected to the influence of the Hydrogen on the stacking fault energy of Pd and the high compressive stresses building up during hydriding.

Waar en wanneer : Vrijdag 18 januari 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Quantitative structure determination of large three-dimensional nanoparticle assemblies

Spreker: Thomas Altantzis, EMAT, University of Antwerp

Abstract:

Nanoparticle assemblies have recently gained increasing interest, since it is an attractive route towards systems in which the properties can be tuned through parameters such as the size of the individual particles and the nature of their 3-dimensional (3D) stacking. In order to investigate these systems in 3 dimensions at the nanoscale, electron tomography can be used. In the case of small assemblies with a size less than 200 nm, electron tomography is performed using HAADF-STEM. For large nanoassemblies however, it is not straightforward to extract quantitative results by applying the conventional approaches of acquisition and reconstruction, due to the appearance of several artefacts, especially in the central part of the volume. We have optimized both the acquisition and reconstruction methodology and this enables us to study large assemblies in a quantitative manner as well.

Waar en wanneer : Vrijdag 11 januari 2013 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT / Seminar organised by EMAT

Onderwerp: ELNES: a powerful tool to investigate local boron environment and distribution in boron-doped nanocrystalline diamond

Spreker: Yinggang Lu, EMAT, University of Antwerp

Abstract:

Boron-doped nanocrystalline diamond films grown under different conditions have been investigated using a combination of high resolution annular dark-field scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy. Using these tools, the local boron environment, distribution and concentration have been determined. The boron energy loss near edge structures reveals a distinct difference in coordination of the B dopants in "pristine" diamond areas and in defective regions. Tetrahedral and trigonal coordination of boron were determined in "pristine" and defective regions respectively. Quantitative B concentration maps evidence no preferential embedding of boron in grain boundaries, but there is preferential embedding at twin boundaries and incoherent defective regions. The energy shift between "pristine" and "defective" areas of the carbon σ* peak demonstrates that there is a bond length variation when different amounts of boron are incorporated into the crystal lattice.

Waar en wanneer : Vrijdag 7 december 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: EM study of precipitates in as-cast and annealed Ni-Ti-Nb shape memory alloys and their influence on the martensitic transformation

Spreker: Hui Shi, EMAT, University of Antwerp

Abstract:

Nb-rich precipitates in the matrix of as-cast and annealed Ni45.5Ti45.5Nb9 alloys are investigated by scanning and scanning transmission electron microscopy, including slice-and-view and geometric phase analysis (GPA). The Nb-rich bcc nano-precipitates in the as-cast alloy have a 10% lattice parameter difference with the B2 matrix and reveal compensating interface dislocations. The 3D reconstruction of the configuration of small Nb-rich precipitates in the annealed alloy reveals a wall-like distribution of precipitates, which may increase the thermal hysteresis of the material.

Waar en wanneer : Vrijdag 30 november 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp : Silver degradation in historical photographs: a TEM case study

Spreker : Eva Grieten, EMAT, University of Antwerp

Abstract:

The development and evaluation of a new non-destructive conservation treatment is often based upon an in-depth study of a conservation treatment in relation to the specific degradation. In this project atmospheric plasma as a non-destructive technique for silver degradation in historical photographs is evaluated. Since a photographic emulsion consists of gelatine (soft matrix) and silver (hard particles), the traditional sample preparation methods cannot be applied. The first part of evaluating the conservation treatment is to fully characterize the degradation. By using diffraction patterns, images and elemental mapping the degradation can be identified with TEM.

Waar en wanneer : Vrijdag 23 november 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp : Quantitative Electron Diffraction

Spreker: Senne Van Rompaey, EMAT, University of Antwerp

Abstract:

Electron diffraction (ED) is often used to determine the unit cell and symmetry of an unknown material. Unfortunately, due to the strong interaction between the sample and the incoming electrons, the intensities of the diffracted beams lose their direct relationship with the structural parameters of the sample. The development of the precession ED and ED tomography partially solved this problem. Intensities measured with these techniques are useable for structure solution and refinement. Processing of this recorded ED data however, is of the utmost importance. During this lecture, different aspects of the data processing will be illustrated and discussed.

Waar en wanneer : Vrijdag 16 november 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Incommensurately modulated scheelite-based structures CAEU_xGD_2-x(BO₄)₄ (x=0 to 2, B=W, Mo)

Spreker: Anne Bertha, EMAT, University of Antwerp

Abstract:

The new materials of the series CaEu_xGd_2-x □ (BO₄)₄ (x=0 to 2) have a scheelite-based structure (ABO₄). Scheelite-based compounds with B=W or Mo are promising new materials for red phosphors in pc-WLEDs (phosphor-converted white-light-emitting-diode) and solid-state lasers. The goal of the current project is to determine the link between the cation distribution and luminescent properties. The structures can be periodically described in four (B=W) and five (B=Mo) dimensions. A model in superspacegroup I2/b(αβ0)00 was built for the first cases already, based on the experimental ED patterns and HAADF-STEM.

Waar en wanneer : Vrijdag 9 november 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door TGM 

Onderwerp: Variational study of Fermi surface deformations in Hubbard models

Spreker: Dr. J. Bünemann, Institut für Physik, DTU Cottbus (Germany)

Abstract:

The study of correlation-induced superconductivity is a notoriously difficult many-particle problem. Despite 20 years of research on superconductivity in the cuprates, there still exists no microscopic theory that is able to describe these systems comprehensively. Even for the simplest theoretical realisation of the cuprates, a Hubbard model in two dimensions, we have not yet obtained a satisfactory understanding of its ground states. There is some evidence, from renormalisation group methods, that two dimensional Hubbard models can have superconducting ground states. Quantitatively, however, little is known about these states. In a recent work, we have therefore introduced a variational method which allows us to study in detail the superconducting as well as other potential ground-states, e.g., the so-called 'Pomeranchuk' or 'nematic' phases with a broken rotational symmetry [1]. In my presentation, I will give a thorough introduction into this new method and its results for the ground states of two-dimensional Hubbard models.
[1] J. Bünemann, T. Schickling, and F. Gebhard, Europhys. Lett. 98, 27006 (2012)
 

Waar en wanneer : Donderdag 8 november 2012 om 16.00 u.  in lokaal U.024 , Campus Groenenborger

Voordracht georganiseerd door EMAT 

Onderwerp : Uncertainty quantification for Predictive Simulations of a Scramjet Combustor

Spreker : Catherine Gorle, EMAT, University of Antwerp

Abstract:

The presentation will give an overview of the overarching goal of the Stanford Predictive Science Academic Alliance Program, which is the quantification of the margin to unstart of the Hyshot II scramjet combustor. The concept of Uncertainty Quantification (UQ) in Computational Fluid Dynamics (CFD) will be introduced, mainly focusing on a methodology to quantify uncertainty in turbulence and turbulent mixing models.

The methodologies presented are the main focus of the research proposed for my Pegasus fellowship. They could be applied to a wide range of computational models and simulations, hence the goal of this presentation is to introduce the methods as a starting point for exploring new ideas and common research interests.

Waar en wanneer : Vrijdag 26 oktober 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door TQC 
 

Onderwerp: Theory and experiments with quantum fl uids of light

Spreker: Iacopo Carusotton, INO-CNR BEC Center and Dipartimento di Fisica, Universita' di Trento ( Italy)

Abstract:

A few years after the fi rst observation of Bose-Einstein condensation, quantum gases of dressed photons in semiconductor microcavities (the so-called exciton-polaritons) are a powerful workbench for the study of phase transitions and many-body effects in a novel non-equilibrium context.

In this talk, I will first briefly review remarkable experiments investigating superfluid hydrodynamics effects in photon fluids hitting localized defects: depending on the flow speed, a wide range of behaviors have been observed, from superfluid flow, to the super-sonic Mach cone, to the nucleation of topological excitations such as solitons and vortices.

I will then illustrate recent theoretical studies in the direction of generating strongly correlated photon gases, from Tonks-Girardeau gases of impenetrable photons in one-dimension, to quantum Hall liquids in the presence of artificial magnetic fields.

