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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Ground State Properties and Applications of Dipolar Ultracold Gases

Dutta, Omjyoti January 2008 (has links)
This thesis contains a study of ultracold paramagnetic atoms or polar molecules characterized by a long-range anisotropic dipolar interaction. We particularly focus on two aspects of ultracold dipolar gases. In the first problem the ground state properties of dipolar Bose-Einstein condensates (BEC) are investigated. This problem has gained importance due to recent experimental advances in achieving a condensate of Chromium atoms and ongoing research to produce quantum degenerate polar molecules. In the second problem, we consider possible applications of ultracold polar molecules to rotation sensing and interferometry. First, we concentrate on the interplay between the trapping geometry and dipole-dipole interaction for a polarized dipolar bosonic condensate. As the dipole-dipole interaction is attractive along the polarized direction, the lowest energy state of the BEC is always a collapsed state. However by applying a trapping potential along the polarization direction it is possible to achieve a metastable dipolar BEC. By numerically solving the Gross-Pitaevskii equation, we show that below a critical interaction strength, a metastable state exists depending on the trapping geometry. We also show that a novel feature of dipolar BEC is the appearance of different structural metastable ground states for certain combinations of trapping geometry and particle number. Next, by mixing in single component fermions we show that dipolar BEC can be stabilized against collapse in pancake shaped or cylindrical traps. We also show that the excitation spectrum of the BEC may have a minimum for non-zero momentum, termed a “roton minimum”. This minimum leads to a transition to stable or metastable density-wave states depending on the density of the bosons and boson-fermion interaction strength. In the second problem, we study a proposal for a large-angle coherent beam splitter for polar molecules. By taking into account the effect of a quasi-static external electric field on the rotational levels of the polarized molecules we show that it is possible to coherently split a stationary cloud of molecules into two counter-propagating components. We then investigate the effect of longitudinal acceleration on the transverse motion of the particles, assuming that the longitudinal motion of the molecules can be approximated classically by a wave packet with some mean velocity while the transverse motion is governed by quantum mechanics. We propose a particular time-dependent shape of acceleration to minimize the excitations in the transverse motion. Our theory is also applicable to the general case of particles moving along a circular guide with time-dependent longitudinal velocity. In addition, we include the effects of velocity fluctuations due to noise in the accelerating field.

Measurement of the ttbar Production Cross Section using the Semileptonic Final States in pp Collisions at √s=7 TeV with 35.3 pb⁻¹ of ATLAS Data

Lei, Xiaowen January 2011 (has links)
A measurement of the production cross section for top quark pairs (tt) in pp collisions at √s = 7 TeV is presented using data recorded with the ATLAS experiment at the Large Hadron Collider in 2010. Events are selected in the tt semileptonic decay channel by requiring a single lepton (electron e or muon μ), large missing transverse energy and at least four jets. In a data sample of 35:3 pb⁻¹, 396 e + jets events and 653 μ + jets events are observed. A multivariate top likelihood is built from four variables including a b-tagging variable. A fit to the top likelihood distribution is employed to extract the tt cross section separately in the e+ jets and μ + jets channel. A combined fit to the top likelihood distribution in both channels is employed to extract a combined cross section. The tt cross section, assuming a top mass of 172.5 GeV, is found to be: e + jets channel: σ(u) = 164.4^+16.2(-15.4)(stat)^31.2(-36.2)(syst)^5.8(-5.4)(lumi) pb; μ + jets channel: σ(u) = 167.8^+1.3.9(-13.4)(stat)^3.9(-34.8)(syst)^5.9(-5.5)(lumi) pb; e + jets and μ + jets channel combined: σ(u) = 166.4^+10.5(-10.2)(stat)^31.7(-33.6)(syst)^5.9(-5.5)(lumi) pb The measurement agrees with approximate NNLO perturbative QCD calculations.

