Dissertations / Theses on the topic 'Electrons – Diffraction'
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1
Krecinic, Faruk [Verfasser]. "Ultrafast electron diffraction and imaging using ionized electrons / Faruk Krecinic." Berlin : Freie Universität Berlin, 2017. http://d-nb.info/1142155447/34.
2
Chen, Yixin. "Electron diffraction analysis of amorphous Ge2Sb2Te5." Thesis, University of Oxford, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.669987.
3
Dogbe, John Kofi. "Comparing cluster and slab model geometries from density functional theory calculations of si(100)-2x1 surfaces using low-energy electron diffraction." abstract and full text PDF (free order & download UNR users only), 2007. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3258835.
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Menzel, Andreas. "Step dynamics measurements with time-resolved low energy electron diffraction." Diss., Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/27870.
5
Chatelain, Robert P. 1982. "RF compression of electron bunches applied to ultrafast electron diffraction." Thesis, McGill University, 2008. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=111943.
Abstract:
The dynamics of atomic scale structures during structural change can be studied by Ultrafast Electron Diffraction (UED). The time resolution needed to reveal the fastest dynamics is 100 fs. Sub-angstrom structural resolution becomes possible with 1-1000 pC of charge necessary for diffraction pattern analysis during subtle structural changes. This combination of requirements cannot currently be realized due to the space-charge temporal broadening inherent to bunches of electrons of high fluence and short temporal duration. Simulations show that the incorporation of a specially designed Radio-Frequncy (RF) cavity into the UED apparatus removes this technical limitation. The RF cavity reverses the near linear position-momentum distribution of the temporally broadened electron bunch, causing the bunch to recompress itself as it propagates. It is found that our proposed method allows for sub-100 fs bunches with maximum charge of 0.6 pC, almost 3 orders of magnitude improvement over today's state of the art.
6
Erasmus, Nicolas. "The development of an electron gun for performing ultrafast electron diffraction experiments." Thesis, Stellenbosch : Stellenbosch University, 2009. http://hdl.handle.net/10019.1/2560.
Abstract:
Thesis (MSc (Physics))--Stellenbosch University, 2009.ENGLISH ABSTRACT: This thesis aims to comprehensively discuss ultrafast electron di raction and its role in temporally resolving ultrafast dynamics on the molecular level. Theory on electron pulses and electron pulse propagation will be covered, but the main focus will be on the method, equipment and experimental setup required to generate sub-picosecond electron pulses, which are needed to perform time resolved experiments. The design and construction of an electron gun needed to produce the electron pulses will be shown in detail, while preliminary pulse characterization experiments will also be illustrated. An introduction into the theory of electron diffraction patterns and how to interpret these diffraction patterns will conclude the thesis.
AFRIKAANSE OPSOMMING: Hierdie tesis het ten doel om ultravinnige elektrondi raksie deeglik te bespreek asook die rol wat dit speel om ultravinnige tyd-dinamika op 'n molekulêre vlak op te los. Die teorie van elektonpulse en die voortplanting van elektronpulse sal gedek word, maar die fokus sal op die metode, gereedskap en eksperimentele opstelling wees wat benodig is om sub-pikosekonde elektronpulse te genereer. Die ontwerp en konstruksie van 'n elektrongeweer, wat benodig word om elektronpulse te produseer, sal in detail bespreek word, terwyl aanvanklike pulskarakterisasie eksperimente ook illustreer sal word. 'n Inleiding tot die teorie van elektrondi raksie patrone en hoe om hulle te interpreteer sal die tesis afsluit.
7
Ho, Wing-kin. "The (3x3) reconstruction of SIC(0001) : a low energy electron diffraction study /." Hong Kong : University of Hong Kong, 1998. http://sunzi.lib.hku.hk/hkuto/record.jsp?B19737105.
8
Kaufmann, Martin. "Electron Diffraction Studies of Unsupported Antimony Clusters." Thesis, University of Canterbury. Physics and Astronomy, 2006. http://hdl.handle.net/10092/1269.
Abstract:
This thesis contains two main parts: the first part focusses on an electron diffraction study on unsupported antimony (Sb) clusters, while in the second part the design and development of a time-of-flight mass spectrometer (TOFMS) is discussed. Electron diffraction is an ideal tool to study the structure of clusters entrained in a beam. The main advantage of this technique is the ability to study the clusters in situ and in an interaction-free environment. It is therefore not necessary to remove the particles from the vacuum system which would lead to oxidation. Since the particles do not have to be deposited on a sample for further investigation, there is also no substrate which could influence the cluster structure. An additional advantage is the short exposure to the electron beam, thereby minimising the likelihood of damaging the particles. Sb clusters were produced using an inert-gas aggregation source. To control the cluster properties the source temperature, pressure and type of cooling gas can be adjusted. In the range of source parameters tested, Sb clusters with three different structures were observed: a crystalline structure corresponding to the rhombohedral structure of bulk Sb, an amorphous structure equivalent to the structure of amorphous Sb thin films, and a structure with the same diffraction signature as Sb4 (Sb evaporates mainly as Sb4). This last structure was found to belong to large particles consisting of randomly oriented Sb4 units. In order to study the size distributions and morphologies of the Sb clusters, the clusters were deposited onto substrates and studied under an electron microscope. The crystalline particles showed a wide variety of strongly faceted shapes. Depending on source conditions, the average cluster diameters ranged from 15 to 130 nm. There was a considerable disagreement between these values and the size estimates from the diffraction results with the latter being smaller by an order of magnitude. This might be due to the existence of domains inside the clusters. The amorphous particles were all found to be spherical with mean sizes between 27 and 45 nm. The Sb4 particles showed a liquid-like morphology and tended to coalesce easily. Their sizes ranged from 18 to 35 nm. To obtain an independent method for determining the cluster size, a TOFMS was designed and developed in collaboration with Dr Bernhard Kaiser. However, the TOFMS failed to detect a cluster signal in the original set-up which is most likely due to a defective ioniser and underestimated cluster energies. Further tests were performed in a new vacuum system and mass spectra for palladium clusters were successfully recorded.
9
何永健 and Wing-kin Ho. "The (3x3) reconstruction of SIC(0001): a low energy electron diffraction study." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1998. http://hub.hku.hk/bib/B31215300.
10
Mok, Cheuk-wai. "Comparing electron and positron scattering factors for applications in diffraction and holography /." Hong Kong : University of Hong Kong, 1997. http://sunzi.lib.hku.hk/hkuto/record.jsp?B18716337.
11
Burgess, William George. "Charge density determination in semiconductors and other materials by electron diffraction." Thesis, University of Cambridge, 1995. https://www.repository.cam.ac.uk/handle/1810/275257.
12
Hu, Hsiu-Lien. "Quantum transport of energetic electrons in ballistic nanostructures." Virtual Press, 2000. http://liblink.bsu.edu/uhtbin/catkey/1178341.
Abstract:
The various electronic phenomena of electrons in the quasi-one-dimensional semiconducotor heterostructures have been largely investigated in the past research, due to its importance both on the theoretical understanding and the design of nanodevices. In particular, most research is currently based on the GaAs-AIGaAs material system with a 2-DEG interface. From the study of Hua Wu, following the Bohm's interpretation of quantum mechanics, energetic electrons approximately approach the classical behavior. The goal of this theoretical study is to investigate how the flow of energetic electrons may be controlled by the use of a tunable reflector. When encountering hard potential walls, energetic electrons in the nanostructure nearly follow the law of reflection. In addition, if the hard potential walls function as a reflector, the bouncing ball trajectory is also predicted. In this project, the fact that energetic electrons demonstrate semi-classical periodical flow motion is conceptually verified.The quantum wire (QW) with a tab and a notch nanostructure is selected as the practical model to achieve the project's goal. The resonant properties of the QW with a tab and the QW with a notch are individually investigated. The tight-binding recursive Green's function method is the theory underlying the numerical computation of the conductance in a nanodevice.Department of Physics and Astronomy
13
Rohwer, Andrea Berenike. "Introducing organic molecular crystals into ultrafast electron diffraction." Thesis, Stellenbosch : Stellenbosch University, 2014. http://hdl.handle.net/10019.1/95819.
Abstract:
Thesis (MSc)--Stellenbosch University, 2014.ENGLISH ABSTRACT: Organic molecular salts have a wide range of physical properties which can be chemically tailored by minor variations of their substituents. These characteristics include high degrees of anisotropy, electrical conductivity ranging from superconducting to insulating, and structural changes in the crystal lattice during first order phase transitions brought about by minimal changes in temperature, effective pressure, and in some cases even light. Hence, these materials are particularly interesting for the development of molecular electronics and also as study materials in solid state physics. The family of copper-dimethyl-dicyanoquinone-diimine (Cu(DMe-DCNQI)2) salts forms part of the radical anion salt subclass of organic molecular crystals and is of particular interest due to its extraordinarily high conductivity compared to other quasi one-dimensional organic conductors. Its metal-to-insulator phase transition is characterised by conductivity jumps across several orders of magnitude within a few kelvin. Over the past three decades the metallic and insulating phases, as well as the transition behaviour have been investigated extensively utilising a broad spectrum of methods amongst others electrical conductivity, electron spin resonance, and re ectivity measurements, x-ray photoelectron and infrared spectroscopy, x-ray diffraction, and dilatometry. Fast light-switching between phases has been observed in partially deuterated forms of Cu(DCNQI)2 on sub-100-ps time scales. Furthermore, the phase transition is believed to be induced by a deformation of the crystalline lattice and a charge density wave formation which are detectable in diffraction images. Therefore we want to investigate this metal-to-insulator phase transition structurally and temporally via ultrafast electron diffraction. The technique of ultrafast electron diffraction employs the fundamentals of pump-probe spectroscopy: One of the two femtosecond pulsed laser beams excites the thin, crystalline sample, while the other - after being converted into a pulsed electron beam via the photoelectric effect - forms a diffraction image of the sample's lattice structure. The arrival time of the two pulses at the sample can be varied by a few femtoseconds with respect to each other enabling the resolution of ultrafast structural dynamics of the crystal's atomic lattice via electron diffraction. During the work presented in this thesis the sample preparation and characterisation leading to a successful introduction of Cu(DCNQI)2 into our ultrafast electron diffraction setup is presented. A diffraction pattern of comparable quality to that of a commercially available transmission electron microscope was recorded of the metallic state of partially deuterated d6 Cu(DCNQI)2. Subsequent analysis of the obtained diffraction data and further studies of the low temperature state { including simulations as well as experiments { have narrowed down the factors still making the diffraction pattern evasive. Possible solutions to experimental challenges are proposed to make the documentation of structural ultrafast dynamics in these organic molecular salts an attainable goal in the future.