Advantages and disadvantages of the different material platforms in view of generating and detecting strongly correlated gases will be reviewed, in particular laterally patterned microcavity and micropillar devices in the optical range, and circuit-QED devices in the microwave domain.

Waar en wanneer : Donderdag 25 oktober 2012 om 16.00 u. in lokaal N.008 , Campus Drie Eiken

Voordracht georganiseerd door EMAT

Onderwerp: Magnetic nanoparticles and anticancer drug (Taxol) encapsulated in biodegradable and biocompatible polymers

Spreker : Maria Filippousi, EMAT, University of Antwerp

Abstract:

Polymer-coated magnetic nanoparticles are materials with ample applications in biomedicine for the treatment of cancer and targeted drug delivery. An effective strategy towards a more site-specific drug delivery could be the entrapment of drug in polymeric carriers with magnetic properties. Thus, incorporation of magnetic nanoparticles (MNPs) in the core of drug-loaded polymeric micelles enables targeting to the desirable area by application of external magnetic field. In the present work multifunctional nanocarriers with multilayer core-shell architecture were prepared by coating superparamagnetic iron oxide nanoparticles encapsulated in the biodegradable and biocompatible triblock copolymers of poly(propylene succinate) (PPSu) and poly(ethylene glycol) (PEG). The drug loading efficiency and drug release properties were investigated using the hydrophobic anticancer drug Taxol. The anticancer hydrophobic drug was loaded into the mPEG–PPSu-mPEG embedded with magnetic nanoparticles. The structural and morphological characterization of the samples was performed by Powder X-Ray Diffraction (XRPD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques while the magnetic hysteresis loop was recorded at room temperature with Vibrating Sample Magnetometer (VSM). Moreover, HAADF-STEM images were acquired from the polymer embedded with MNPs and confirmed the encapsulation of the iron oxide nanoparticles in the polymer matrix. For quantitatively measurements of the nanoscale morphology of the soft materials embedded with magnetite we performed elemental mapping using EELS.

Waar en wanneer : Vrijdag 19 oktober 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Onderwerp : Recent developments on data mining the exit wave: residual aberrations, resolution and a new idea to determine the "weight" of the atom columns

Spreker : Amy Wang, EMAT, University of Antwerp

Abstract:

As suggested by the title, the presentation consists of three parts. The first part is about determining the residual aberrations of the microscope in an experimentally reconstructed exit wave. In principle, the exit wave is reconstructed from a series of HRTEM images and is supposed to be free of microscope aberrations assuming the transfer function of the microscope is fully understood. However, due to instrumental instabilities, we cannot determine lens aberrations with sub-Å precision. Thus, there remain residual aberrations in the reconstructed exit wave which influences the quantification of structure parameters from the exit wave. In the second part, a method to determine the resolution in an object-independent way is introduced. In general, the resolution is determined related to the point spread function of the microscope where the atom is considered as a point object. However, as the resolution of the microscope enters the sub-Å regime which is of the same order as the width of an atom, the resolution is entangled with the information about the object. In the third part, a new method will be presented to determine the "weight" of the atom columns.

Waar en wanneer : Vrijdag 12 oktober 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door  EMAT

Onderwerp : Oxidation state mapping: from bulk to the atomic scale

Spreker: Haiyan Tan, EMAT, University of Antwerp

Abstract:

Transition metals (TMs) in oxide materials are known to adopt many different oxidation states, leading to a wide range of chemical and physical properties. We present a thorough study of the link between EELS fine structure and oxidation state on a wide variety of bulk TM oxides. Combining this knowledge with the available atomic resolution in modern aberration corrected STEM instruments provides us with the possibility to extend oxidation state mapping to the atomic level. We demonstrated this exciting possibility on single crystals of Mn₃O₄ which are known for their ordered arrangement of Mn²⁺ and Mn³⁺ sites. This technique has also been applied to a super - lattice structure of SrVO₃ and LaVO₃ film to study the oxidation state of V at their interfaces. A fundamental asymmetry of V oxidation states was unambiguously revealed at atomic level between the LVO/SVO and SVO/LVO interfaces.

Waar en wanneer : Vrijdag 5 oktober 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door  EMAT 

Onderwerp : Strain modified charge order at the oxide interface

Spreker : He Tian, EMAT, University of Antwerp

Abstract:

Oxide materials show an amazing variety of electronic and ionic phenomena, exhibiting a wide range of phenomena such as magnetism, superconductivity, ionic conduction and ferroelectricity. In particular, oxide interfaces, offer the unique opportunity to control and enhance the effects by controlling the interaction between layers. One important goal is to tailor the amount of strain at the interface to tune the balance among the competing interactions and structural degrees of freedom available to oxides. Normally, strain is induced by an appropriately chosen substrate and is coherently maintained up to a critical thickness. Here, we use HAADF STEM in combination with mono EELS to investigate the chemical and magnetic properties of the Fe3O4/ZnO interface. A charge order of Fe 3+ and Fe 2+ are present at the monolayer of Fe3O4/ZnO interface, which are modified by strain due to lattice mismatch.

Waar en wanneer : Vrijdag 28 september 2012 om 11.30 u.  in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door  EDF 

Onderwerp : the theory of Wilson lines and loops and properties of the loop space

Spreker : Tom Mertens, EDF, University of Antwerp

Waar en wanneer : Vrijdag 28 september 2012 om  17.15 u.  in lokaal  U.203, Campus Groenenborger

Voordracht georganiseerd door  TGM 

Onderwerp : Aspects of many-electron physics in graphene (I-III)

Spreker : Prof. Tapash Chakraborty, University of Manitoba, Canada and CMT, University of Antwerp

Abstract:

In this lecture a brief background will be presented on the interacting electron gas in a magnetic field, the Laughlin state, and other materials relevant for graphene, and also the magnetic field effects on monolayer graphene.

Waar en wanneer : Donderdag 6 september 2012 om 15.00 u.  in lokaal U.203 , Campus Groenenborger

Voordracht georganiseerd door  EGM

Onderwerp: Synthesis of Carbon Materials by Plasma Enhanced Chemical Super Critical Fluid Deposition / Highly Efficient Long-Lifetime Room Temperature Phosphorescence in Metal-free and Halogen-free Materials under Ambient Conditions

Spreker: Prof. Toshiyuki Watanabe, Tokyo University of Agriculture and Technology

Abstract:

Synthesis of Carbon Materials by Plasma Enhanced Chemical Super Critical Fluid Deposition

We will demonstrate a novel material synthesis process by plasma enhanced chemical supercritical fluid deposition (PE-CSCFD). Nano carbon materials and metal oxides were synthesized from carbon dioxide as a raw material by radio frequency plasma discharge under supercritical condition. This approach is not only unique method for carbon materials synthesis, but also promising candidate for new carbon dioxide fixation. The products were characterized by several techniques such as scanning electron microscopy, Raman spectroscopy, energy dispersive X-ray analyzer, X-ray photoelectron spectroscopy, X-ray diffraction and high-resolution transmission electron microscopy. These analyses reveal that only nano diamonds were synthesized with high growth rate from carbon dioxide.

Highly Efficient Long-Lifetime Room Temperature Phosphorescence in Metal-free and Halogen-free Materials under Ambient Conditions

Phosphorescence with a long lifetime (>1 s) can only be observed at low temperature from pure (metal-free and halogen-free) organic materials, because of the significant nonradiative deactivation pathway that occurs at room-temperature (RT). If pure organic materials with long-lifetime RT phosphorescence (LL-RTP) in air could be developed, they could be potentially utilized for a variety of applications. Here, we present pure organic host-guest materials with LL-RTP by minimizing the nonradiative deactivation pathway. The nonradiative deactivation pathway depended on both nonradiative deactivation of the guest and quenching by diffusional motion of the host. The rigidity and oxygen barrier properties of steroidal compounds used as the host suppressed the quenching, and aromatic hydrocarbons used as the guest were highly deuterated to minimize the nonradiative deactivation of the guest. In the pure organic materials, red-green-blue phosphorescence were realized with a quantum yield >10 % and a lifetime >1 s at RT in air.