Structure and Decay in the QED Vacuum

Labun, Lance January 2011 (has links)
This thesis is a guide to a selection of the author's published work that connect and contribute to understanding the vacuum of quantum electrodynamics in strong, prescribed electromagnetic fields. This theme is elaborated over the course of two chapters: The first chapter sets the context, defining the relevant objects and conditions of the study and reviewing established knowledge upon which this study builds. The second chapter organizes and explains important results appearing in the published work. The papers 1. (Labun and Rafelski, 2009) "Vacuum Decay Time in Strong External Fields" 2. (Labun and Rafelski, 2010a) "Dark Energy Simulacrum in Nonlinear Electrodynamics" 3. (Labun and Rafelski, 2010b) "QED Energy-Momentum Trace as a Force in Astrophysics" 4. (Labun and Rafelski, 2010c) "Strong Field Physics: Probing Critical Acceleration and Inertia with Laser Pulses and Quark-Gluon Plasma" 5. (Labun and Rafelski, 2010d) "Vacuum Structure and Dark Energy" 6. (Labun and Rafelski, 2011) "Spectra of Particles from Laser-Induced Vacuum Decay" are presented in their published format as appendices. Related literature is cited throughout the body where it directly supports the content of this overview; more extensive references are found within the attached papers. This study begins with the first non-perturbative result in quantum electrodynamics, a result obtained by Heisenberg and Euler (1936) for the energy of a zero-particle state in a prescribed, long-wavelength electromagnetic field. The resulting Euler-Heisenberg effective potential generates a nonlinear theory of electromagnetism and exhibits the ability of the electrical fields to decay into electronpositron pairs. Context for phenomena arising from the Euler-Heisenberg effective potential is established by considering the energy-momentum tensor of a general nonlinear electromagnetic theory. The mass of a field configuration is defined, and I discuss two of its consequences pertinent to efforts to observe vacuum decay. I develop a method for non-perturbative evaluation of a trace component of the energymomentum tensor and discuss its significance and consequences. I study the effect of the energy-momentum trace as part of a Euler-Heisenberg-generated modification to the Lorentz force. Modifications of the energy-momentum tensor from the Maxwell theory are evaluated numerically and compared to those arising from Born-Infeld electromagnetism and the Euler-Heisenberg effective potential for a scalar electron. Finally, I explore how this study guides investigation into how vacuum structure can generate the cosmological dark energy.

Extensions to the No-Core Shell Model: Importance-Truncation, Regulators and Reactions

Kruse, Michael Karl Gerhard January 2012 (has links)
The No-Core Shell Model (NCSM) is a first-principles nuclear structure technique, with which one can calculate the observable properties of light nuclei A ≤ 20. It is considered ab-initio as the only input to the calculation is the nuclear Hamiltonian, which contains realistic two or three-nucleon (NN or NNN) interactions. Provided the calculation is performed in a large enough basis space, the ground-state energy will converge. For A ≤ 4 convergence has been demonstrated explicitly. The NCSM calculations are computationally very expensive for A ≥ 6, since the required basis size for convergence often approaches on the order of a billion many-body basis states. In this thesis we present three extensions to the NCSM that allow us to perform larger calculations, specifically for the p-shell nuclei. The Importance-truncated NCSM, IT-NCSM, formulated on arguments of multi-configurational perturbation theory, selects a small set of basis states from the initially large basis space, in which the Hamiltonian is now diagonalized. Previous IT-NCSM calculations have proven reliable, however, there has been no thorough investigation of the inherent error in the truncated IT-NCSM calculations. We provide a detailed study of IT-NCSM calculations and compare them to full NCSM calculations in an attempt to judge the accuracy of IT-NCSM in heavier nuclei. Even when IT-NCSM calculations are performed, one often needs to extrapolate the ground-state energy from the finite basis (or model) spaces to the infinite model space. Such a procedure is common-place but does not necessarily have the ultraviolet (UV) or infrared (IR) physics under control. We present a potentially promising method that maps the NCSM parameters into an effective-field theory framework, in which the UV and IR physics is treated appropriately. The NCSM is well suited to describing bound-state properties of nuclei, but is not well adapted to describe loosely bound systems, such as the exotic nuclei near the neutron drip line. With the inclusion of the resonating group method (RGM), the NCSM/RGM can provide a first-principles description of exotic nuclei. The NCSM/RGM is also the first extension of the NCSM that can describe dynamic processes such as nuclear reactions.