AFRIKAANSE OPSOMMING: Organiese molekulêre soute het `n wye verskeidenheid van fisiese eienskappe wat chemies verander kan word deur geringe variasie in die samestelling van die sout. Hierdie eienskappe sluit in `n hoë graad van anisotropie, elektriese geleidingsvermoë wat strek van supergeleiding tot elektriese isolasie, en strukturele veranderinge in die kristalstruktuur tydens eerste orde fase-oorgange wat veroorsaak word deur geringe veranderinge in temperature, effektiewe druk en in sommige gevalle selfs lig. Gevolglik is hierdie material besonder interessant vir die ontwikkeling van molekulêre elektronika en ook as studiemateriaal in vastetoestandfisika. Die familie van koperdimetieldisianokinoondiimien (Cu(DMe-DCNQI)2) soute vorm `n deel van die radikaal-anioon-sout subklas van organiese molekulêre kristalle en is van besondere belang as gevolg van hulle buitengewone hoë elektriese geleidingsvermoë in vergelyking met ander kwasi-eendimensionele organiese geleiers. Die metaal-na-isolator fase-oorgang van hierdie kristal word gekenmerk deur die verandering van die geleidingsvermoë met verskeie ordegroottes binne `n paar kelvin. Gedurende die laaste drie dekades is die metaal en isolator fases, asook die oorgangsgedrag deeglik ondersoek met behulp van `n wye verskeidenheid van metodes wat onder andere elektriese geleidingsvermoë, elektron-spin resonans en reeksiemetings, x-straal fotoelektron en infrarooi spektroskopie, x-straal diffraksie en dilatometrie insluit. Vinnige skakeling tussen fases is waargeneem in gedeeltelik gedeuteerde vorms van Cu(DCNQI)2 op `n sub-100-ps tydskaal. Daar word verder geglo dat die fase-oorgang geïnduseer word deur `n deformasie van die kristalstruktuur en die vorming van `n ladingsdigtheidgolf wat meetbaar is in elektrondiffraksiebeelde. Om hierdie rede wil ons die metaal-na-isolator fase-oorgang se struktuur- en tydafhanklikheid ondersoek deur gebruik te maak van ultra-vinnige elektron diffraksie. Die tegniek van ultra-vinnige elektron diffraksie maak gebruik van die beginsels van pomp-toets spektroskopie: Een van die twee femtosekonde laserpulse wek die dun kristallyne monster op, terwyl die ander na omskakeling in `n elektronpuls via die foto-elektriese effek `n diffraksiebeeld van die monster se kristalstruktuur vorm. Die aankomtyd van die twee pulse by die monster kan met `n paar femtosekondes ten opsigte van mekaar verander word om die tydresolusie van die ultra-vinnige strukturele dinamika van die kristal se atoomstruktuur deur elektrondiffraksie moontlik te maak. In hierdie tesis word die monstervoorbereiding en karakterisering wat gelei het tot suksesvolle eksperimente op Cu(DCNQI)2 in ons ultra-vinnige elektron diffraksie opstelling behandel. `n Diffraksie patroon waarvan die kwaliteit vergelykbaar is met die van `n kommersiëel beskikbare transmissie elektron mikroskoop is gemeet vir die metaalfase van gedeeltelik gedeuteerde d6 Cu(DCNQI)2. Daaropvolgende analiese van die gemete diffraksiedata en verdere studies van die lae temperatuur toestand wat simulasies sowel as eksperimente insluit het `n klein aantal faktore uitgewys wat steeds die deteksie van die isolatorfase se ladingsdigtheidgolf se kenmerkende diffraksiepatroon verhoed. Moontlike oplossings tot eksperimentele uitdagings word voorgestel om die dokumentering van strukturele ultra-vinnige dinamika in hierdie organiese molekulêre soute `n haalbare toekomstige doelwit te maak.
14
CORNIER-QUIQUANDON, MARIANNNE. "Theorie dynamique de la diffraction des electrons rapides par les cristaux et quasicristaux." Paris 6, 1988. http://www.theses.fr/1988PA066167.
Abstract:
Description de la diffraction des electrons par les solides ordonnes a longue distance. Le formalisme necessite d'exprimer l'hamiltonien de diffraction sur une base discrete d'ondes planes. Le spectre des etats propres de l'hamiltonien de diffraction est dense non uniforme. Les calculs dynamiques peuvent donc etre effectues en echantillonant le spectre par valeurs discretes. Application de la methode a la simulation des images de microscopie electronique en haute resolution sur une phase icosaedrique en utilisant la description 6d de la structure. Interpretation des resultats experimentaux par la representation 6d
15
Suleiman, Aminat Oyiza. "Structural dynamics of 1T-TiSe2 using femtosecond electron diffraction." Thesis, Stellenbosch : Stellenbosch University, 2014. http://hdl.handle.net/10019.1/95990.
Abstract:
Thesis (MSc)--Stellenbosch University, 2014.ENGLISH ABSTRACT: Trilayered transition metal dichalcogenides such as our sample (1T-TiSe2) have been studied for many years as systems with strong electron-electron and electron-phonon correlations. The main attraction to this family of compound is its potential to exhibit ground state phenomena known as charge density waves whose detailed physical origin has been controversially determined. In this study, we have used an ultrafast femtosecond laser based on a pump-probe technique, namely ultrafast electron diffraction, to investigate these exotic features associated with the crystal. A pump laser pulse photo-excites the crystal from its ground state and the probe pulse (ultrashort electron pulse) takes the snapshot of the evolution of the lattice generating an electron diffraction pattern of the crystal. Hence the dynamical structural behaviour can be observed in time with a subpicosecond temporal resolution. As a hexagonal close-packed structure, its signature is expected to be seen in the diffraction pattern in both a steady-state and electron time-resolved femtosecond electron diffraction. In addition, simulations of electron diffractions pattern for room and low temperature structural data via a software called Simulation and Analysis of Electron diffraction (SAED) have been carried out. Clear signatures of charge density waves were seen at low temperature.
AFRIKAANSE OPSOMMING: Drie-laag oorgangsmetaal dikhalkogeniedes soos ons voorbeeld (1T-TiSe2), word reeds vir baie jare bestudeer as sisteme met sterk elektron-elektron en elektron-fonon korrelasies. Die hoof aantrekkingskrag van hierdie sisteme is die verskynsel van ladingdigtheidsgolwe in die grondtoestand. Die fisiese oorsprong van hierdie ladingdigtheidsgolwe was bepaal te midde van verskeie teenstrydighede. In hierdie studie, maak ons gebruik van die ultravinnige femtosekonde laser gebaseerde aktiveer-interogeer tegniek, genaamd ultravinnige elektron diffraksie (UED) om unicke eienskappe wat met die kristal geassosieer is te bestudeer. In UED wek ’n ultravinnige laserpuls (aktivering) die kristal op vanaf die grondtoestand waartydens n ultravinnige elektronpuls (interogering) ’n foto neem van die evolusie van die elektron diffraksiepatroon wat deur die kristalrooster gegenereer word. Hierdie wisselwerking van die interogerings elektronpuls en die sisteem kan gevolglik teen verskeie vasgetelde tye toegelaat word. Dus kan die dinamiese strukturele gedrag waargeneem word met ’n tydresolusie in die orde van die elektronpuls (sub-pikosekondes). Siende dat die kristal ’n diggepakte-heksagonale struktuur vorm, behoort die kenmerkende diffraksiepatroon daarvan waarneembaar te wees in beide die bestendige diffraksie en femtosekonde elektron diffraksie tegnieke. In hierdie konteks was duidelike tekens van ladingdigtheidsgolwe waargeneem. Benewens was daar ook simulasies uitgevoer om die elektron diffraksiepatrone asook die strukturele data by kamer en lae temperature vas te pen. Die sagteware wat hiervoor gebruik word is genaamd Simulasie en Ontleding van Elektronendiffraksie (SAED) - Simulation and Analysis of Electron Diffraction (SAED).
16
Plana, Ruiz Sergi. "Development & Implementation of an Electron Diffraction Approach for Crystal Structure Analysis." Doctoral thesis, Universitat de Barcelona, 2021. http://hdl.handle.net/10803/670887.