Waar en wanneer : Woensdag 5 september 2012 om  16.00 u.  in lokaal N.008 , Campus Drie Eiken

Voordracht georganiseerd door TQC 

Onderwerp: Itinerant magnetism in 2D ultracold Fermi atoms with Spin-Orbit coupling: variational approach

Spreker : G. Lombardi, Univ. Padova (Italy)

Abstract:

 The exploration of Rashba spin-orbit (SO) interaction is stimulated by its increasing interest for spintronic applications.

The first experimental evidences of SO interactions have been found in semiconductor devices. We investigate the behaviour of a 2D ultracold Fermi gas in presence of a Rashba-like spin-orbit coupling.

At ultralow temperatures the fermions interact via contact (repulsive) interactions. We determine the ground state of the system perturbatively up to second order terms in the coupling constant. First we employ the functional integral method that enables us to highlight clearly the similarities with the well known (3D) Stoner model. Then, to overcome some difficulties related to the dimensionality, we analyse the behaviour of the 2D system with a variational approach, both without and with the SO coupling. We focus in particular on the aspects related to the magnetization, and as main result we show how the presence of the Rashba coupling frustrates the emergence of a magnetized phase.

Waar en wanneer : Donderdag 12 juli 2012 om 16.00 u.  in lokaal N.008 , Campus Drie Eiken

Voordracht georganiseerd door TGM
 
Onderwerp: Phonon Heat Transport in One-Dimensional Quantum-Dot Superlattices and Cross-Section Modulated Nanowires
Spreker: Prof. dr. V.M. Fomin, Institute for Integrative Nanosciences, IFW Dresden, Germany
 
Abstract:
 Theory of phonon heat transport is developed for Si-based acoustically mismatched one-dimensional (1D) quantum-dot superlattices [1] and cross-section modulated nanowires [2]. The phonon spectra are calculated in the framework of Face-centered cubic cell and Born – von Karman models of lattice dynamics. The phonon transport is analyzed using the Boltzmann transport equation.
In acoustically-mismatched Si-based 1D quantum-dot superlattices the phonon thermal conductivity can be suppressed by a factor of 5 to 30. The effect is explained by the removal of the high-energy and highvelocity  phonon modes from the heat flux due to their localization in quantum-dot superlattice segments and  the phonon scattering on the interfaces. For instance, in Si/Ge 1D quantum-dot superlattices a strong  reduction of the lattice thermal conductivity down to 0.5 W m^-1 K^-1 can be achieved depending on quantumdot  sizes and interface quality. These findings are in line with the recently measured values of the thermal conductivity in multilayered quantum-dot arrays [3].
Suppression of the thermal fluxes in the cross-section modulated nanowires as compared to the  homogeneous Si nanowire increases with temperature and reaches the values 1.5 to 5. Increase of the wide cross-section enhances the suppression of the thermal flux due to a rise in the number of the trapped highenergy  phonon modes, which do not carry heat in the cross-section modulated nanowires, and, correspondingly, to a drop in the phonon group velocities in cross-section modulated nanowires compared  with those of a homogeneous nanowire. The obtained results indicate that segmented and cross-section modulated nanowires are promising candidates for thermoelectric and thermoisolation applications.
References
1. D. L. Nika, E. P. Pokatilov, A. A. Balandin, V. M. Fomin, A. Rastelli, O. G. Schmidt, Physical Review B84, 165415 (2011).
2. D. L. Nika, A. I. Cocemasov, C. I. Isacova, A. A. Balandin, V. M. Fomin, O. G. Schmidt, Physical Review B, 85, 205439 (2012).
3. G. Pernot, M. Stoffel, I. Savic, F. Pezzoli, P. Chen, G. Savelli, A. Jacquot, J. Schumann, U. Denker, I. Monch, C. Deneke, O. G. Schmidt, J. M. Rampnoux, S. Wang, M. Plissonnier, A. Rastelli, S. Dilhaire, and N. Mingo, Nature Materials 9, 491 (2010).

Waar en wanneer : Donderdag 28 juni 2012 om 11.00 u. in lokaal U.024, Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp : Computer simulations used for a search for Sb-Te, Bi-Te and Bi-Se groundstate structures

Spreker: Kirsten Govaerts, EMAT, University of Antwerp

Abstract:

It is common now to consider computation as a third branch of science, besides theory and experiment. The purpose of doing modeling and simulation is to gain understanding and insight, and the benefits are that modeling and simulation can be cheaper, quicker, and easier than experimentation alone.  The aim of our work was to find the Sb-Te, Bi-Te and Bi-Se groundstate structures, with ab initio techniques, and to investigate their energetics and electronic properties. These materials have different important applications in thermoelectrics, phase change materials, and as topological insulators. Using an effective one-dimensional cluster expansion in combination with first-principles electronic structure calculations, groundstates of Sb-Te, Bi-Te and Bi-Se can be found without making the dataset of ab initio calculated structures unreasonably large. In order to get a fast convergence we used a (pseudo)ternary cluster expansion, which takes into account the formation of Sb or Bi bilayers after relaxation due to a Peierls distortion of the lattice. The first-principles electronic structure calculations are done within the Density Functional Theory (DFT)-formalism and we explicitly took into account the van der Waals forces, known to be important in these structures.

Waar en wanneer : Vrijdag 22 juni 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door TGM 

Onderwerp: The gamma-alpha phase transition in cerium as a hidden structural phase transformation

Spreker: Prof. Dr. A.V. Nikolaev, Institute of Physical Chemistry of RAS, 117915, Moscow, Russia

Abstract:

 Usually different crystallographic forms of a same element are distinguished by their crystal space symmetry. In some cases, for example in metallic cerium, there are two well defined thermodynamic phases: gamma and alpha, which apparently have the same face centered cubic lattice. The existence of such twin phases in cerium has become a challenge to the theory and prompted search of a main factor responsible for the difference. In the talk on the basis of theoretical and experimental studies it is advocated that the well known gamma-to-alpha phase transition in cerium is not really isostructural. This finding makes cerium the first element where the symmetry change is driven exclusively by the valence electron degrees of freedom while the atomic centers of mass (cerium nuclei) occupy the face centered cubic positions.

References:

[1] G. Elisashberg and H. Capellmann, JETP 67, 125 (1998).

[2] A.V. Nikolaev and K.H. Michel, Eur. Phys. J. B 9, 619-634 (1999).

[3] A.V. Nikolaev and K.H. Michel, Phys. Rev. B 66, 054103 (2002).

[4] A.V. Tsvyashenko et al., JETP 111, 627 (2010).

[5] A.V. Tsvyashenko et al., Phys. Rev. B 82, 092102 (2010).

[6] M. Krisch et al., PNAS 108, 9342 (2011)

Waar en wanneer : Donderdag 21 juni 2012 om 14.00 u.  in lokaal  U.024, Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp : Advanced TEM investigation of the plasticity mechanisms in Aluminum nanocrystalline thin films

Spreker: Hosni Idrissi, EMAT, University of Antwerp

Abstract:

Nanocrystalline (nc) thin metallic films receive actually special attention from the scientific community because of their use in a variety of technologies ranging from flexible electronics, MEMS, microelectronics, and thin functional coatings. However, the use of these films is still limited by their mechanical properties (the weak ductility and the related strain hardening capacity). Understanding the origin of such feature constitutes a real challenge because: i) The plastic deformation of nc metallic films is very complex and usually involve the simultaneous activation of different elementary mechanisms (dislocation activity, grain boundaries mediated processes, shear band, twinning, etc). ii) Due to the small grain size (below 100 nm) as well as the resolution limit that long persisted in standard transmission electron microscopy (TEM) , the characterisation of the fundamental mechanisms controlling the plastic deformation of nc films at the atomic scale is very difficult.

The aim of the present work is to use panoply of conventional and advanced TEM techniques (HRTEM and HRSTEM including aberration corrected microscopy, ACOM-TEM, in-situ TEM nano-mechanical testing, diffraction contrast electron tomography, etc)  to uncover the fundamental properties of structural defects involved in the plastic deformation of nc Al thin films. This constitutes the key for the improvement of the mechanical behavior and the reliability of these films.  