Transverse Energy Flow in the Underlying Event in the Proton-Proton Collisions at √s = 7 TeV with the ATLAS Calorimeter

Paleari, Chiara January 2012 (has links)
The European Organization for Nuclear Research (CERN) operates the world's largest and highest energy proton-proton collider at a center of mass energy of √s = 7 TeV, the Large Hadron Collider (LHC). ATLAS is one of the four detectors operating at the LHC. The Underlying Event (UE), which is an unavoidable background at any hadron collider, includes particles from various sources generated in each proton collision. The particle flow in the underlying event is characterized by low transverse energies reflecting the long range character of the individual processes generating them. This regime cannot be described by the usual perturbative models provided by Quantum Chromodynamics (QCD), the theory of the strong force. To model this flow, phenomenological models have to be applied, as provided by Monte Carlo simulations. In this work I define new discriminating variables to constrain these UE models in the new kinematical regime available at LHC. Using calorimeter data from the ATLAS experiment, several Monte Carlo models are tested by comparing the data to these predictions for three different final states (minimum bias, di-jet and direct photon production). The experimental data are fully unfolded to the hadron level within the full acceptance of the ATLAS detector, thus for the first time including the forward direction in hadron collisions. The final results are presented in the context of previous measurements on the characteristics of the strong force in the proton, in deep inelastic lepton-proton scattering.

Applications of Effective Field Theory at the intersection of Nuclear and Particle Physics

de Azevedo, Regina Staropoli January 2012 (has links)
We apply effective field theory (EFT) techniques to solve problems at the intersection of nuclear and particle physics. In particular, we study three problems that span different energy scales. First, we probe scattering processes, like ππ scattering and e⁺e⁻ annihilation into pions, around the rho mass, m(p) ≃ 770 MeV, which is comparable to the characteristic QCD scale, M(QCD) ∼ 1 GeV. Second, we analyze the nucleon-nucleon potential, especially the renormalization of its binding energy, whose momentum scale, κ ∼ 45 MeV, is much smaller than the pion mass, m(π) ≃ 140 MeV, which in turn is much smaller than M(QCD). Finally, we address a decay problem of a bottomonium into charmed mesons covering two different scales larger than M(QCD), the bottom mass m(b) ∼ 4.5 GeV and the charm mass m(c) ∼ 1.5 GeV V. At energies comparable to M(QCD), rho mesons with masses parametrically comparable to the pion masses are incorporated in a controlled low-energy EFT of QCD, with the assumption that the latter has a dynamical "vector symmetry". We enlarge the "Vector EFT" of pions and rhos to include electromagnetic gauge invariance, and the connections to vector-meson-dominance models are discussed. We develop a power counting for reactions at energies near the rho mass, which requires selective resummations of the momentum expansion. We calculate the amplitudes for ππ → ππ and e⁺e⁻ → π⁺π⁻ in leading order and compare them with data. At lower energies, we use an EFT with momentum of the order of the mass of the pion, Chiral Perturbation Theory (ChPT), to develop a framework to study the renormalization of the two-nucleon problem. To eliminate the 1/r³ singularity in the isovector tensor force from the one-pion-exchange potential in leading order (LO), we add auxiliary-field potentials. The latter could possibly substitute the various counterterms necessary to renormalize the amplitude arising from the one-pion-exchange potential. By treating the short-range interaction of the nucleon-nucleon potential as the dominant piece, we provide an exact solution of the Schrödinger equation and find the binding energy of the deuteron, B, in a first approximation. The long-range interaction is treated in perturbation theory in order to calculate the first correction to B. At higher energies, we take advantage of the separation between the scales 2 m(b) ≫ m(c) ≫ M(QCD) and use a sequence of EFTs to develop a framework to study the exclusive two-body decays of bottomonium into two charmed mesons and apply it to study the decays of the C-even bottomonia. We prove that, at LO in the EFT power counting, the decay rate factorizes into the convolution of two perturbative matching coefficients and three non-perturbative matrix elements, one for each hadron. We calculate the relations between the decay rate and non-perturbative bottomonium and D-meson matrix elements at LO, with next-to-leading-log resummation. The phenomenological implications of these relations are discussed.

Hybrid Atomic-Optomechanical Systems -- Observing Quantum Effects in Macroscopic Oscillators