Abstract:
The application of electron diffraction to crystallographically characterize all kinds of materials has experienced new developments that have attracted some attention in recent years. A large number of structural analyses from different compounds have been already carried out with the help of 3D electron diffraction data that were not possible with the available X-ray methods. The use of a transmission electron microscope as an electron nano-diffractometer has proved to be advantageous when diffraction data from single nanocrystals are required, for instance in phase mixtures. In this way, the individual phases do not have to be purely synthesized, which always involves the risk of structural changes. The work presented here includes the development and implementation of a novel and universal routine for the accurate and reliable acquisition of electron diffraction data. It contains the alignment of the transmission electron microscope in order to set a quasi-parallel and precessed beam suitable for the acquisition of this kind of data, and the description and comparison of the developed technique to the other methods already available. This novel acquisition method has been called fast and automated diffraction tomography (Fast-ADT) and it is based on two consecutive tilt scans of the goniometric stage of the microscope; one to image the crystal and generate a crystal tracking file, and a second one to acquire the diffraction patterns while the beam is automatically shifted to follow the crystal at the different tilt angles. Such technique has been implemented in different experimental setups to test different detectors as well as different samplings of the diffraction space to point out their advantages and hindrances. The potential of this new data collection strategy to solve various crystallographic problems is illustrated using three known materials: barite, an inorganic salt stable under the electron beam used to check the level of accuracy that can be obtained from electron diffraction, RUB-5, a layered silicate with disordered features to show the advantage of high frame-rate cameras, and the pi-ferrosilicide, a crystalline phase from a commercial alloy measured to demonstrate how electron diffraction can reliably determine absolute structures. In addition, two unknown crystal structures from commercial products are fully determined and refined. The first one is the dehydrated DRED1 molecular crystal, an inorganic dye used in textile applications, which has been solved ab initio revealing the 46 non-hydrogen atoms of the two independent molecules in P-1. Furthermore, the structure refinement based on intensities calculated according to the dynamical diffraction theory has proved the position of some of the hydrogens, confirming the square-like H-bond network initially found from a Rietveld refinement based on a X-ray powder diffractogram. Finally, the incommensurate modulated structure of one of the polymorphs of dicalcium silicate (belite), a major constituent of cement, has been comprehensively characterized only by means of electron diffraction. Such crystal structure has been known since 1971 but the observed modulation has never been fully studied. Here, the Fast-ADT technique is presented as a robust acquisition method to systematically identify and properly characterize different crystallographic phases and features of powder mixtures. In this particular case, the precise knowledge of the different crystal structures in cement clinkers, such as the alpha’H-C2S, enables the exact phase analysis of these industrial phase mixtures directly from the manufacturing process, and paves the way for its CO2 emissions reduction that, at the moment, adds up to 5% of the global anthropogenic emission. In this context, a novel technique for the collection of 3D electron diffraction datasets has been successfully developed and implemented to increase the range of reliable available tools for the characterization of atomic structures from different materials.L’aplicació de la difracció d’electrons per caracteritzar cristal·logràficament tot tipus de materials ha experimentat nous desenvolupaments que han cridat certa atenció durant aquests últims anys. Un gran nombre d’anàlisis estructurals en diferents compostos ja s’han dut a terme amb l’ajut de dades tridimensionals de difracció d’electrons que no eren possibles amb els habituals mètodes de raigs X. L’ús d'un microscopi electrònic de transmissió com a nano-difractòmetre d’electrons ha demostrat ser molt més beneficiós quan es requereixen dades de difracció de nanocristalls individuals, per exemple en mescles de fases cristal·logràfiques. D’aquesta manera, les fases individuals no s’han de sintetitzar en estat pur, fet que sempre comporta el risc de canvis estructurals. El treball presentat aquí inclou el desenvolupament i implementació d’una nova rutina universal per l’adquisició precisa i fiable de dades de difracció d’electrons. El potencial d’aquesta nova estratègia de recopilació de dades per resoldre diversos problemes cristal·logràfics s’il·lustra mitjançant tres materials coneguts. A més a més, dues estructures cristal·lines desconegudes de productes comercials han estat determinades i refinades completament; un colorant orgànic de baixa simetria i una estructura modulada incommensurada d’un component principal del ciment. En particular, el coneixement precís de les diferents estructures cristal·lines dels clinkers de ciment, com ara la alpha’H-C2S, permet l’anàlisi exacta de les fases d'aquestes mescles industrials extretes directament del procés de fabricació, i facilita el seu estudi per reduir les emissions de CO2.
17
Faulkner, Helen Mary Louise. "Studies in phase and inversion problems for dynamical electron diffraction." Connect to thesis, 2003. http://repository.unimelb.edu.au/10187/2880.
Abstract:
This thesis examines problems in electron diffraction and related areas of theoretical optics. It begins with a study of the phase of a quantum mechanical wave function and the behaviour of phase vortices and vortex cores. Several rules for vortex core evolution are given and simulated vortex trajectories are studied. These simulations show that in electron microscopy at atomic resolution and in other similar situations, vortices occur in the wave functions very frequently. This means any image processing methods which deal with the wave function phase must permit vortices to occur. In this context a number of methods of phase retrieval are compared and evaluated. The criteria of evaluation are the accuracy of the phase retrieval, its ability to cope with vortices, its numerical stability and its required computational resources. The best method is found to be an iterative algorithm similar in approach to the Gerchberg-Saxton method, but based on a through focal series of images.Using this phase retrieval method as an essential tool, the thesis continues with a study of inverse problems in electron optics. The first problem considered is that of using a set of images taken to characterise the coherent aberrations present in a general imaging system. This problem occurs in many areas of optics and is studied here with a focus on transmission electron microscopy. A method of using software to simultaneously determine aberrations and subsequently remove them is presented and tested in simulation. This method is found to have a high level of accuracy in aberration determination. The second inverse problem studied in this thesis is the inversion problem in dynamical electron diffraction. This problem is solved for a periodic object, giving an accurate and unique solution for the projected potential in the multiple scattering case. An extension of this solution to objects which are non-periodic in the direction of the incident wave is investigated. Finally a model computation solving the general inversion problem for dynamical diffraction in an aberrated transmission electron microscope is performed, illustrating this and previous material and summing up the advances presented in this work.
18
Kassier, Gunther Horst. "Ultrafast electron diffraction : source development, diffractometer design and pulse characterisation." Thesis, Stellenbosch : University of Stellenbosch, 2010. http://hdl.handle.net/10019.1/5359.
Abstract:
Thesis (PhD (Physics))--University of Stellenbosch, 2010.ENGLISH ABSTRACT: Ultrafast Electron Diffraction (UED) is a rapidly maturing field which allows investigation of the evolution of atomic arrangement in solids on timescales comparable to the vibrational period of their constituent atoms (~10-13 s). The technique is an amalgamation of conventional high energy electron diffraction methods and pump-probe spectroscopy with femtosecond (1 fs = 10-15 s) laser pulses. Ultrafast pulsed electron sources generally suffer from limitations on the attainable electron number per pulse (brightness) due to Coulomb repulsion among the electrons. In this dissertation, the design and construction of a compact UED source capable of delivering sub-300 fs electron pulses suitable for diffraction experiments and containing about 5000 electrons per shot is described. The setup has been characterised by measurement of the transverse beam size and angular spread, and through recording and analyzing an electron diffraction pattern from a titanium foil. Measurement of the temporal duration of fs electron pulses is not trivial, and a specialised compact streak camera operating in accumulation mode has been developed as part of this study. A sub-200 fs temporal resolution has been achieved, and the dependence of temporal duration on electron number per pulse was investigated for the current UED source. The observed trends correlate well with detailed electron bunch simulations. In order to investigate ultrafast processes on samples that cannot be probed repeatedly, it becomes necessary to significantly increase the brightness of current state of the art compact sources such as the one constructed in the present study. UED sources employing electron pulse compression techniques offer this possibility. Traditional pulse compression schemes based on RF cavities, while simple in principle, pose significant technical challenges in their realisation. The current thesis describes two novel UED pulse compression methods developed by the author: achromatic reflectron compression and pulsed cavity compression. Both concepts are expected to be easier to realise than conventional RF compression. Detailed simulations predict that such sources can attain a brightness improvement of more than one order of magnitude over compact sources that do not employ compression techniques. In addition, such sources show much promise for the attainment of pulse durations in the sub-100 fs range.
AFRIKAANSE OPSOMMING: Ultra vinnige elektron diffraksie is ‘n meettegniek wat tans in die proses is om vinnige ontwikkeling te ondergaan. Die tegniek het ten doel om strukturele omsettingsprosesse op ‘n lengteskaal van atoombindings en ‘n tydskaal van die vibrasie periode van atome in materie, ongeveer 10-13 s, te ondersoek. Dit word bewerkstellig deur die spasieresolusievermoë van gewone hoë energie elektron diffraksie met die tydresolusievermoë van femtosekonde (1 fs = 10-15 s) laserspektroskopie te kombineer. Die aantal elektrone per puls (intensiteit) van ultravinnige gepulsde elektronbronne word beperk deur die Coulomb afstootingskragte tussen die elektrone. Hierdie dissertasie beskryf die ontwerp en konstruksie van ‘n kompakte ultravinnige elektron bron. Die elektronpulse wat geproduseer word bevat tot 5000 elektrone per puls met ‘n tyd durasie van minder as 300 fs, en is geskik vir diffraksie eksperimente. Die aparaat is gekarakteriseer deur die volgende metings: elektronpulsdiameter, straaldivergensie, en ‘n titaan foelie se statiese diffraksie patroon. Dit is nie triviaal om die durasie van femtosekonde elektronpulse te meet nie, en n spesiale kompakte akkumulerende “streak camera” is vir die doeleindes van hierdie projek onwikkel. Die tydresolusie van hierdie “streak camera” is beter as 200 fs, en die afhanklikheid van die pulsdurasie wat deur die ultravinnige elektron bron geproduseer word as n funksie van die elektrongetal per puls is met behulp van hierdie toestel bepaal. Die resultate klop redelik goed met gedetaileerde simulasies van die elektron puls dinamika. Die karakterisasie van monsters wat nie herhaaldelik gemeet kan word nie vereis verkieslik ‘n nog hoër pulsintensiteit as wat met huidige bronne bereik kan word. ‘N verdere doelstelling is dus om ultravinnige elektron bronne te ontwikkel wat pulse met meer elektrone per puls kan genereer. Dit kan bewerkstellig word deur bronne wat van elektron puls kompressie tegnieke gebruik maak. Die tradisionele manier waarop dít gedoen word is deur middel van n kontinu gedrewe radio frekwensie holte. Hierdie metode gaan egter gepaard met aanmerklik hoë tegniese uitdagings. Om hierdie rede het die outeur twee alternatiewe puls kompressie konsepte ontwikkel: akromatiese reflektron kompressie and gepulsde holte kompressie. Albei konsepte sal waarskeinlik makliker wees om te realiseer as die tradisionele radio frekwensie kompressie, en is deur middel van gedetaileerde simulasies geverifiseer. Hierdie simulasies voorspel dat die intensiteit van genoemde bronne met ten minste n grooteorde meer kan wees as wat tans met kompakte ultravinnige elektron bronne moontlik is. Verder blyk dit dat sulke bronne n pulsdurasie van minder as 100 fs kan bereik.