Waar en wanneer : Vrijdag 15 juni 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door TGM

Onderwerp: Functional  Oxides: From Multiferroic to Fluxonic Concepts

Spreker: Prof. dr. Roger Wördenweber, Peter Grünberg Institut (PGI-8), Forschungszentrum Jülich (FZJ), Germany

Waar en wanneer : Donderdag 14 juni 2012 om 11.00 u. in lokaal U.244 , Campus Groenenborger

Voordracht georganiseerd door EMAT
 

Onderwerp : The influence of coating of silicon nanoparticles on the electrolyte decomposition in Li-ion batteries

Spreker : Kristof Van Havenbergh, EMAT, University of Antwerp

Abstract:

Largely based on its much higher rechargeable capacity compared to graphite, silicon nanopowder appears as an ideal candidate for the next generation of negative electrodes for Li-ion batteries. However, a crucial problem with silicon is the large volume expansion undergone upon alloying with lithium, which results in stability problems. Means to avoid such problems are mostly linked to the understanding of the interfacial chemistry during charging/discharging. In this work, the influence of a coating on the surface of silicon particles on the electrolyte decomposition is examined. Silicon nanopowders with different coating are systematically characterised using transmission electron microscopy, before and after battery usage. Simultaneously, the materials are examined using electrochemical impedance spectroscopy. The aim is to couple both techniques into one powerful screening tool for future battery materials.

In this talk, a brief introduction to battery chemistry will be presented. Then, the first results and encountered obstacles will be discussed. Finally, the future objectives will be shown.

Waar en wanneer : Vrijdag 8 juni 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door WN en TQC 

Onderwerp: Propagation of single-excitation quantum states through spin chains and James-Cummings-Hubbard arrays

Spreker : Prof. Dr. Ranabir Chakrabarti, Department of Theoretical Physics, University of Madras

Abstract:

 It is intended to discuss the time-dependent correlation function describing the evolution of a single excitation state on a linear spin chain with isotropic nearest-neighbor XY coupling, where the Hamiltonian is related to the Jacobi matrix of a set of orthogonal polynomials. For instance, in the case of Krawtchouk polynomial, an arbitrary element of

the correlation function can be calculated in closed form showing exact transmission of the state at pre-prescribed time. A similar problem is studied in the context of an one-dimensional array of the James-Cummings-Hubbard system of arbitrary number of coupled cavities, each containing a two-level atom that interacts with a field mode.

References:

1) R. Chakrabarti and J. Van der Jeugt, J. Phys. A 43, 085302 (2010)

2) R. Chakrabarti and G. Sreekumari, J. Phys. B 44, 115505 (2011)

Waar en wanneer : Woensdag 6 juni 2012 om 16 u. in lokaal  N.008, Campus Drie Eiken

Voordracht georganiseerd door EMAT

Onderwerp: Molecular imaging meets imaging molecules

Spreker: Dimitri Roose, EMAT, University of Antwerp

Abstract:

Contrast particles are used for contrast enhancement in magnetic resonance imaging, especially for in vivo cellular imaging where cells are loaded with these particles. For applications such as the follow-up of stem cell migration, these contrast particles have to reside for long times in the cell. Therefore, the interaction between the cell and the particles becomes important. Toxicity studies have been performed to investigate the cellular response on the presence of these particles, but this interaction has not yet been studied at the electron microscopic level.

In this lecture, an overview of molecular imaging will be presented with a focus on in vivo cellular imaging and the particle studied so far. Afterwards the developed protocol to prepare the samples will be shown, to end with the results achieved this far.

Waar en wanneer : Vrijdag 1 juni 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door  TQC  

Onderwerp : Spectroscopic study on quantum phonon dynamics in correlated systems
Spreker : Kai Ji, Institute of Theoretical Physics, University of Heidelberg
 
Abstract:
The nano-scaled ferroelectric clusters come into being in paraelectric BaTiO3 when the temperature decreases close to Curie temperature (Tc).  These ultrafast dipole fluctuations have been discerned in the soft x-ray pump-probe speckle spectroscopy recently. In this talk, I will present a theoretical study on the ultrafast relaxation dynamics of polarization clusters by the path integral Monte Carlo approach. It is shown that the time evolution of speckle pattern is related to the relaxation of ferroelectric clusters. The relaxation rate slows down on decreasing temperature towards Tc as a result of the enhanced critical fluctuations. Application of this theoretical approach to some other phonon related spectroscopic issues will also be briefly discussed.
 
Waar en wanneer : woensdag 16 mei 2012 om  13.00 u.  in lokaal N.108 , Campus Drie Eiken

Voordracht georganiseerd door EMAT 

Onderwerp: Describing matter through theory: e--e- interaction, symmetry and chirality from the low to the high loss response

Spreker : Fabiana Da Pieve, EMAT, University of Antwerp

Abstract:

In this seminar I will talk about what theory can do for 1) the interpretation of spectra 2) the interpretation of structural properties 3) for providing  experimentalists reference spectra and new ideas coming from prediction. I will first do a short introduction about how the description of electronic, structural and excited state properties properties is tackled by theory. Then I will focus on the high loss and low loss response in absorption, making connections between the use of light and the use of electrons to probe matter. I will talk about the mercury chalcogenide alpha-HgS, a system which constitutes the red pigment in paintings, plays a role in some photochemical cells and is at the basis of some fundamental reactions on earth. I will show how hidden symmetries in the probe-matter interaction (multipoles) and in the material itself (chirality) can be unravelled both at the core edges and the valence region. Possible future investigations in connection with new forms of dichroism and optical activity are discussed from a mixed theoretical-experimental perspective.  

Waar en wanneer : Vrijdag 11 mei 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Density functional theory: Why it matters to *you*, what you *don't* know about its history, and some striking applications

Spreker: R.O. Jones, Forschungszentrum Julich, Germany

Abstract:

The density functional (DF) formalism is the basis of most calculations in condensed matter physics and materials science that avoid using adjustable parameters and experimental input. Applications in chemistry have grown dramatically in the past 15 years, and related theories are important in plasma and nuclear physics. Almost all presentations of DF theory assume that the world was created in 1964, but the real story begins long before this. I shall trace the developments (and the reactions to them!) over the last 80 years, and I shall conclude with some recent applications, including studies of the alloys that make DVD-RW and the Blu-Ray Disc optical memories possible.

Waar en wanneer : Vrijdag 4 mei 2012 om 11.30 u in lokaal U.408 , Campus Groenenborger

Voordracht georganiseerd door EMAT 

Onderwerp : Microstructure study of the precipitation behaviour in Co₃₈Ni₃₃Al₂₉ ferromagnetic shape memory alloys

Spreker : Jiangbo Lu, EMAT, University of Antwerp

Abstract:

Ferromagnetic shape memory alloys have attracted considerable attentions because of their potential applications in industry as sensors and actuators. In this talk, transmission electron microscopy was performed to investigate the microstructures of g ’ L1₂ precipitates and their surrounding matrix in two kinds of Co₃₈Ni₃₃Al₂₉ ferromagnetic shape memory alloys. The fastest grown sample with the shortest annealing time reveals eutectic lamellar and dendritic shaped phase in a martensite matrix, with a precipitate-austenite-martensite sandwich morphology at the precipitate-matrix interface. The formation of the sandwich morphology can be explained by the strong martensitic start temperature dependency on element composition. The slower grown sample annealed for a longer time, shows extra facetted sheet-like precipitates with a central ‹111›_p twin plane in the austenite matrix apart from the eutectic dendritic phase. These facetted precipitates have a specific orientation distribution due to the existence of the twin planes inside the precipitates plus the Kurdjumov-Sachs orientation relationship between precipitates and matrix. Lorentz microscopy was also applied to study the magnetic domain structures of precipitate and matrix.

Waar en wanneer : Vrijdag 27 april 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door  Dirk Van Dyck

Onderwerp: A History of Max Born and Robert Oppenheimer Approximations in Molecular Physics, Solid State Physics and Mathematics

Spreker: Lucas Lathouwers, Prof. Emeritus University of Antwerp, Visiting Prof. FORTH University of Crete and Visiting scholar Harvard University

Abstract:

 The 1926 paper by Julius Robert Oppenheimer and Max Born is by far the most cited publication in theoretical, molecular and solid state physics. It is remarkable in its physical, chemical and mathematical contents.