Singh, Swati January 2012 (has links)
This thesis concentrates on generating and measuring non-classical states of mechanical oscillators by coupling them to atomic and molecular quantum systems. We start with a discussion of what novel physics can be explored by mechanical systems operating in the quantum regime. We then discuss one technique making it a possibility- cavity optomechanics, particularly optomechanical cooling. We investigate the limits of optomechanical cooling and review how the coupling of mechanical oscillators to external heat baths limits the minimum attainable phonon occupation number. As a possible alternative for circumventing clamping losses, we consider an all-optical approach where the mechanical element (in this case, a Bragg mirror) is suspended via optical forces, and discuss some limitations of this approach. We explore several schemes aimed at the generation of quantum states in mechanical oscillators. We consider specifically two examples: one in which the mechanical oscillator is coupled to polar molecules via dipole-dipole interaction, and another where it is magnetically coupled to a Bose condensate. The first example emphasizes that such an interaction can generate parametric squeezing and entanglement. The second scheme demonstrates that the back action of BEC spin measurements can be used to generate quantum states of motion of a mechanical oscillator. We then discuss possible methods for measuring the entire density matrix of a mechanical oscillator. The first method achieves the tomographic reconstruction of the mechanical Wigner function by coupling it simultaneously to a classical optical oscillator and a qubit. The second approach involves a state transfer scheme between momentum excitations of a bose-condensate in a cavity and a moving mirror of the cavity that is entirely mediated by the light field. We conclude with a discussion of the broader implications of this work, and some future research directions.


Ahmed, Hassan Abdelhalim Abdallah Seed 17 May 2013 (has links)
Amorphous silica powders doped with lanthanide ions were synthesised by the sol-gel method and their cathodoluminescence (CL) and photoluminescence (PL) emissions were compared. Interesting differences depending on the type of excitation were observed for Tb and Ce-doped samples. For Tb-doped samples blue 5D3®7FJ emission was measured during CL in samples for which PL results showed this emission to be concentration quenched due to cross-relaxation, while for Ce-doped samples luminescence occurred for CL but not during PL measurements. Unlike the other lanthanides, Tb and Ce ions are sometimes found in the tetravalent rather than the trivalent state, and these differences were attributed to the possibility of electron capture of tetravalent ions possible during CL but not PL. A scheme for the energy levels of divalent and trivalent lanthanide ions relative to the conduction and valence bands in silica was proposed, making use of experimental data and the known relative positions of the energy levels for the lanthanides. Although the location of the divalent europium ion f-level above the valence band can be located by using the charge transfer energy of trivalent europium, this process cannot be generalized to find the location of the trivalent cerium ion f-level above the valence band using the charge transfer energy of tetravalent cerium as has been suggested. Initial investigations of the luminescence properties of Ce doped silica were complicated by overlapping luminescence from oxygen deficiency defects from the host itself and the fact that Ce took the tetravalent state which is nonluminescent for PL measurements. Spectra obtained using a wide variety of excitation methods, including synchrotron radiation, were compared and evaluated in the light of previously published data. Radically improved results were obtained by annealing in a reducing atmosphere instead of air. X-ray photoelectron spectroscopy as well as ultraviolet reflectance spectroscopy provided evidence of the conversion of Ce from the tetravalent to trivalent state and this was accompanied by strong luminescence of these sample during PL measurements. Ce,Tb co-doped silica was used to study the energy transfer from Ce to Tb ions. Initial results were disappointing when measurements showed that adding Ce quenched the Tb emission intensity instead of increasing it. However, after annealing the samples in a reducing atmosphere, a quantum efficiency of 97% for energy transfer from Ce to Tb was achieved. The mechanism for energy transfer was investigated by comparing experimental measurements of the changes in donor (Ce) emission intensity and lifetime as a function of the amount of acceptor (Tb) with numerical simulations of various models. Measurements correlated well to models for dipole-dipole and exchange interactions, but the critical transfer distance obtained was not appropriate for the exchange interaction, hence dipole-dipole interaction was identified as the interaction mechanism. Nanocrystalline LaF3 powders were synthesized by the hydrothermal method and strong luminescence was obtained from samples doped with Ce and Tb. Photoluminescence spectra from co-doped samples revealed that the emission from Ce was quenched and the emission from Tb was enhanced, showing that energy transfer from Ce to Tb occurred. The energy transfer mechanism was investigated in a similar way as for the silica samples, but in this case the experimental results fitted models for the quadrupolequadrupole and exchange interactions. Much higher concentrations of Tb were required to significantly affect the Ce luminescence properties than in the case for silica, and the critical transfer distance obtained was appropriate for the exchange interaction. Either or both of these interaction mechanisms are therefore possible. The results show that the interaction mechanism for energy transfer between lanthanide ions depends not only on the ions themselves, but also on the lattice hosting them.