19
Jones, Daniel M. "A convergent beam electron diffraction study of some rare-earth perovskite oxides /." Connect to this title, 2007. http://theses.library.uwa.edu.au/adt-WU2008.0057.
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Jones, Daniel M. "A convergent beam electron diffraction study of some rare-earth perovskite oxides." University of Western Australia. School of Physics, 2008. http://theses.library.uwa.edu.au/adt-WU2008.0057.
Abstract:
This work describes detailed convergent beam electron diffraction (CBED) studies of GdAlO3 and LaAlO3 perovskites. CBED patterns tilted away from major zone axes have been found to have high sensitivity to the presence of mirror or glide mirror symmetry. Such patterns confirm to high accuracy that the space group of GdAlO3 is orthorhombic, Pnma. Tilted patterns from this well characterised structure also serve as benchmarks against which similar patterns may be compared. In the case of LaAlO3, tilted patterns enable the space group to be confirmed as rhombohedral R3c, previously claimed to be cubic (Fm3c) by CBED. Furthermore, no evidence for the low symmetry (I2/a or F1) phases proposed for LaAlO3 has been observed. The LaAlO3 study also gives a careful assessment of the influence of tilted specimen surfaces on the CBED data. Within the qualitative scope of these experiments, no symmetry degrading effects could be observed. Some preliminary Quantitative CBED (QCBED) data from LaAlO3 is also presented. This shows it will be possible to make a detailed study of the bonding charge density (Δρ) in this material when combined with X-ray diffraction data. Also included is a brief CBED study of LaFeO3, a material that is isostructural with GdAlO3. Although this is restricted to exact zone axis patterns, it is noted that tilted patterns have significant potential to improve the quality of the symmetry determination.
21
Chandril, Sandeep. "In situ structural and compositional analysis using RHEED electrons induced x-rays." Morgantown, W. Va. : [West Virginia University Libraries], 2009. http://hdl.handle.net/10450/10641.
Abstract:
Thesis (Ph. D.)--West Virginia University, 2009.Title from document title page. Document formatted into pages; contains ix, 97 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 95-97).
22
莫卓威 and Cheuk-wai Mok. "Comparing electron and positron scattering factors for applications indiffraction and holography." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1997. http://hub.hku.hk/bib/B31214459.
23
Rusz, Ján, Shunsuke Muto, and Kazuyoshi Tatsumi. "Energy Loss by Channeled Electrons: A Quantitative Study on Transition Metal Oxides." Cambridge University Press, 2013. http://hdl.handle.net/2237/20834.
24
Lau, Wai-ping, and 劉偉平. "Minimising the calculation time of the cluster program by choosing theminimum convergent cluster size and the best relaxation factor." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1999. http://hub.hku.hk/bib/B29760392.
25
Haeberlé, Olivier. "Etude du rayonnement electromagnetique genere par des electrons relativistes en interaction avec un reseau de diffraction." Université Louis Pasteur (Strasbourg) (1971-2008), 1994. http://www.theses.fr/1994STR13238.
Abstract:
L'effet smith-purcelle a ete largement etudie depuis la prediction de son existence en 1942 et sa premiere confirmation experimentale en 1953. Les modeles theoriques precedemment existants supposent un electron se propageant parallelement a la surface du reseau de diffraction et perpendiculairement aux traits du reseau. Dans le chapitre premier est exposee dans le cadre d'une theorie electromagnetique la description de l'effet smith-purcell pour un electron se propageant parallelement a la surface d'un reseau de diffraction avec un angle arbitraire par rapport aux traits du reseau. Le modele propose est restreint au cas des surfaces parfaitement conductrices, pour lesquelles quelques techniques modernes de resolution du probleme de reseau ont ete adaptees a la configuration particuliere du champ incident. Ces techniques sont decrites dans le second chapitre. Dans le chapitre trois, le rayonnement de smith-purcell produit par des electrons relativistes d'energie 1 a 100 mev en interaction avec des reseaux millimetriques est calcule en utilisant les theories precedemment developpees. Les proprietes du rayonnement sont decrites et quelques applications potentielles sont proposees. L'accelerateur gelina utilise pour la realisation des experiences smith-purcell est decrit dans le chapitre quatre. Dans le chapitre cinq, les experiences smith-purcell sont decrites. Les distributions spectrales et angulaires, la polarization et la dependance par rapport a l'energie des electrons du rayonnement obtenu par interaction d'electrons de 20 a 110 mev avec des reseaux optiques sont presentees
26
Haupt, Kerstin Anna. "Phase transitions in transition metal dichalcogenides studied by femtosecond electron diffraction." Thesis, Stellenbosch : Stellenbosch University, 2013. http://hdl.handle.net/10019.1/85608.
Abstract:
Thesis (PhD)--Stellenbosch University, 2013.ENGLISH ABSTRACT: Low-dimensional materials are known to undergo phase transitions to differently or- dered states, when cooled to lower temperatures. These phases often show a periodic modulation of the charge density (called a charge density wave – CDW) coupled with a periodic perturbation of the crystal lattice (called a periodic lattice distortion – PLD). Although many experiments have been performed and much has been learnt about CDW phases in low-dimensional materials, the reasons for their existence are still not fully understood yet. Many processes, involving either strong electron–electron or electron–lattice coupling, have been observed which all might play a role in explaining the formation of different phases under different conditions. With the availability of femtosecond lasers it has become possible to study materials under highly nonequilibrium conditions. By suddenly introducing a known amount of energy into the system, the equilibrium state is disturbed and the subsequent relax- ation processes are then observed on timescales of structural and electronic responses. These experiments can deliver valuable information about the complex interactions between the different constituents of condensed matter, which would be inaccessible under equilibrium conditions. We use time resolved electron diffraction to investigate the behaviour of a CDW system perturbed by a short laser pulse. From the observed changes in the diffraction patterns we can directly deduce changes in the lattice structure of our sample. A femtosecond electron diffraction setup was developed at the Laser Research In- stitute in Stellenbosch, South Africa. Short laser pulses produce photo electrons which are accelerated to an energy of 30 keV. Despite space charge broadening effects, elec- tron pulses shorter than 500 fs at sample position can be achieved. Technical details of this system and its characterisation as well as sample preparation techniques and analysis methods are described in detail in this work. Measurements on two members of the quasi-two-dimensional transition metal di- chalcogenides, namely 4Hb-TaSe2 and 1T-TaS2, are shown and discussed. Both show fast (subpicosecond) changes due to the suppression of the PLD and a rapid heating of the lattice. When the induced temperature rise heats the sample above a phase tran- sition temperature, a complete transformation into the new phase was observed. For 4Hb-TaSe2 we found that the recovery to the original state is significantly slower if the PLD was completely suppressed compared to only disturbing it. On 1T-TaS2 we could not only study the suppression of the original phase but also the formation of the higher energetic CDW phase. Long (100 ps) time constants were found for the tran- sition between the two phases. These suggest the presence of an energy barrier which has to be overcome in order to change the CDW phase. Pinning of the CDW by de- fects in the crystal structure result in such an energy barrier and consequently lead to a phase of domain growth which is considerably slower than pure electron or lattice dynamics.
AFRIKAANSE OPSOMMING: Dit is bekend dat lae-dimensionele materie fase oorgange ondergaan na anders ge- ori¨enteerde toestande wanneer afgekoel word tot laer temperature. Hierdie fases toon dikwels ’n periodiese modulasie van die elektron digtheid (genoem ’n “charge density wave” – CDW), tesame met ’n periodiese effek op die kristalrooster (genoem ’n “peri- odic lattice distortion” – PLD). Alhoewel baie eksperimente al uitgevoer is en al baie geleer is oor hierdie CDW fase, is die redes vir hul bestaan nog steeds nie ten volle verstaan nie. Baie prosesse, wat of sterk elektron–elektron of elektron–fonon interaksie toon, is al waargeneem en kan ’n rol speel in die verduideliking van die vorming van die verskillende fases onder verskillende omstandighede. Met die beskikbaarheid van femtosekonde lasers is dit nou moontlik om materie onder hoogs nie-ewewig voorwaardes te bestudeer. Deur skielik ’n bekende hoeveel- heid energie in die stelsel in te voer, word die ewewigstaat versteur en word die daar- opvolgende ontspanning prosesse waargeneem op die tydskaal van atomies struktu- rele en elektroniese bewiging. Hierdie eksperimente kan waardevolle inligting lewer oor die komplekse interaksies tussen die verskillende atomiese komponente van ge- kondenseerde materie, wat ontoeganklik sou wees onder ewewig voorwaardes. Ons gebruik elektrondiffraksie met tyd resolusie van onder ’n pikosekonde om die gedrag van ’n CDW stelsel te ondersoek nadat dit versteur is deur ’n kort laser puls. Van die waargenome veranderinge in die diffraksie patrone kan ons direk aflei watse veranderinge die kristalstruktuur van ons monster ondergaan. ’n Femtosekonde elektronendiffraksie opstelling is ontwikkel by die Lasernavors- ingsinstituut in Stellenbosch, Suid-Afrika. Kort laser pulse produseer foto-elektrone wat dan na ’n energie van 30 keV versnel word. Ten spyte van Coulomb afstoting ef- fekte, kan elektron pulse korter as 500 fs by die monster posisie bereik word. Tegniese besonderhede van hierdie opstelling, tegnieke van die voorbereiding van monsters asook analise metodes word volledig in hierdie tesis beskryf. Metings op twee voorbeelde van kwasi-tweedimensionele semi-metale, naamlik 4Hb-TaSe2 en 1T-TaS2, word gewys en bespreek. Beide wys ’n vinnige (subpikosekon- de) verandering as gevolg van die versteuring van die PLD en ’n vinnige verhitting van die kristalrooster. Wanneer die ge¨ınduseerde temperatuur bo die fase oorgang tempe- ratuur styg, is ’n volledige transformasie na die nuwe fase waargeneem. Vir 4Hb-TaSe2 het ons gevind dat die herstelling na die oorspronklike toestand aansienlik stadiger is as die PLD heeltemal viernietig is in vergelyking met as die PLD net versteur is. Met 1T-TaS2 kon ons nie net alleenlik die vernietiging van die oorspronklike fase sien nie, maar ook die vorming van ’n ho¨er energie CDW fase. Lang (100 ps) tydkonstante is gevind vir die oorgang tussen die twee fases. Hierdie dui op die teenwoordigheid van ’n energie-versperring wat eers oorkom moet word om die CDW fase voledig te ver- ander. Vaspenning van die CDW deur defekte in die kristalstruktuur veroorsaak so’n energie versperring en gevolglik lei dit tot ’n fase van groeiende CDW gebiede wat heelwat stadiger as pure elektron of kritalrooster dinamika is.