The Born-Oppenheimer and adiabatic approximations break down when electronic states are nearly degenerate. The generator coordinate approximation, introduced by Lathouwers and Van Leuven, on the other hand is capable, in spite of the fact that it is based on a single electronic state, to produce good results when potential energy surfaces closely approach each other. This will be illustrated for so-called curve crossing problems and Jahn-Teller situations. Finally, an extension of the aforementioned coupled oscillator system to a linear chain of alternating masses (to simulate lattice dynamics) will be outlined.

(uitgebreide abstract in bijlage / an extended abstract is included with this message)

Waar en wanneer : Dinsdag 24 april 2012 om  16.00 u. in lokaal U.024 , Campus Groenenborger

Voordracht georganiseerd door EMAT  

Onderwerp: Spectroscopic Imaging of (Solvated) Soft Materials in the Cryo-STEM

Spreker: Matthew Libera, Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, New Jersey, USA

Abstract:

Traditional methods for studying the morphology of soft materials – both synthetic polymers and biological structures - in the transmission electron microscope (TEM) principally rely on differential heavy-element staining (e.g. Os/Ru tetroxide among others) to induce amplitude contrast based on elastic electron scattering.  This imaging approach has had substantial impact on the electron-optical imaging of soft-materials morphology over many decades of practice, but it is increasingly unsuited for the quantitative mapping of composition with the necessary near-nanometer spatial resolution demanded by a broad range of technologically forward-looking applications.  Our work concentrates on an alternate method based on quantitative maps of composition in unstained soft materials using electron energy-loss spectroscopy to study the inelastically scattered electrons.  Soft materials offer rich electronic structure with which to spectroscopically differentiate between specimen components.  Here we use two examples – synthetic polymer nanoparticles and frozen-hydrated tissue – to illustrate some of the problem-solving possibilities afforded by this imaging approach. More generally, we have achieved 1 nm resolution studying interfaces in poly(styrene)-poly(2 vinyl pyridine) homopolymer blends, 5 nm resolution studying epoxy-alumina composite interphases, and 8-15 nm resolution studying hydrated polymers and tissue where cryo-TEM/STEM techniques are needed.  In contrast to traditional imaging of radiation resistant hard materials where the resolution is ultimately limited by the quality of the electron optics, the achievable spatial resolution associated with imaging of unstained soft materials is instead determined by the radiation sensitivity of the specimen.  Improving the so-called dose-constrained resolution thus concentrates on enhanced data acquisition and spectral-analysis techniques rather than on minimizing the spherical aberration of the objective lens needed to image atomic structure in radiation-resistant hard materials.

Waar en wanneer : Vrijdag 20 april 2012 om  11.30 u.   in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: The Biomaterial–Bone Interface: Understanding it in 3D with Electron Tomography

Spreker : Kathryn Grandfield, EMAT, University of Antwerp

Abstract:

Interfacial relationships between biomaterials and tissues strongly influence the success of implant materials and their long-term functionality. Due to the inhomogeneity of biological tissues at an interface, in particular bone tissue, two-dimensional images often lack detail on the interfacial complexity. Furthermore, the increasing use of nanotechnology in the design and production of biomaterials demands characterization techniques on a similar length scale. Electron tomography can meet these challenges by enabling three-dimensional imaging of biomaterial interfaces.

This talk will present a background on biomaterials designed for bone interfacing applications, and how electron tomography has been used to reveal details at the biomaterial-bone interface.

Waar en wanneer : Vrijdag 13 april 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door TQC

Onderwerp:    Condensate fraction in the BCS-BEC crossover: dimensionality, spin-orbit and multi-components

Spreker : Prof.dr. Luca Salasnic (Universita di Padova, Italy)

Abstract:

We analyze the condensate fraction of fermionic pairs for a uniform two-component atomic Fermi gas in the crossover from the Bardeen-Cooper-Schrieffer (BCS) state of weakly-interacting Cooper pairs to the Bose-Einstein Condensate (BEC) of molecular dimers. We derive explicit formulas for condensate density and condensate fraction as a function of the chemical potential and the energy gap both in three [1] and two dimensions [2]. We extend these results taking into account the effect of a laser-induced Rashba-like spin-orbit coupling [3]. Finally, we consider the case of three-component atomic Fermi superfluid with U(3) invariant attractive interaction.

[1] L. Salasnich, N. Manini, A. Parola, Phys. Rev. A 72, 023621 (2005).

[2] L. Salasnich, Phys. Rev. A 76, 015601 (2007).

[3] L. Dell'Anna, G. Mazzarella, L. Salasnich, Phys. Rev. A 84, 033633 (2011).

[4] L. Salasnich, Phys. Rev. A 83, 033630 (2011).

Waar en wanneer : Donderdag 12 april 2012 om 16.00 u. in lokaal N0.08 , Campus Drie Eiken

Voordracht georganiseerd door EMAT
 

Onderwerp: Strain-Driven Oxygen Deficiency in Nanostructured Superconductors

Spreker: Leopoldo Molina, EMAT, University of Antwerp

Abstract:

Precipitates in YBCO thin films yield a significant improvement of the magnetic flux pinning properties and a reduced anisotropy of the critical current density in coated conductors. Therefore, controlling precipitates in size and density, but also understanding their associated strain fields is of crucial importance for applications. YBa2Cu3O7-δ; (YBCO) superconducting films containing nanosized BaHfO3 precipitates were prepared by Metal Organic Deposition (MOD). Strain-fields of crystal defects associated with these precipitates were measured quantitatively by High-Angle Annular Dark-Field (HAADF) aberration-corrected Scanning Transmission Electron Microscopy (STEM). Experimental images were evaluated with respect to the strain tensor components by a geometrical phase analysis and a model-based parameter estimation method. EELS spectra provide fingerprints of the oxygen sublattices of YBCO. Changes in the O K-edge electron energy loss near-edge fine structure (ELNES), in particular of the first peak (which originates from the interaction of the O 2p and the partially occupied Cu d orbital), allowed to distinguish superconducting from non-superconducting areas with a sub-nm spatial resolution. The possibilities of the applied techniques for a structure-property correlation will be discussed.

Waar en wanneer : Vrijdag 30 maart 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door  EDF

Onderwerp: Asymmetries associated with higher twists: Gauge invariance of polarized Drell-Yan, gluonic poles and twist three

Spreker: Dr. I. V. Anikin, Univ. Regensburg, Germany

Abstract:

 We explore the electromagnetic gauge invariance of the hadron tensor of the Drell-Yan process with one transversely polarized hadron. The special role is played by the contour gauge for gluon fields. The prescription for the gluonic pole in the twist 3 correlator is related to causality property and compared with the prescriptions for exclusive  hard processes. As a result we get the extra contributions, which naively do not have an imaginary phase. The single spin asymmetry for the Drell-Yan process is accordingly enhanced by a factor of two.

Waar en wanneer : Donderdag 29 maart 2012 om 16.00 u.  in lokaal U.203 , Campus Groenenborger

Voordracht georganiseerd door  TGM

Onderwerp: Oxides in Electronics

Spreker: Chris G. Van de Walle, Materials Department, University of California, Santa Barbara

Abstract:

Oxides have many applications, ranging from gas sensors to varistors.  They are increasingly being used in electronics and optoelectronics.  Transparent conducting oxides (TCOs) such as ZnO, SnO₂, and In₂O₃, provide metallic conductivity while absorbing little or no visible light; they are essential for devices such as solar cells, light-emitting diodes, and flat-panel displays.  TiO₂ has applications in solar cells and (photo)catalysis, and also, along with Al₂O₃, Ga₂O₃, SiO₂ and MgO, as a dielectric. Controllable p-type doping would in principle allow for optoelectronic devices such as light emitting diodes and detectors. … (klik hier voor de volledige abstract)

Waar en wanneer : Vrijdag 23 maart 2012 om 14.00 u.  in lokaal U.026 , Campus Groenenborger

Voordracht georganiseerd door EMAT
 

Onderwerp: Big-Bang tomography : a new route to atomic resolution tomography

Spreker: D. Van Dyck, EMAT, University of Antwerp

Abstract:

Classical tomographic schemes make use of a large series of projections of an object taken at small angular increments. However for atomic resolution tomography this can pose many problems. First it is not straightforward to perform mechanical tilting in an EM with sub-Angstrom precision over a very large angular range. Secondly many real-life objects such as e.g. an amorphous dielectric layer in a microelectronic device have a geometry that does not give this degree of freedom. And thirdly many objects such as proteins are sensitive to radiation damage so that they do not allow sufficient experimental time and dose for a full tomographic reconstruction at high resolution.