van Soelen, Brian 27 May 2013 (has links)
The high-mass binary system PSR B1259-63/LS 2883 is one of only six known gamma-ray binaries, and the only one where the compact object is known from the direct observation of a pulsed radio signal to be a 48 ms pulsar. During itâs eccentric 3.4 year orbit, the pulsar moves through the circumstellar disc of the optical companion, approximately twenty days before and after periastron. This results in conditions for complicated interactions between the material in the disc, the fast rotating pulsar, the pulsar wind, and the radiation field from the star and disc. The system has been the object of multi-wavelength campaigns with telescopes such as the VLT, H.E.S.S. and Fermi. The interaction between the stellar and pulsar wind results in the formation of a radiating pulsar wind nebula within the binary system, which has been detected from radio to TeV gamma-ray energies. The spectral energy distribution is dominated by the emission at gamma-ray energies, classifying this system as a gammaray binary. The interaction between the stars is greater near periastron where the pulsar passes closest to the optical companion. Approximately twenty days from periastron the pulsar passes through or behind the Be starâs circumstellar disc, obscuring the pulsed radio signal. During this period there is a corresponding increase in the unpulsed emission from the system. The TeV gamma-rays are believed to be produced by electrons in the pulsar wind which cool via the inverse Compton up-scattering of stellar photons from the optical companion. The circumstellar disc associated with the Be star produces an infrared flux below ! 1015 Hz, which is greater than that expected from the blackbody distribution associated with star, providing additional target photons which could increase the inverse Compton scattering rate. The scattering of infrared photons can occur in the Thomson limit with its significantly larger cross-section and should produce GeV energy gamma-rays in the energy range observed by the Fermi telescope. A curve of growth method is presented to model the infrared free-free and free-bound emission from the circumstellar disc, taking into account the changing viewing angle as observed from the pulsar. The curve of growth model is fitted to archive near-infrared and optical data and mid-infrared data obtained with the Very Large Telescope during January 2011. The effect of this infrared excess on the inverse Compton scattering rate is considered for an isotropic and anisotropic photon distribution, considering preâ and postâshock electron distributions. The anisotropic modelling considers the effects of the changing size and orientation of the circumstellar disc relative to the pulsar, as well as the change in the inverse Compton scattering angle during the orbit. The inverse Compton scattering rate for three disc orientations is modelled over a period of approximately 160 days around periastron, including the disc crossing epochs before and after periastron. The maximum disc contribution is found to occur close to periastron and not near the discâcrossing where the low infrared flux from the disc, at a radius of ! 45 stellar radii, has a less significant effect. It is found that the inclusion of the infrared flux from the circumstellar disc can increase the GeV flux from the system by a factor ! 2 near periastron, for favourable disc orientations. The predicted increase is, however, less than was detected with Fermi during the 2011 periastron passage. The observations showed a flare which cannot be explained by this, or any current model.