27
周紀文 and Kee-man Chow. "[square root of three] x [square root of three] reconstruction of SiC(0001) surface and 2x1 reconstruction of Si(111) cleaved surface: a LEED study." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1999. http://hub.hku.hk/bib/B31222456.
28
梁艾芝 and Ai-chi Angela Leung. "Atomic structure studies of holmium on SI(111) surface by low energy electron diffraction Patterson inversion at multiple incident angles." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2007. http://hub.hku.hk/bib/B39558824.
29
HENRI, PASCAL. "Rayonnement de transition par un reseau rayonnement electromagnetique emit par des electrons traversant un reseau de diffraction." Université Louis Pasteur (Strasbourg) (1971-2008), 1999. http://www.theses.fr/1999STR13170.
Abstract:
Le rayonnement de transition est emis lorsqu'un electron traverse la frontiere separant deux milieux de proprietes optiques differentes. Ce phenomene trouva sa description theorique des 1945 et fut largement etudie experimentalement depuis le rayonnement x jusqu'a l'infrarouge. La majorite des travaux jusqu'ici, ont ete consacres au cas du rayonnement de transition emis par une surface plane (otr). Nous proposons d'etudier dans le domaine du visible, le cas ou le miroir plan est remplace par un reseau optique, la periode du reseau etant de l'ordre de la longueur d'onde d'observation. Nous appellerons ce cas particulier, rayonnement de transition par un reseau (rtr). Le chapitre deux est consacre a la description theorique de notre travail. Un phenomene tres proche de celui que nous etudions est l'effet smith-purcell (sp), ou le rayonnement est produit par la diffraction du champ coulombien d'un electron se propageant parallelement a la surface d'un reseau. Dans le
30
Leung, Ai-chi Angela. "Atomic structure studies of holmium on SI(111) surface by low energy electron diffraction Patterson inversion at multiple incident angles." Click to view the E-thesis via HKUTO, 2007. http://sunzi.lib.hku.hk/HKUTO/record/B39558824.
31
Pancotti, Alexandre. "Propriedades estruturais e eletrônicas de filmes ultra finos de In, Sn e Sb, crescidos sobre Pd (111), estudados por PED e XPS." [s.n.], 2005. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277061.
Abstract:
Orientador: Richard LandersDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
Made available in DSpace on 2018-08-09T10:50:19Z (GMT). No. of bitstreams: 1 Pancotti_Alexandre_M.pdf: 2977317 bytes, checksum: 914d690fda86677fcc9c894c6f408409 (MD5) Previous issue date: 2005
Resumo: Nesse trabalho nos propomos a estudar a estrutura eletrônica e geométrica de ligas de superfície à partir de filmes ultra finos, da ordem de monocamada atômica , crescidos por MBE sobre substratos monocristalinos. Os filmes finos foram crescidos in situ e analizados por XPS (X-Ray Photoelectron Spectroscopy), PED (Photoelectron Diffraction), LEED (Low Energy Electron Diffraction) e UPS (Ultra Violet Photoelectron Spectroscopy). As ligas de superfície estudas foram InPd, SnPd e SbPd sempre sobre um substrato de Pd(111). Os resultados das medidas PED dos sistemas InPd e SnPd foram interpretadas usando programas tipo MSCD[1]. Foi mostrado que Sn e In em baixa cobertura formam uma estrutura (raiz2 3 x raiz2 3) R 30o ( e o In mostrou uma estrutura (1x1) para filmes com mais de 2 monocamadas, provavelmente na forma de ilhas sobre a superfície do Pd. Estudos anteriores mostraram que Sb sobre Pd(111) também apresenta a fase (raiz2 3 x raiz2 3)R 30o para baixas coberturas[2]. Os modelos que melhor se adequaram aos dados sugerem fortemente que os metais sp(In, Sn,e Sb) não difundem além da segunda monocamada. Durante estes estudos foi observado que a intensidade do satélite de shake-up do Pd tendia a zero para os átomos em contato direto com os metais s
Abstract: The purpose of this report is to present a study of the electronic and geometric structure of surface alloys grown by MBE (Molecular Beam Epitaxy) on single crystal substrates in the sub monolayer regime. The films were grown "in-situ" in the analysis chamber and analyzed by XPS (X-Ray Photoelectron Spectroscopy), PED (Photoelectron Diffraction), LEED (Low Energy Electron Diffraction) and UPS (Ultra Violet Photoelectron Spectroscopy). The alloys studied were InPd, SnPd e SbPd, all grown on the (111) face of a Pd crystal. The PED measurements for InPd and SnPd were analyzed using the MSCD code[3]. It was possible to show that for low coverages both metals formed a (raiz2 3 x raiz2 3) R 30o ( reconstruction and the first inter-planar distances were determined. For films with over two monolayers the In grew with a (1x1) structure probably in the form of islands separated by clean Pd. Previous studies showed that Sb[4] at low coverage also forms a (raiz2 3 x raiz2 3) R 30o ( structure. Our simulations suggest that In, Sn and Sb for the coverages studied do not diffuse beyond the second atomic layer. We also observed that the intensity of the Pd shake up satellite tends to zero for the Pd atoms in contact with these sp metals
Mestrado
Física da Matéria Condensada
Mestre em Física
32
Py, Christophe. "Construction d'un interféromètre électronique pour l'étude des propriétés émissives de micropointes à effet de champ : diffraction et interférences d'électrons lents." Grenoble 1, 1993. http://www.theses.fr/1993GRE10176.
Abstract:
La substitution des filaments chauffants par des micropointes a effet de champ comporte des avantages dans plusieurs applications d'electronique: le leti a deja prouve l'interet de ce principe pour les ecrans plats; de nombreuses personnes croient que les micropointes pourraient fournir des sources micro-ondes innovantes. De plus, les proprietes de l'emission permettent d'aller plus loin en envisageant des applications d'optique electronique que n'etaient pas possible avec les filaments chauffants. En vue d'evaluer les potentialites des micropointes fabriquees au leti pour de telles applications, une experience d'interferometrie avec des electrons lents a ete imaginee, concue, realisee et caracterisee. L'instrumentation developpee est composee d'un biprisme de mollenstedt, d'un systeme de lentilles electrostatiques et d'un systeme d'imagerie compose de plaques de deflexion, d'un diaphragme de petite taille (1 m) et d'un multiapplicateur d'electrons. Des effets ondulatoires ont ete observes confirmant les bonnes proprietes de coherence de la source. L'experience permet par ailleurs potentiellement d'avancer dans la comprehension de phenomene d'emission par les pointes fabriquees au leti, et d'envisager des ameliorations technologiques pour les applications visees. Elle doit egalement permettre d'etudier l'application a l'holographie electronique, et par la suite d'imaginer des dispositifs d'optique electronique compacts tenant compte des proprietes ondulatoires des electrons emis par les micropointes
33
Zeybek, Orhan. "Surface studies of magnetic thin films." Thesis, University of Liverpool, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367247.
34
Amara, Mehdi. "Ordres conjugués à l'antiferromagnétisme dans les composés cubiques de terres rares." Habilitation à diriger des recherches, Université de Grenoble, 2011. http://tel.archives-ouvertes.fr/tel-00968333.
Abstract:
Les composés cubiques à base de terres rares sont un terrain favorable à l'expression de degrés de liberté alternatifs, liés à la dégénérescence d'orbite et/ou aux instabilités cristallographiques. Du fait du caractère multiaxial de la symétrie cubique, une simple anisotropie à un ion associée à des couplages d'échange isotrope y conduirait à des états d'ordre magnétique de dégénérescence élevée. Celle-ci étant systématiquement levée, on est forcé de mettre en cause des degrés de liberté autres que celui de spin. Cet impératif est confirmé par l'observation dans ces systèmes de transitions magnétiques du premier ordre, de structures magnétiques exotiques (notamment multiaxiales), d'effets magnétoélastiques, d'une anisotropie ne relevant pas du seul champ cristallin etc., phénomènes tous impossibles à appréhender sur la base de l'échange isotrope et de l'anisotropie à un ion. Nous étudions l'influence de deux types de degrés de liberté, qui interviennent par une redistribution de la charge dans le cristal : - la "déformation" de la couche 4f , décrite par l'émergence de multipôles électriques. - le déplacement des ions terre rare, lorsque la structure cristalline le permet. Dans les états d'ordre magnétique, ces deux modes correspondent à l'émergence de paramètres d'ordre secondaires, composantes multipolaires 4f et distribution de déplacement, qui participent à la définition des états ordonnés. Ils en déterminent largement les propriétés et donnent lieu à des phénomènes spécifiques qui, via la charge, trahissent le magnétisme. A l'état paramagnétique, ils sont également actifs, modifiant l'anisotropie magnétique, les propriétés magnétoélastiques et de transport etc.. Ils peuvent même y induire des transitions de phase non-magnétiques, où seule intervient la distribution de charge (ordres quadrupolaires, effet Jahn-Teller).