Hence there is need for a new tomographic scheme that can is able to deduce 3D information from only one projection, or from 2 projections using stereoscopy.

In this lecture I will present a totally new tomographic scheme that in principle allows to determine not only the position of individual atoms within the plane of observation but also the vertical position with sub Angstrom precision. For reasons of analogy and intuitive understanding we have called it “big-bang tomography”. We have applied it successfully to the study of a two-layer graphene object.

Waar en wanneer : Vrijdag 23 maart 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door TQC 

Onderwerp : Dynamics and collective behavior in ultracold neutral matter and plasmas

Spreker: Hugo Terças, Instituto Superior Técnico, Lisboa

Abstract:

 In this talk, I will summarize some of the aspects of three different aspects of cold and ultracold matter: cold atomic traps, Bose-Einstein condensates and ultracold plasmas. I will firstly explain why the long-range forces mediated by light have put the physics of magneto-optical traps (MOTs) again into the stage. Multiple scattering of photons in the trap is responsible for the emergence of a Coulomb-like interaction between the atoms, which is the root for different collective features, namely the emergence of a cut-off frequency  and a roton minimum in the excitation spectrum, dynamical instabilities and normal modes. Secondly, I will discuss the effects of advection in rotating Bose-Einstein condensates, and how it may open the door for a new collective mode, analogous to the Rossby wave, which is a drift acoustic mode ubiquitous in geophysics. The effects of this drift term is added to the dispersion relation of the usual Tkachenko modes. Finally, I will consider the dynamics of expansion of ultracold neutral (Rydberg) plasmas, by deriving a Thomas-Fermi potential for the electrons. I will conclude the presentation by suggesting the theoretical possibility of observing slow-light in ultracold plasmas, an effect resulting from the combined dielectric response of electrons and two-level Rydberg atoms.

Waar en wanneer : Donderdag  23 maart 2012 om  16.00 u. in lokaal N.008 , Campus Drie Eiken

Voordracht georganiseerd door  Dirk Van Dyck

Onderwerp: About some oddities in applying standard error propagation laws to climatological measurement. - Is an extreme narrow uncertainty band in time series of global temperature trends feasible?

Spreker: Michael Limburg

Abstract:

 The lecture is about some fundamentals in error treatment theory and its application to historical climatological temperature data which are used to calculate mean global temperature anomaly time series. By using simple practical examples it will be demonstrated that the total achievable uncertainty can not be lower than that what the proved rules of error detection, attribution and propagation allow. This means that it must be at least as great as total mean global temperature anomaly variation of the last 150 years, but very likely much greater, which would make any attribution an illusory task.

Waar en wanneer : Vrijdag 16 maart 2012 om 16.00 u.  in lokaal  U.024, Campus Groenenborger

Voordracht georganiseerd door EMAT / Seminar org anised by EMAT

Onderwerp: Transmission electron microscopy and inorganic chemistry needs

Spreker: O.I. Lebedev, Laboratoire CRISMAT, CNRS ENSICAEN, Universite Caen Basse Normandie

Abstract:

It is impossible to overestimate the role of XRD and neutron powder diffraction (NPD) as reliable and extremely precise techniques for solving complex structures in inorganic chemistry. However, there are more and more examples of complex structures for which XRD and NPD are unable to find an unambiguous structure solution and explain physical properties, and more input from the nanoscale level of the materials structure (possible superstructure, short range order- disorder effects and possible chemical and structural inhomogeneities) are needed. Indeed, TEM is one of the most powerful and reliable techniques to provide this type of information at an atomic level. Moreover, recent advances in aberration-corrected microscopy allow the acquisition of chemical, structural and even electronic information from single atomic columns. Combining conventional, analytical and advance TEM with XRD and NPD is therefore highly effective in completely solving new structures. The use of different methods of electron microscopy for the needs of inorganic chemistry will be discussed for several examples of new complex materials.

Waar en wanneer : Vrijdag 16 maart 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT 
 
Onderwerp: 3D structure determination of amorphous  objects using electron wave tomography
Spreker: C. Tang, EMAT, University of Antwerp
 
Abstract:
At present it is possible to retrieve the electron wave in a plane of projection of an object. The basic idea of wave tomography is to backpropagate this electron wave through the object. By repeating this over several projection angles the usual “filtered backprojection algorithm” can be replaced by a “filtered backpropagation algorithm”.
It is the ultimate goal of the project to apply these ideas to reconstruct the 3D structure of an amorphous object. In the first phase of this project we analysed the possibility to use a single graphene sheet as support of a small object such as a nanoparticle. Therefore we started from simulations in which a single atom was positioned on top of a graphene sheet and we tried to reconstruct the position of the atom from the knowledge of the exit wave. In a sense the graphene support was “peeled off”. Next, we could study the precision of this reconstruction as a function of the weight of the atom, the resolution and the noise.
 
Waar en wanneer : Vrijdag 9 maart 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT 

Onderwerp : Perovskite-based anion deficient homologous series AnBnO3n-2

Spreker : M. Batuk, EMAT, University of Antwerp.

Abstract:

In this presentation, results of our ongoing work on the perovskite-based AnBnO3n-2 homologous series will be demonstrated. We investigate how the structure of the members with n=4-6 depends on the size of the A and B cations by studying the crystal structure of compounds with different chemical composition (A=Pb/Ba/Bi/Sr, B=Fe/Ti/Sn/Sc).

The crystal structure is built from perovskite blocks with a thickness of (n-2) octahedra connected to each other by chains of edge-sharing tetragonal pyramids. From TEM data combined with synchrotron X-ray powder diffraction data we have established that all compounds are orthorhombic at high temperatures and undergo a phase transition upon cooling, forming a complex monoclinic twinned structure. The model implementing the defect structure into the Rietveld refinement is suggested, allowing a next step towards understanding the structure of these compounds.

Waar en wanneer : Vrijdag 2 maart 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door TQC

Onderwerp: Quantum phase transition of a low-dimensional weakly interacting Bose gas

Spreker: Luca Fontanesi, Ecole Polytechnique Federale de Lausanne

Abstract:

We study the 1D repulsive Bose gas at zero temperature in presence of spatially correlated disorder. In particular, we focus on the quasi-condensate to Bose glass quantum phase transition due to the interplay of interaction and disorder. The Bose gas is described via an extended density-phase Bogoliubov formalism suitable to treat low-dimensional degenerate systems in the regime of weak interaction. By studying the long-range behaviour of the one-body density matrix and the superfluid component of the gas, we draw the mean  field phase diagram on the interaction-disorder plane. We identify two different power-law relations of the phase boundary that can be explained by scaling arguments valid in the white-noise and Thomas-Fermi limits. In addition, we give a criterion to determine the onset of the phase transition by studying the probability distribution of the density in presence of disorder: this criterion links the fragmentation of the density profile to the loss of superfludity. The last part of this talk is a brief introduction about the physics of the 2D Bose gas in presence of disorder. As this is a very challenging problem both from the conceptual and  numerical point of view, this last part sketches the main open problems and some numerical techniques that we adopt to study thermodynamic properties in the two dimensional case.

Waar en wanneer : Donderdag 16 februari 2012 om 16.00 u. in lokaal N.108 , Campus Drie Eiken

Voordracht georganiseerd door EMAT 

Onderwerp: Characterization of semiconductor devices through electron microscopy

Spreker: Tyché Perkisas, EMAT, University of Antwerp

Abstract:

This lecture will be divided into two parts. The first part will give a brief overview of Scanning Confocal Electron Microscopy (SCEM) and its advantages and disadvantages. The second (and main) part of the presentation will consider semiconductor devices, namely the FinFET and parallel trench devices. In the FinFET device the continuity of the high-k HfO2/TiN layer is investigated through HAADF-STEM tomography, while in the parallel trench structure the presence of defects is studied through HAADF-STEM and simulation. 