Barnard, Pieter Egbert 27 May 2013 (has links)
A systematic study was conducted to investigate the diffusion and segregation of S in bcc Fe using (i) DFT modelling and (ii) the experimental techniques Auger Electron Spectroscopy (AES) and XRay diffraction (XRD). The aim of this study was to obtain the activation energies for the segregation of sulfur (S) in bcc iron (Fe), both computationally and experimentally in order to explain the diffusion mechanism of S in bcc Fe as well as the influence the surface orientation has on surface segregation. The Quantum ESPRESSO code which performs plane wave pseudopotential Density Functional Theory (DFT) calculations was used to conduct a theoretical study on the segregation of S in bcc Fe. To determine the equilibrium lattice sites of S in bcc Fe, the tetrahedral-interstitial, octahedralinterstitial and substitutional lattice sites were considered. Their respective binding energies were calculated as -1.464 eV, -1.660 eV and -3.605 eV, indicating that the most stable lattice site for S in bcc Fe is the substitutional lattice site. The following mechanisms were considered for the diffusion of S in bcc Fe: tetrahedral-interstitial, octahedral-interstitial, nearest neighbour (nn) substitutional and next nearest neighbour (nnn) substitutional with migration energies, Em, of respectively 4.438 kJ/mol (0.046 eV), 22.48 kJ/mol (0.233 eV), 9.938±6.754 kJ/mol (0.103±0.007 eV) and 96.49±0.579 kJ/mol (1.000±0.006 eV). According to the binding and migration energy calculations, S will diffuse via a substitutional mechanism with a migration energy of 9.938±6.754 kJ/mol (0.103±0.007 eV). The three low-index planes of bcc Fe were investigated to determine the stability, the vacancy formation energy and the activation energy for each surface. Structural relaxation calculations showed that the surfaces in order of decreasing stability are: Fe(110)>Fe(100)>Fe(111) which is in agreement with surface energy calculations obtained from literature. The formation of a vacancy in bcc Fe was modelled as the formation of a Schottky defect in the lattice. Using this mechanism, the vacancy formation energies, Evac, for the Fe(110), Fe(100) and Fe(111) surfaces were respectively calculated as 267.4 kJ/mol (2.772 eV), 256.8 kJ/mol (2.662 eV) and 178.2 kJ/mol (1.847 eV). The activation energy, Q, of S diffusing via the substitutional mechanism for the Fe(100), Fe(110) and Fe(111) surfaces were respectively calculated as 277.4 kJ/mol (2.875 eV), 266.8 kJ/mol (2.765 eV) and 188.1 kJ/mol (1.950 eV). Thus it was found that the vacancy formation energy is dependent on the surface orientation and thus the structural stability of the Fe crystal. Experimental values for the activation energy of S in bcc Fe (232 kJ/mol (2.40 eV) and 205 kJ/mol (2.13 eV)) were obtained from literature confirming the nearest neighbour substitutional diffusion mechanism of S in bcc Fe. No indication is given regarding the orientation of the crystal in which the value of 232 kJ/mol (2.40 eV) was obtained while the value of 205 kJ/mol (2.13 eV) is for a Fe(111) crystal orientation. For the experimental investigation of the Fe/S system polycrystalline bcc Fe samples were studied. These samples were prepared by a new doping method by which elemental S is diffused into Fe. In order to prepare the samples by this method a new system was designed and build. Auger depth profile analysis confirms the successful doping of Fe with S using the newly proposed doping method. It was found that the S concentration was increased by 89.38 % when the doping time was doubled from 25 s to 50 s. An Fe sample doped for 50 s was annealed at 1073 K for 40 days after which the effects induced by S and the annealing of the sample were investigated by Secondary Electron Detector (SED) imaging. Results showed a 36±11 % decrease in the grain sizes of the polycrystalline Fe sample due to the presence of S. It was found that the re-crystallization rate of Fe is increased due to the presence of S. Using XRD, the Fe (100), Fe(211), Fe(110), Fe(310) and Fe(111) orientations were detected for both the un-doped and the annealed S doped Fe samples. The annealed sample showed the following percentage changes in the concentrations of the respective orientations compared to the un-doped sample: -5.180, +2.030, +16.41, +0.400, -13.66. Taking the calculated trend in surface stability for the three low-index orientations of Fe into consideration, it was found that the more stable Fe(110) orientation had increased in concentration during annealing, while the less stable Fe(100) and unstable Fe(111) orientations had decreased in concentration during annealing. AES measurements on the two samples were performed using the linear programmed heating method. The segregation parameters of S for the un-doped Fe sample are: D0=4.90Ã10-2 m2/s, Q=190.8 kJ/mol (1.978 eV), ÎG=-134 kJ/mol (-1.39 eV) and ΩFe/S=20 kJ/mol (0.21 eV). The segregation parameters of P obtained for the un-doped Fe sample are: D0=0.129 m2/s, Q=226.5 kJ/mol (2.348 eV). For the S doped Fe sample the segregation parameters of S were determined as: D0=1.79Ã10-2 m2/s and Q=228.7 kJ/mol (2.370 eV), ÎG=-145 kJ/mol (-1.50 eV) and ΩFe/S=8 kJ/mol (0.08 eV). These results showed that for the doped sample, with an increased concentration in the stable Fe(110) and a decreased concentration in the less stable Fe(100) and unstable Fe(111) orientations, a higher activation energy was obtained. Comparing the measured activation energies to the calculated values indicates that the diffusion of S occurs via a vacancy mechanism, where the S atom occupies a substitutional lattice site. Despite the fact that polycrystalline samples were analysed, the activation energies are still in the same order as the calculated activation energies of the single crystals. This confirms the theoretical prediction of a substitutional diffusion mechanism of S in bcc Fe. During this study the diffusion mechanism of S was determined as the substitutional diffusion mechanism whereby a S atom would diffuse from a substitutional lattice site to a nearest neighbour vacancy. The different Fe orientations considered in the calculations can be arranged from highest to lowest activation energy as Fe(110)>Fe(100)>Fe(111). These calculations are in agreement with the AES results which showed an increased activation energy for the doped sample having a higher Fe(110) concentration and lower Fe(111) and Fe(100) concentrations

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