35
Ihee, Hyotcherl Zewail Ahmed H. Zewail Ahmed H. "Ultrafast electron diffraction /." Diss., Pasadena, Calif. : California Institute of Technology, 2001. http://resolver.caltech.edu/CaltechETD:etd-04072008-112244.
36
Morishita, Shigeyuki, Jun Yamasaki, Keisuke Nakamura, Takeharu Kato, and Nobuo Tanaka. "Diffractive imaging of the dumbbell structure in silicon by spherical-aberration-corrected electron diffraction." American Institite of Physics, 2008. http://hdl.handle.net/2237/11975.
37
Lahme, Stefan. "Femtosecond single-electron diffraction." Diss., Ludwig-Maximilians-Universität München, 2014. http://nbn-resolving.de/urn:nbn:de:bvb:19-178346.
Abstract:
Die grundlegenden Funktionsprinzipien der Natur zu verstehen, ist seit jeher Antrieb der Naturwissenschaften. Verhalten und Eigenschaften von Festkörpern werden dabei häufig von dynamischen Prozessen auf atomarer Skala (< 10^-10 m) bestimmt, welche typischerweise auf Zeitskalen im Bereich von zehn Femtosekunden (10^-15 s) bis hin zu vielen Picosekunden (10^-12 s) ablaufen. Zeitaufgelöste Elektronenbeugung an kristallinen Festkörpern ermöglicht die direkte Beobachtung solcher Prozesse in Raum und Zeit. Die bislang mit diesem Verfahren erreichte Zeitauflösung von etwa 100 fs eignet sich jedoch nicht zur Beobachtung der schnellsten Prozesse in Festkörpern. Auch die, zur zuverlässigen Auflösung von großen Elementarzellen molekularer Kristalle erforderliche, transversale Kohärenz ist unzureichend. Eine wesentliche Ursache für diese beiden Probleme liegt in der gegenseitigen Coulomb-Abstoßung der Elektronen innerhalb eines Pulses und den daraus resultierenden Veränderungen der Geschwindigkeitsverteilungen in radialer und longitudinaler Richtung. Während erstere zu verringerter transversaler Kohärenz führt, hat letztere längere Elektronenpulsdauern und damit eine begrenzte Zeitauflösung zur Folge.
In dieser Arbeit wird ein Messaufbau zur zeitaufgelösten Elektronenbeugung vorgestellt, welcher auf der Erzeugung von nur einem Elektron pro Puls basiert. Aufgrund der Vermeidung von Coulomb-Abstoßung innerhalb der Pulse ist dieser Ansatz eine vielversprechende Basis zur konzeptionell nahezu unbegrenzten Verbesserung der Zeitauflösung. Eine hier eigens entwickelte, thermisch stabilisierte Elektronenquelle garantiert einen hohen Grad an Kohärenz bei gleichzeitig hervorragender Langzeitstabilität der Photoelektronenausbeute. Insbesondere letzteres ist für zeitaufgelöste Beugungsexperimente mit Einzeleelektronen aufgrund der längeren Integrationszeit unerlässlich, konnte jedoch durch vorhergehende Quellen nicht erreicht werden. Darüber hinaus werden in dieser Arbeit die besonderen Ansprüche der Einzelelektronenbeugung an die zu untersuchenden Materialien diskutiert und Strategien zur Vermeidung von Schäden an der Probe durch akkumulierte Anregungsenergie entwickelt. Diese erfordern neue Schwerpunkte bei der Probenpräparation, welche entwickelt und diskutiert werden. Die Beobachtung der komplexen Relaxationsdynamik in Graphit-Dünnfilmen mit zeitaufgelöster Einzelelektronenbeugung demonstriert abschließend die generelle Eignung dieses Verfahrens als zuverlässige Methodik zur Untersuchung von reversibler, struktureller Dynamik in Festkörpern mit atomarer Auflösung.
Nicht-relativistische Einzelelektronenpulse können mit Hilfe von zeitabhängigen Feldern bei Mikrowellenfrequenzen bis in den 10 fs-Bereich komprimiert werden, eventuell sogar
bis in den Attosekundenbereich. Die hier demonstrierte langzeitstabile und hochkohärente Elektronenquelle, sowie die Methodiken zur Probenpräparation und zeitaufgelösten Beugung
mit Einzelelektronenpulsen liefern die Basis für zukünftige Experimente dieser Art.The understanding of nature’s fundamental processes has always been the goal of science. Often, the behavior and properties of condensed matter are determined by dynamic pro- cesses on the atomic scale (< 10^-10 m). The relevant time scales for these processes range from tens of femtoseconds (10^−15 s) to several picoseconds (10^−12 s). Time-resolved electron diffraction on crystalline solids allows the direct observation of such processes in space and time. However, the state-of-the-art temporal resolution is insufficient to observe the fastest processes in solids. The transverse coherence is insufficient to resolve large unit cells of molecular crystals. One major origin for both of these problems is that the electron within the pulse repel each other, resulting in a change of the radial and longitudinal velocity distribution. The former leads to a decrease transverse coherence while the former leads to a significant increase in electron pulse duration, limiting temporal resolution. In this work, a setup for time-resolved electron diffraction is introduced that works with electron pulses each containing only a single electron. Circumventing Coulomb repulsion, this approach can lead to in principle nearly unlimited, improvement of temporal resolu- tion. The novel, thermally stabilized single-electron gun developed here provides a high degree of transverse coherence and excellent long-term stability of the photoemission yield at the same time. The latter is crucial for time-resolved diffraction experiments due to the long integration times required when working with single-electron pulses and has not been achieved prior to this work. Furthermore, the special requirements of single-electron diffraction on the materials under study are discussed. Strategies for avoidance of sam- ple damage from accumulated excitation energy are developed, requiring new emphases in sample preparation. The observation of the complex relaxation dynamics of graphite thin films using time-resolved single-electron diffraction finally demonstrates the general feasi- bility of this technique as a reliable methodology for investigation of reversible, structural dynamics in solids with atomic resolution. Using time-dependent fields at microwave frequencies, non-relativistic single-electron pulses can be compressed to 10 fs and possibly even down to the attosecond regime. The long-term stable and high-coherence electron gun demonstrated here as well as the method- ology developed for sample preparation and time-resolved electron diffraction using single- electron pulses provide the basis for such experiments in the future.
38
White, Thomas Ashley. "Structure solution using precession electron diffraction and diffraction tomography." Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611748.
39
Rivera, Felipe. "Solid Phase Crystallization of Vanadium Dioxide Thin Films and Characterization Through Scanning Electron Microscopy." Diss., CLICK HERE for online access, 2007. http://contentdm.lib.byu.edu/ETD/image/etd2233.pdf.
40
Diehl, Markus. "Diffraction in electron-proton collisions." Thesis, University of Cambridge, 1996. https://www.repository.cam.ac.uk/handle/1810/265482.
Abstract:
Subject of this thesis is diffractive collisions of electrons and protons, i.e. collisions wher~ the proton remains intact or is slightly excited, and loses only a small fraction of its initial energy. Such processes can be described by the exchange of a pomeron, which is well-known from the phenomenology of highenergy hadron scattering but far from being understood in the framework of quantum chromodynamics. We use the simple model of Landshoff and Nachtmann, where the pomeron is approximated by two nonperturbative gluons, to calculate the cross section for diffractive production of a quark-antiquark pair from a real or virtual photon. This allows us to give detailed predictions for diffractive electro- or photoproduction of a jet pair, for diffractive charm production, and for the inclusive cross section in diffractive deep inelastic scattering at not too large masses of the diffractive system. For the latter we find rather good agreement with the data taken at the HERA collider, taking into account the uncertainties of our calculation. We then turn our attention to azimuthal correlations in diffractive scattering. The dependence of the electron-proton cross section on the angle between the lepton plane and some direction in the hadronic final state has a simple relation with cross sections and interference terms of the photon-proton subprocess for definite photon helicity. Its measurement can be used to constrain the cross section for longitudinally polarised photons. We compute the distribution of the azimuthal jet angle in diffractive dijet production in the Landshoff-Nachtmann model and find that useful bounds on the longitudinal cross section for such events might be obtained from its measurement. Going further we extend our calculation of quark-antiquark production to finite squared momentum transfer from the proton. This enables us to investigate the distribution of another important azimuthal angle, that of the scattered proton, which contains information about the helicity of the pomeron. The last part of this thesis is devoted to the phenomenological pomeron model of Donnachie and Landshoff. When applied to diffractive quark-antiquark production it violates electromagnetic gauge invariance. We suggest a prescription to deal with this problem and compare the results we then obtain with those of the Landshoff-Nachtmann model. We find remarkable similarities between the two models, but they also show important differences, especially in details of kinematical distributions.
41
Grasset, Fabien. "Propriétés structurales, magnétiques et catalytiques de nouveaux oxydes à base de ruthénium ou de platine à empilements mixtes dérivés de la structure perovskite." Phd thesis, Université Sciences et Technologies - Bordeaux I, 1998. http://tel.archives-ouvertes.fr/tel-00007279.