Waar en wanneer : Vrijdag 10 februari 2012 om 11.30 u. in lokaal U.244 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Formation of the crystallographic shear plains in the Pb-based perovskites

Spreker: Batuk Dmitry, EMAT, University of Antwerp

Abstract:

In this talk the formation of crystallographic shear (CS) plains in the Pb-Fe-O and Pb-Fe-Ti-O will be considered. It will be demonstrated that not a simple perovskite, but a more complex partially disordered layered intergrowth structure with the composition Pb₂Fe₂O₅ is first formed in the Pb-Fe-O system and then serves as a parent structure for nucleation and growth of the CS planes. Concerning the Pb-Fe-Ti-O system, it will be shown how the substitution of Ti⁴⁺ by Fe³⁺ in the PbTiO₃ perovskite provokes the formation of the CS planes, and how those plains evolve upon the increase of the anion deficiency up to the composition “Pb₂Fe₂O₅”

Waar en wanneer : Vrijdag 3 februari 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Hierarchically Structured Porous Materials and Low Dimensional Nanomaterials: from Synthesis to Applications

Spreker: Bao-Lian Su, Center for Nanomaterials Chemistry, Laboratory of Inorganic Materials Chemistry, University of Namur (FUNDP), Namur, Belgium

Abstract:

This talk will be divided into two parts, one on the advanced hierarchically structured porous materials, their synthesis, structure characterization, formation mechanism and different outstanding properties. The second part concerns a series of new and unprecedented nanostructures. This part of the talk will illustrate the importance and the need of accurate and multitechnical characterization.

Waar en wanneer : Vrijdag 27 januari 2012 om 11.30 u. in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door  EMAT 

Onderwerp: A Thorough Structural Study of 150nm CNT Cu Damascene Contacts

Spreker: Xiaoxing Ke, EMAT, University of Antwerp

Abstract:

 Carbon nanotubes as promising interconnects in integrated circuits have been studied for some years. Now the challenge comes as the process advances into the integration of structure with top and bottom contacts. Therefore subsequent structural and electrical characterization is required in order to realize a real technological breakthrough in this field. Here a simulated interconnect structure with CNTs integrated in contact holes of 150nm, with underlying TiN contact layer and a Cu single damascene top contact, is characterized by TEM. The special structure is prepared by FIB in order to obtain both cross-section sample and plan-view sample. A thorough investigation of the structure from bottom contact to top contact is performed by combination of cross-section sample and plan-view sample. The study of the structure not only confirms the presence and distributions of CNTs in the contact hole, but also reveals the interface of CNTs in contact with metal layers and substrate. Moreover, the presence of CNTs inside top contact layer by using plan-view sample is successfully demonstrated which shows significant learning obtained with electrical measurements of the structure.

Waar en wanneer : Vrijdag 13 januari 2012 om 11.30 u.  in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EGM

Onderwerp: Carbon nanosystems under extreme conditions

Spreker: Prof. Alfonso San Migu, Université de Lyon et CNRS

Abstract:

 In carbon nanotubes or graphene the balance between elastic energy and internal or external interaction energies (van der Waals for instance with the environment for instance) can be modified by pressure and determines the stability of the nanosystem. In the case of carbon nanotubes, the nanotube wallinteraction energy can be modified by pressure application, leading to the collapse of carbon nanotubes.

We have provided the first clear experimental evidence of pressure induced collapse on SWNT[1] and demonstrated the ovalization [2] and collapse [3] in double wall carbon nanotubes. We will compare the collapse of carbon nanotubes having different fillers and show the main mechanism governing the high pressure stability of carbon nanotubes in general and double-wall carbon nanotubes more particularly. Our studies involve both high pressure Raman spectroscopy combined and theoretical studies.

In the case of supported graphene, the elastic energy depends on the number of graphene layers. We have shown that the numbers of layers, together with the surface roughness of the substrate are the key parameter determining the high pressure behavior of supported graphene.

In addition to changes between a 2D to a 3D mechanical response, we have observed a giant pressure induced charge injection in graphene [4].

[1] Ch. Caillier, D. Machon, A. San Miguel, G. Montagnac, H. Cardon, M. Kalbac,M. Zukalova, L. Kavan, Phys. Rev. B, 2008, 77, 125418

[2] Ch. Caillier, A. Ayari, V. Gouttenoire, J.-M. Benoit, V. Jourdain, M. Picher, M. Paillet, S. Le Floch, S.T. Purcell, J.-L. Sauvajol, and A. San Miguel, Advanced Functional Materials, 2010, 20, 3330.

[3] A.L. Aguiar, E.B. Barros, R.B. Capaz,A.G. Souza Filho, P.T.C. Freire,J. Mendes Filho, D. Machon, Ch. Caillier, Y.A. Kim, M. Muramatsu, M.Endo and A. San Miguel, J. Phys. Chem. C, 115, 5378.5384 (2011)

[4] J.Nicolle, D. Machon, P. Poncharal, O. Pierre-Louis and A. San Miguel, Nano Letters, 11, 3564 (2011)

Waar en wanneer : woensdag 21 december 2011 om 11.00 u.  in lokaal N.008 , Campus Drie Eiken

Voordracht georganiseerd door EMAT

Onderwerp: Precession electron diffraction: state-of-the-art

Spreker: Joke Hadermann, EMAT, University of Antwerp

Abstract:

 An overview will be given of the status of precession electron diffraction in 2011, what were recent achievements, on what kinds of systems has precession electron diffraction proven to be useful already, which different types of information can be achieved, … and this inside as well as outside EMAT.  In short, what can already or cannot yet be done using precession electron diffraction and what is the place of the research on precession electron diffraction in EMAT in all this?

Waar en wanneer : vrijdag 16 december 2011 om 11.30 u.  in lokaal X.009, Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwer: Various applications of quantitative electron microscopy

Spreker: Wouter Van den Broek, EMAT, University of Antwerp

Abstract:

 In quantitative electron microscopy, object parameters like for example particle diameters, are estimated from the absolute image intensities. In order to estimate these parameters correctly, extensive use needs to be made of models that describe the object and/or the image process, image analysis and statistics. In this talk four different applications of quantitative electron microscopy are presented: Optimal spot size selection in STEM; Measurement of the modulation transfer function of the CCD; Correction of non-linear thickness effects in tomography; and Estimation of atom positions from various projections.

Waar en wanneer : Vrijdag 2 december 2011 om 11.30 u.  in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door  EMAT 

Onderwerp: Advanced applications of SEM-based electron diffraction techniques

Spreker: Stefan Zaefferer, Max Planck Institute for Iron Research, Düsseldorf

Abstract:

There are 3 diffraction techniques available in the scanning electron microscope (SEM), backscatter electron diffraction (EBSD), electron channelling contrast imaging (ECCI) and Kossel diffraction, an X-ray diffraction technique. The most popular technique is EBSD which is well known for its application in EBSD-based orientation microscopy a method for quantitative description of microstructures. Despite its established use to display microstructures, EBSD still has great development potential: spatial resolution can be driven into the domain of classical transmission electron microscopy (TEM) by low-kV application. Angular resolution may be improved by new techniques like the cross-correlation of patterns (delta-EBSD), 3D orientation microscopy can be performed by combination of serial sectioning with EBSD. Finally new applications arise from the possibility to simulate EBSD patterns using dynamic diffraction theory.

The ECCI technique allows the observation of lattice defects like dislocations, stacking faults and fine twins with surprisingly good contrast on bulk samples. The technique is comparable to dark field imaging under well defined diffraction conditions in TEM. It enables the easy assessment of dislocation densities, the determination of Burgers vectors and the observation of defect evolution during in-situ straining experiments in the SEM.

The Kossel technique finally, enables, in principle, the measurement of lattice constants with very good accuracy. Nevertheless, the technique suffers from a relatively bad spatial resolution (about 10 times worse than EBSD) and long exposure times. It is therefore to date not yet used on a regular basis.

The talk will introduce the different techniques and illustrate some of their aspects with examples from research on different steels.