Abstract:
Lors de l'étude de la série A3A'RuO6 (membre n = infini de la famille A3n+3A'nBn+3O9+6n) (A = Ca, Sr ; A'=Li, Na), quatre nouvelles phases ont été isolées. La structure, constituée de chaînes [RuA'O6] parallèles à l'axe c, est isotype de Sr4PtO6. Une étude comparative des propriétés magnétiques et des expériences de diffraction neutronique montre que ces phases ne semblent pas supporter de description en terme de système magnétique monodimensionnel. L'investigation du système Ba-Pt-O a permis de mettre en évidence la solution solide Ba12[BaxPt3-x]Pt6O27 (O £ x £ 3) où x représente la proportion de Pt2+ occupant les sites prismatiques. La structure de base correspond au membre n = 3 de la famille A3n+3A'NBn+3O9+6n. Il a été montré par diffraction des rayons X et des électrons que la structure de ces phases est modulée incommensurable pour x £ 1 et modulé commensurable pour x > 1. L'activité catalytique de ces composés pour l'oxydation totale du CO en CO2 a été étudiée. Dans le système Ba-Ru-O, les phases Ba5Ru2O9(O2) et Ba5Ru2O10 ont été caractérisées sur le plan structurale et magnétique. L'originalité dans la phase Ba5Ru2O9(O2) est l'existence d'ions peroxydes (O2)2- dans les couches [Ba2(O2)]. Une filiation structurale avec la structure perovskite est proposée. Ba5Ru2O9(O2) et Ba5Ru2O10 correspondant au membre n = 3 de la série [A2O2](AnBn-1O3n).
42
Pancotti, Alexandre. "Determinação estrutural da superfície de óxidos ordenados por difração de fotoelétrons: o caso de CrxOy sobre Pd(111) e SrTiO3(100)." [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277013.
Abstract:
Orientadores: Richard Landers, Abner de SiervoTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
Made available in DSpace on 2018-08-14T22:49:38Z (GMT). No. of bitstreams: 1 Pancotti_Alexandre_D.pdf: 18876572 bytes, checksum: 607e084dcdd143737b311eaaf705dc73 (MD5) Previous issue date: 2009
Resumo: Este trabalho apresenta um estudo sobre a estrutura atômica da superfície de dois materiais com potencial para suportar nano partículas metálicas num catalisador modelo, o primeiro, Óxido de Cromo é um suporte usado comercialmente, e o segundo, SrTiO3(100) (STO) tem a característica interessante de induzir o crescimento de nano partículas de alguns metais com formatos definidos pelo tipo de tratamento térmico que o suporte recebeu. O Óxido de cromo estava na forma de filmes ordenados crescidos epitaxialmente sobre um cristal de Pd(111) enquanto que o SrTiO3 consistia de cristal dopado com Nb cortado segundo a face (100). Os filmes de óxido de cromo sobre Pd(111) foram crescidos "in-situ" na câmara de análises pela deposição de cromo metálico numa atmosfera de oxigênio (1,0.10-6 mBar) sobre o substrato aquecido (623K) o que produziu filmes com boa cristalinidade e estequiometria. Foram estudados filmes com duas estruturas diferentes: óxido de cromo com 3.5 Å de espessura que apresentava uma reconstrução tipo p(2x2) e um filme mais espesso com 12.0 Å que mostrava uma reconstrução (V3xV3)R30o, ambos determinados por LEED (Low Energy Electron Diffraction). A composição e a estrutura atômica foram determinadas por XPS (X-Ray Photoelectron Spectroscopy) e XPD (X-Ray Photoelectron Diffraction), respectivamente. Titanato de Estrôncio, SrTiO3 normalmente é um material isolante, mas a adição de Nb como dopante o transformou em um material suficientemente condutor para permitir o uso das mesmas técnicas para o estudo da estrutura atômica e composição de sua superfície. Nestes estudos foram utilizados como fontes de excitação radiação Síncrotron de 700 eV de energia da linha SGM do Laboratório Nacional de Luz Síncrotron e fontes convencionais de raios-X (Al, Mg) Ka. Para o modelamento teórico das estruturas superficiais empregou se o pacote MSCD [1] juntamente com um algoritmo genético [2] para acelerar a procura dos parâmetros estruturais. Determinou se que o filme mais fino de óxido de cromo correspondia a CrO com uma reconstrução p(2x2). A primeira e a segunda distâncias interplanares foram determinadas como sendo iguais a 0,16 Å e 1,92 Å, respectivamente. O filme mais grosso foi estudado em duas situações, como crescido e após tratamento térmico a 973K. Nas duas situações o filme se apresentou como a-Cr2O3(0001), mas com terminações diferentes. Antes do aquecimento do filme os resultados de XPD mostraram que a superfície é terminada por uma camada de "O" com a primeira distância interplanar expandida de 9,5% em relação ao seu valor no volume. Depois do aquecimento, a superfície é terminada em uma dupla camada formada por átomos de Cr, com a primeira distância interplanar reduzida de 68% em relação ao seu valor no volume. Os resultados para a superfície do STO mostraram regiões de SrO e TiO2. Em ambos os casos todas as camadas de cátions relaxam para dentro e os átomos da segunda camada relaxam para fora resultando na corrugação das superfícies do TiO2e SrO. Estes resultados se comparam muito bem com os resultados encontrados por LEED. Usando o algoritmo genético foi possível determinar que 30% da superfície do cristal é recoberta por ilhas de SrO(100)
Abstract: This thesis presents a study of the atomic structure of the surface of two materials with potential as supports for metallic nanoparticles in model catalysts. The first is Chromium Oxide that is used as a support for commercial catalysts, and the second is SrTiO3(100) (STO), which has the very interesting characteristic of inducing the growth of nanoparticles of some metals with different shapes depending on the type of heat treatment of the support. The Chromium Oxide used in this study was in the form of ordered films grown epitaxially on a Pd(111) crystal, while the SrTiO3 consisted of a bulk crystal doped with Nb cleaved along the (100) face. The epitaxial Chromium Oxide films were grown in situ in a surface analysis chamber by evaporating metallic Cr under 1.0x10-6 mBar of O2 pressure on to the substrate heated to 623K. The films as grown showed clear LEED (Low Energy Electron Diffraction) patterns and constant stoichiometry. Two different reconstructions were studied: p(2x2), that was present for thin films of about 3.5 Å, and (V3xV3)R30o, which is characteristic of thicker films (above 12.0 Å). The composition and detailed surface structures were determined by XPS (X-Ray Photo electron Spectroscopy) and XPD (X-Ray Photoelectron Diffraction). SrTiO3 is an insulator, but doping with Nb makes it sufficiently conducting so that it is possible to use LEED, XPS and XPD without charging problems inherent to insulating samples. Two types of radiation were used for exciting the samples: synchrotron radiation (700eV) from the SGM beam line at the Brazilian National Synchrotron Radiation Light Laboratory and conventional X-rays from Al and Mg anodes. To simulate the surface structures the MSCD package [1] was used. To accelerate the optimization of the structural parameters a genetic algorithm [2] was used in conjunction with the MSCD package. The thinner Chromium Oxide film was shown to consist of CrO with a p(2x2) reconstruction, having as first and second interlayer distances 0,16 Å and 1,92 Å respectively. The thicker Chromium Oxide film was studied as grown and after annealing at 973K. In both cases the structure was determined to be a-Cr2O3(0001), but with different surface termination. XPD revealed that the film as grown was terminated by an O monolayer, with the first interlayer distance expanded by 9.5% relative to bulk values. After annealing the film was shown to terminate in two atomic layers of Cr, who¿s interlayer distance was reduced by 68% relative to the bulk. Results for the surface of STO showed regions covered by SrO and TiO2. In both cases the cation layers relaxed inwards and the atoms of the second layer outwards, resulting in corrugated surfaces. These results compare very well with results obtained by LEED. By using the genetic algorithm [2], it was possible to show that 30% of the surface was covered by SrO(100) islands
Doutorado
Física da Matéria Condensada
Doutor em Ciências
43
Hyslop, Michael. "Electron diffraction studies of unsupported clusters." Thesis, University of Canterbury. Physics and Astronomy, 2002. http://hdl.handle.net/10092/5563.
Abstract:
The motivation to study the structure of clusters is the possible observation of unusual structures for small cluster sizes. The presence of such structural size effects is generally associated with the optimization of surface energy in clusters. For metals that have face centred cubic (FCC) structure in the bulk, optimization of the surface energy typically results in icosahedral or decahedral structures being preferred for nanometre sized clusters.
The inert gas aggregation (IGA) technique has been used to produce a beam of clusters (diameter ≤10 nm) for structural studies using electron diffraction. Studying the clusters while in a molecular beam, as opposed to on a substrate, means that the clusters are unsupported and thus free of any perturbing effects due to a substrate. The use of a beam also means each cluster is subjected to only a brief exposure to the electron beam, minimizing effects due to the electron beam. Attempts to obtain diffraction patterns from Zn clusters were unsuccessful, however using Pb it was possible to obtain diffraction patterns from clusters using a wide range of parameters in the IGA source.
The experimental diffraction patterns result from the range of different sized and structured clusters produced by the source. The analysis reflects the distribution of cluster sizes and structures by combining diffraction patterns from model clusters with a range of sizes and structures to produce a best fit to the experimental pattern. In general, two sets of model clusters are used: the first set contains models with up to ~6500 atoms, created using bulk and symmetry properties for clusters with FCC, decahedral and icosahedral structure. The second set contains the same sizes and structures as the first, however each model has been relaxed using molecular dynamics (MD) simulations. In the analysis of several experimental diffraction patterns, models with twinned FCC, liquid, anti-Mackay icosahedral and shaved icosahedral structures are also considered. Domain size estimates are obtained using the fit results; cluster size estimates are made from samples collected from the beam and observed in a TEM. Size estimates are also made using the Scherrer formula and the Fourier inversion method .
Analysis of diffraction patterns from Pb clusters shows that changing the type of inert gas produces the greatest variation in the size and structure of the clusters. The small clusters produced using He are found to be based on icosahedral structures. The clusters produced using Ar are larger than those produced using He and the diffraction patterns are difficult to interpret. The patterns bear a strong resemblance to those from decahedra, but diffraction patterns from decahedra are similar to those from twinned FCC structures, and from a combination of shaved icosahedra and FCC structures. FCC structure is not observed, which is both interesting and surprising.
44
Papathomas, Paul Michael. "Developments in gas-phase electron diffraction." Thesis, University of Edinburgh, 1998. http://hdl.handle.net/1842/12760.
Abstract:
The first molecular scattering observed using the new Edinburgh electron diffraction apparatus is detailed in this thesis. The new apparatus utilises a phosphor screen/CCD detection system rather than the photographic plates more commonly used in electron diffraction studies. The electron beam is provided by a telefocus electron gun. Two molecular target sources have been investigated: a Campargue-type molecular beam and an effusive needle source. Calibration of the apparatus has been attempted using argon gas. Carbon tetrafluoride, CF4, has been used as a typical gas-phase molecule and its scattering investigated extensively, while preliminary results for a more complex molecule, 1,2,4,5-tetrafluorobenzene, are also reported. Finally, the structure refinement of di-t-butyl(trichlorosilyl)phosphane using data from the existing electron diffraction apparatus is reported.
45
Fender, Robert Scott. "Advances in gas-phase electron diffraction." Thesis, University of Edinburgh, 1996. http://hdl.handle.net/1842/14833.
Abstract:
A new gas-phase electron diffraction apparatus is reported in this thesis. The machine complements the existing electron diffraction set-up at Edinburgh University. The new apparatus utilises an electron counting device consisting of a pair of stacked microchannel plates and a novel, position-sensitive anode counter rather than the photographic plate-rotating sector method more commonly used in electron diffraction studies. The work carried out with this detector is discussed together with results from a simulation program designed to evaluate the operational capabilities of the device under a range of experimental conditions. The molecular target source was provided by a Campargue-type molecular beam and the electron beam was produced by a telefocus electron gun. Both of these beams have been fully characterised and the results are presented in this work. A short review is given of the current developments in gas-phase electron diffraction. Finally, the structural refinements of two molecules studied using the photographic method are reported. These are 1,2-di-tert-butyldisilane and 1,2-dicarbapentaborane(7).
46
Xu, Junliang. "Ultrafast imaging: laser induced electron diffraction." Diss., Kansas State University, 2012. http://hdl.handle.net/2097/13616.
Abstract:
Doctor of PhilosophyDepartment of Physics
Chii-Dong Lin
Imaging of molecules has always occupied an essential role in physical, chemical and biological sciences. X-ray and electron diffraction methods routinely achieve sub-angstrom spatial resolutions but are limited to probing dynamical timescales longer than a picosecond. With the advent of femtosecond intense lasers, a new imaging paradigm emerges in last decade based on laser-induced electron diffraction (LIED). It has been placed on a firm foundation by the quantitative rescattering theory, which established that large-angle e-ion elastic differential cross sections (DCS) can be retrieved from the LIED spectrum. We further demonstrate that atomic potentials can be accurately retrieved from those extracted DCSs at energies from a few to several tens of electron volts. Extending to molecules, we show mid-infrared (mid-IR) lasers are crucial to generate high-energy electron wavepackets (> 100 eV) to resolve the atomic positions in a molecule. These laser-driven 100 eV electrons can incur core-penetrating collisions where the momentum transfer is comparable to those attained in conventional keV electron diffraction. Thus a simple independent atom model (IAM), which has been widely used in conventional electron diffractions, may apply for LIED. We theoretically examine and validate the applicability of IAM for electron energies above 100 eV using e-molecule large-angle collision data obtained in conventional experiments, demonstrating its resolving powers for bond lengths about 0.05 angstrom. The Validity of IAM is also checked by an experimental LIED investigation of rare gas atoms in the mid-IR regime. We show that the electron’s high energy promotes core-penetrating collisions at large scattering angles, where the e-atom interaction is dominated by the strong short range atomic-like potential. Finally, we analyze the measured LIED spectrum of N[subscript]2 and O[subscript]2 at three mid-IR wavelengths (1.7, 2.0, and 2.3 μm). As expected, the retrieved bond lengths of N[subscript]2 at three wavelengths are about same as the equilibrium N[subscript]2 bond length. For O[subscript]2, the data is also consistent with a bond length contraction of 0.1 angstrom within 4-6 fs after tunnel ionization. This investigation establishes a foundation for this novel imaging method for spatiotemporal imaging of gas-phase molecules at the atomic scale.
47
McMorran, Benjamin James. "Electron Diffraction and Interferometry Using Nanostructures." Diss., The University of Arizona, 2009. http://hdl.handle.net/10150/194029.
Abstract:
Here it is demonstrated that nanofabricated structures can be used as electron optical elements in new types of electron interferometers. This enables novel investigations with electrons analogous to experiments in light and atom optics. Far field diffraction from a single nanograting is used to examine the force on a charge moving in close proximity to a surface. Near field diffraction from the nanograting is investigated in a Talbot interferometer. It is found that electron waves form replicas of the grating in free space, and these replicas can be de-magnified using illumination by a converging beam. An electron Lau interferometer has the same grating configuration as the Talbot interferometer, but uses spatially incoherent beams that give rise to drastically different interference behavior. A single optical theory is developed to efficiently model a variety of grating interferometers under a diverse set of illumination conditions, and it is used to understand the experiments described here. Applications for these new interferometers are discussed, as well as possible directions for future research.
48
Bernu, Sylvain. "Etude structurale sous pression de BaVS3, un composé quasi-1d à électrons corrélés, et effets des substitutions chimiques." Phd thesis, Université Paris Sud - Paris XI, 2011. http://tel.archives-ouvertes.fr/tel-00647329.
Abstract:
BaVS3 est un système quasi-unidimensionnel constitué de chaînes de vanadium. C'est aussi un système multi-bandes dans lequel plusieurs types d'électrons coexistent au voisinage du niveau de Fermi (les bandes dz2 et e(t2g)), qui sont fortement corrélés. Ce système s'appuie donc à la fois sur la physiques des systèmes unidimensionnels et sur celle des systèmes à électrons fortement corrélés. La transition métal-isolant de BaVS3, qui fut l'objet de nombreuses études, a récemment été interprété comme s'apparentant à une onde de densité de charge des électrons dz2 : celle-ci s'accompagne d'une quadrimérisation de la chaîne de vanadium et de l'apparition d'une sur-structure indexée (1,0,1/2)O dans le réseau réciproque. La composante commensurable de la sur-structure selon l'axe des chaînes ne peut s'expliquer qu'en considérant une redistribution des électrons entre les différentes bandes au voisinage du niveau de Fermi sous l'effet des corrélations électroniques. La transition métal-isolant disparaît en un point critique quantique sous pression à 2 GPa. D'un autre point de vu, une autre instabilité isolante d'un autre type, indexée (1/3,1/3,0.8)H avait été observée dans des échantillons substitués chimiquement. Cette thèse en deux temps s'est d'abord intéressée à la compétition entre les deux phases (1,0,1/2)O et (1/3,1/3,0.8)H dans deux séries d'échantillons chimiquement modifié BaVS3-δ et Ba1-xSrxVS3. Cette étude permet de montrer que la phase (1/3,1/3,0.8)H est associée à un ordre de charge des électrons e(t2g) et de conclure sur le rôle des modifications chimiques en temps qu'impuretés quand au mécanisme d'apparition de la seconde phase, par opposition à une théorie de "pression chimique". Le deuxième temps de cette thèse présente l'étude structurale de la transition métal-isolant sous pression. Cette étude à nécessité la mise en place d'un dispositif de diffraction sous pression à basse température avec mesure in-situ de la pression. Cette étude a montrée que la modulation structurale reste commensurable sur les 3/4 du diagramme de phase puis devient incommensurable lorsque l'on s'approche du point critique quantique. Ceci permet d'interpréter l'évolution de la transition métal-isolant sous pression en terme de locking à la commensurabilité, et donne une information sur la physique du composé au voisinage du point critique quantique.
49
Hayes, Stuart A. "Development of experimental gas electron diffraction technique." Thesis, University of Edinburgh, 2008. http://hdl.handle.net/1842/2581.
Abstract:
A state-of-the-art gas electron diffraction (GED) apparatus has been reassembled in the school of chemistry at the University of Edinburgh. This combines molecularbeam and telefocus-electron-gun technologies and the alignment of the electron beam produced by the latter has been discussed. A new custom-made CCD detector has also been installed and electron diffraction patterns for a few small molecules have been recorded. In analogy to the rotating sector in a conventional GED apparatus, the new camera contains an optical filter and a procedure for its calibration is outlined and followed step by step to produce an estimate of the filter transmittance. The data have been shown to be of less than ideal quality and the probable root of the problem is discussed. GED refinements of two pairs of compounds (arachno-6,9-decaboranes, and a covalent sulfonate and thiosulfonate) are presented, using data collected with the conventional Edinburgh GED apparatus.
50
Haydon, S. K. "Structure determination using electron diffraction and microscopy." Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.603876.
Abstract:
This thesis presents a program of work initiated to implement, assess and test a difference potential methodology of structure refinement using quantitative electron diffraction. The results of a parallel synthetic and structural study of the pentagonal tunnel (PT) structure, tetragonal lead tungsten bronze (lead-TTB), and the unexplored quaternary Pb-Nb-W-O system are also presented. Quaternary Pb-Nb-W-O phases are synthesised via fully oxidised and reduced reaction mechanisms. The phases observed in both experiment types are similar and based on superstructures of basic TTB, formed by the ordering of pentagonal columns (PCs) within the host framework. The formation of a fully oxidised quaternary analogue of lead-TTB is discussed. Reduced phase preparations prove it is possible to incorporate lead into PC structures whilst maintaining the parent framework PC distribution. In other experiments an alternative PT/PC ordering scheme is found. A model for this phase and its hidden supercell is presented and discussed. A new TTB related phase is also observed possessing a tetragonal cell repeat five times the (210)TTB plane spacing. A possible model for this phase is proposed. Methods of extracting experimental intensities for the difference potential calculation from an exposure series of diffraction patterns are discussed. A software package (EDIM) for performing the intensity extraction automatically is presented. Tests performed to investigate the characteristics of the difference potential are described, and the results of a limited refinement of lead-TTB using EDIM generated experimental electron diffracted intensities are presented. These calculations confirm that the electron difference potential is a viable means of determining light atom positions in structures containing heavy atom components.