Waar en wanneer : Vrijdag 25 november 2011 om 11.30 u. in lokaal X.009 , Campus Groenenborger

  Voordracht georganiseerd door TQC

Onderwerp: Feynman diagrams versus Feynman quantum emulator

Spreker: Kris Van Houcke , Universiteit Gent

Abstract:

Precise understanding of strongly interacting fermions, from electrons in modern materials to nuclear matter, presents a major goal in modern physics.

However, the theoretical description of interacting Fermi systems is usually plagued by the intricate quantum statistics at play. Here we present a cross-validation between a new theoretical approach, Bold Diagrammatic Monte Carlo (BDMC), and precision experiments on ultra-cold atoms. Specifically, we compute and measure with unprecedented accuracy the normal-state equation of state of the unitary gas, a prototypical example of a strongly correlated fermionic system. Excellent agreement demonstrates that a series of Feynman diagrams can be controllably resummed in a non-perturbative regime using BDMC.

This opens the door to the solution of some of the most challenging problems across many areas of physics.

Waar en wanneer : Woensdag 23 november 2011 om  16.00 u.  in lokaal N0.08, Campus Drie Eiken

Voordracht georganiseerd door EGM

Onderwerp: Optical spectroscopy of low-dimensional carbon materials:  individual carbon nanotubes and graphene monolayers

Spreker: Stéphane Berciaud,   Université de Strasbourg & CNRS

Abstract:

Over the past two decades, carbon nanotubes, then graphene have risen as outstanding systems to investigate the behavior of electrons and phonons in reduced dimensions. Furthermore, these materials stand among the most promising candidates for realistic applications in electronic and optoelectronic nano-devices [1,2].

We will show how the intrinsic properties of graphene and carbon nanotubes can be probed using relatively simple optical techniques. For both materials, inelastic light (or Raman) scattering provides quantitative information about the electronic structure, the position of the Fermi level (i.e. doping), as well as the influence of extrinsic factors such as disorder, strain, temperature... [3-5]. In the particular case of carbon nanotubes, the 1D confinement gives rise to a large variety of electronic structures and strong excitonic effects that can be probed at the single nanotube level by means of broadband elastic light

(Rayleigh) scattering spectroscopy [4].

Combined with electron transport measurements, optical spectroscopy reveals invaluable information on the operation of graphene-based devices. As a classic example, we will examine the energy dissipation mechanisms (electron-electron, electron-phonon, phonon-phonon

couplings) in electrically-biased graphene channels. By combining in-situ measurements of the spontaneous optical emission with a Raman study of the graphene sample under conditions of current flow, we obtained independent information on the energy distribution of the electrons and phonons. These results help understand the mechanisms responsible for current saturation in graphene field-effect transistors [6].

Some references:

[1] P. Avouris, Z. Chen & V. Perebeinos, Nature Nanotechnology 2, 605 (2007).

[2] P. Avouris, M. Freitag & V. Perebeinos, Nature Photonics 2, 341 (2008).

[3] S. Berciaud, S. Ryu, L.E. Brus & T.F. Heinz,  Nano Letters 9, 346 (2009).

[4] S. Berciaud, C. Voisin et al. Phys. Rev. B 81, 041414(R) (2010).

[5] H. Farhat, S. Berciaud, M. Kalbac, R. Saito, T.F. Heinz, M.S. Dresselhaus & J. Kong. Phys. Rev. Lett. 107, 157401 (2011).

[6] S. Berciaud, M.Y. Han, K.F. Mak, L.E. Brus, P. Kim & T.F. Heinz Phys. Rev. Lett. 104, 227401 (2010).

Waar en wanneer : Maandag 21 november 2011 om  11.00 u.  in lokaal N1.08, Campus Drie Eiken

Voordracht georganiseerd door EMAT

Onderwerp: Quantitative electron microscopy analysis of oxide hetero-structures

Spreker  : Ricardo Egoavil, EMAT, Universiteit Antwerpen

Abstract:

Oxides of transition metals with perovskite-type structure have gained quite large amount of attention due to their exceptional properties for oxide-electronics devices and the possibility to combine all functionalities of the perovskite oxide building blocks. Polar discontinuity is a general boundary condition at the oxide hetero-interfaces, and for a significant number of materials this phenomenon is unwanted as they reduce the intrinsic material properties at the interface. In this talk we will discuss about a strategy to eliminate the reconstruction of the polar discontinuity at the La_0.67Sr_0.33MnO₃/SrTiO₃ (LSMO/STO) interface from interpretation of the experimental STEM-EELS datasets obtained on the TITAN 50-80. These results will be compared with software simulations.

Waar en wanneer : Vrijdag 7 oktober 2011 om 11.30 u.  in lokaal  X.009, Campus Groenenborger

Voordracht georganiseerd door EDF 

 Onderwerp : Role of anomalous quark-gluon chromomagnetic interaction in hadron physics
Spreker: Nikolai Kochelev, Joint Institute for Nuclear Research, Dubna, Russia
 
Abstract:
 It is shown that the existence of strong fluctuations of the gluon fields in the nonperturbative QCD vacuum leads to appearance of the anomalous quark chromomagnetic moment (AQCM).  The  AQCM contribution to the spin-dependent quark-quark interaction is found to be large. A new model for soft Pomeron exchange based on AQCM is suggested and possible manifestations of new interaction in high energy reactions are discussed.
 
Waar en wanneer : Maandag 10 oktober 2011 om 16.00 u.  in lokaal  U.203, Campus Groenenborger

Voordracht georganiseerd door EMAT

  Onderwerp: Multivariate Statistical Analysis: application to TEM / EELS
Spreker: Stijn Lichtert, EMAT, Universiteit Antwerpen.
 
Abstract:
 With the arrival of new (S)TEM’s, the amount of digital information generated through techniques as STEM-EELS increased significantly. New problems rise how to efficiently interpret these data, sometimes containing high noise levels. To some of these problems multivariate statistical analysis can provide an answer by extracting statistically relevant information from a dataset. In this talk several techniques (PCA, ICA, factor analysis) from this domain are explained briefly. Specific problems in application to HR(S)TEM and STEM-EELS are discussed together with potential solutions. Several examples will show the possibilities of using these techniques in TEM. 
  
Waar en wanneer : Vrijdag 7 oktober 2011 om 11.30 u.  in lokaal  X.009, Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Synthesis and characterization of gold-ruthenium bimetallic catalysts

Spreker: Lidia Esther Chinchilla, Faculty of Sciences, University of Cádiz

Abstract:

Since the early 1940s the Water gas shift reaction (WGS) has represented an important step in the industrial production of hydrogen via the steam reforming of hydrocarbons. Is the reaction of water and carbon monoxide to produce hydrogen and carbon dioxide. This reaction also plays a role in the overall chemistry of hydrocarbon oxidation reactions and auto exhaust combustion catalysis.

Recent work has suggested that supported gold catalysts are very active in the WGS reaction at low temperatures. Along with their promising technical performance, the presence of a second component as Ruthenium induces significant changes in activity and stability as bimetallic catalyst. The bimetallic catalyst preparation can play a major role in controlling particle size of the nanoparticles of gold and ruthenium system.

Transmission electron microscope in High-resolution STEM mode, and nanometer resolution X-ray energy dispersive spectroscopy techniques, is critical to the fundamental studies of sizes, shapes, defects, surface structures, and compositions of the nanostructured catalysts. This is an example to illustrate the critical role of TEM-related techniques in heterogeneous catalysis.

Waar en wanneer : Vrijdag 30 september 2011 om  11.30 u.   in lokaal X.009 , Campus Groenenborger

Voordracht georganiseerd door EMAT

Onderwerp: Strain measurement at the nanoscale using transmission electron microscopy

Spreker: Armand Béché, EMAT, University of Antwerp

Abstract:

 For the last ten to fifteen years, developing new structural materials or new devices has required the control of strain at the nanometer scale. The needs for characterizations have greatly increased with more and more stringent criteria on spatial resolution and strain sensitivity, especially in the microelectronic industry . Development and improvement of techniques fulfilling such criteria is then necessary. The transmission electron microscope (TEM) is one of the only tools able to measure quantitative strain at the nanoscale. Four different TEM techniques will be presented: geometrical phase analysis (GPA) using high resolution imaging, convergent beam electron diffraction (CBED), nanobeam electron diffraction (NBED), dark field electron holography (DFEH).