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41 
Ground State Properties and Applications of Dipolar Ultracold GasesDutta, Omjyoti January 2008 (has links)
This thesis contains a study of ultracold paramagnetic atoms or polar molecules characterized by a longrange anisotropic dipolar interaction. We particularly focus on two aspects of ultracold dipolar gases. In the first problem the ground state properties of dipolar BoseEinstein 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 dipoledipole interaction for a polarized dipolar bosonic condensate. As the dipoledipole 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 GrossPitaevskii 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 nonzero momentum, termed a “roton minimum”. This minimum leads to a transition to stable or metastable densitywave states depending on the density of the bosons and bosonfermion interaction strength. In the second problem, we study a proposal for a largeangle coherent beam splitter for polar molecules. By taking into account the effect of a quasistatic 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 counterpropagating 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 timedependent 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 timedependent longitudinal velocity. In addition, we include the effects of velocity fluctuations due to noise in the accelerating field.

42 
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 DataLei, 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 btagging 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.

43 
Structure and Decay in the QED VacuumLabun, 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 EnergyMomentum Trace as a Force in Astrophysics" 4. (Labun and Rafelski, 2010c) "Strong Field Physics: Probing Critical Acceleration and Inertia with Laser Pulses and QuarkGluon Plasma" 5. (Labun and Rafelski, 2010d) "Vacuum Structure and Dark Energy" 6. (Labun and Rafelski, 2011) "Spectra of Particles from LaserInduced 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 nonperturbative result in quantum electrodynamics, a result obtained by Heisenberg and Euler (1936) for the energy of a zeroparticle state in a prescribed, longwavelength electromagnetic field. The resulting EulerHeisenberg 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 EulerHeisenberg effective potential is established by considering the energymomentum 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 nonperturbative evaluation of a trace component of the energymomentum tensor and discuss its significance and consequences. I study the effect of the energymomentum trace as part of a EulerHeisenberggenerated modification to the Lorentz force. Modifications of the energymomentum tensor from the Maxwell theory are evaluated numerically and compared to those arising from BornInfeld electromagnetism and the EulerHeisenberg effective potential for a scalar electron. Finally, I explore how this study guides investigation into how vacuum structure can generate the cosmological dark energy.

44 
Extensions to the NoCore Shell Model: ImportanceTruncation, Regulators and ReactionsKruse, Michael Karl Gerhard January 2012 (has links)
The NoCore Shell Model (NCSM) is a firstprinciples nuclear structure technique, with which one can calculate the observable properties of light nuclei A ≤ 20. It is considered abinitio as the only input to the calculation is the nuclear Hamiltonian, which contains realistic two or threenucleon (NN or NNN) interactions. Provided the calculation is performed in a large enough basis space, the groundstate 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 manybody basis states. In this thesis we present three extensions to the NCSM that allow us to perform larger calculations, specifically for the pshell nuclei. The Importancetruncated NCSM, ITNCSM, formulated on arguments of multiconfigurational perturbation theory, selects a small set of basis states from the initially large basis space, in which the Hamiltonian is now diagonalized. Previous ITNCSM calculations have proven reliable, however, there has been no thorough investigation of the inherent error in the truncated ITNCSM calculations. We provide a detailed study of ITNCSM calculations and compare them to full NCSM calculations in an attempt to judge the accuracy of ITNCSM in heavier nuclei. Even when ITNCSM calculations are performed, one often needs to extrapolate the groundstate energy from the finite basis (or model) spaces to the infinite model space. Such a procedure is commonplace 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 effectivefield theory framework, in which the UV and IR physics is treated appropriately. The NCSM is well suited to describing boundstate 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 firstprinciples description of exotic nuclei. The NCSM/RGM is also the first extension of the NCSM that can describe dynamic processes such as nuclear reactions.

45 
Transverse Energy Flow in the Underlying Event in the ProtonProton Collisions at √s = 7 TeV with the ATLAS CalorimeterPaleari, Chiara January 2012 (has links)
The European Organization for Nuclear Research (CERN) operates the world's largest and highest energy protonproton 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, dijet 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 leptonproton scattering.

46 
Applications of Effective Field Theory at the intersection of Nuclear and Particle Physicsde 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 nucleonnucleon 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 lowenergy 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 vectormesondominance 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 twonucleon problem. To eliminate the 1/r³ singularity in the isovector tensor force from the onepionexchange potential in leading order (LO), we add auxiliaryfield potentials. The latter could possibly substitute the various counterterms necessary to renormalize the amplitude arising from the onepionexchange potential. By treating the shortrange interaction of the nucleonnucleon 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 longrange 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 twobody decays of bottomonium into two charmed mesons and apply it to study the decays of the Ceven 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 nonperturbative matrix elements, one for each hadron. We calculate the relations between the decay rate and nonperturbative bottomonium and Dmeson matrix elements at LO, with nexttoleadinglog resummation. The phenomenological implications of these relations are discussed.

47 
Hybrid AtomicOptomechanical Systems  Observing Quantum Effects in Macroscopic OscillatorsSingh, Swati January 2012 (has links)
This thesis concentrates on generating and measuring nonclassical 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 alloptical 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 dipoledipole 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 bosecondensate 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.

48 
LUMINESCENCE FROM LANTHANIDE IONS AND THE EFFECT OF CODOPING IN SILICA AND OTHER HOSTSAhmed, Hassan Abdelhalim Abdallah Seed 17 May 2013 (has links)
Amorphous silica powders doped with lanthanide ions were synthesised by the solgel
method and their cathodoluminescence (CL) and photoluminescence (PL) emissions were
compared. Interesting differences depending on the type of excitation were observed for
Tb and Cedoped samples. For Tbdoped samples blue 5D3Â®7FJ emission was measured
during CL in samples for which PL results showed this emission to be concentration
quenched due to crossrelaxation, while for Cedoped 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 flevel 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 flevel 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. Xray 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 codoped 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 dipoledipole and exchange
interactions, but the critical transfer distance obtained was not appropriate for the
exchange interaction, hence dipoledipole 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 codoped 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.

49 
GAMMARAY PRODUCTION IN THE BETYPE STARâPULSAR BINARY SYSTEM PSR B125963van Soelen, Brian 27 May 2013 (has links)
The highmass binary system PSR B125963/LS 2883 is one of only six known gammaray 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
multiwavelength 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 gammaray energies. The spectral energy
distribution is dominated by the emission at gammaray 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 gammarays are believed
to be produced by electrons in the pulsar wind which cool via the inverse Compton upscattering 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 crosssection and should produce GeV energy gammarays in the energy range observed by the
Fermi telescope. A curve of growth method is presented to model the infrared freefree and freebound
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 nearinfrared and optical data and midinfrared
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.

50 
EXPERIMENTAL AND COMPUTATIONAL STUDY OF S SEGREGATION IN FEBarnard, 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 tetrahedralinterstitial, 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: tetrahedralinterstitial, octahedralinterstitial, 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), 9.938Â±6.754 kJ/mol (0.103Â±0.007 eV) and
96.49Â±0.579 kJ/mol (1.000Â±0.006 eV). According to the binding and migration energy calculations,
S will diffuse via a substitutional mechanism with a migration energy of 9.938Â±6.754 kJ/mol
(0.103Â±0.007 eV).
The three lowindex 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 36Â±11 % decrease in the grain sizes of the
polycrystalline Fe sample due to the presence of S. It was found that the recrystallization 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 undoped 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
undoped sample: 5.180, +2.030, +16.41, +0.400, 13.66. Taking the calculated trend in surface
stability for the three lowindex 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 undoped Fe sample are: D0=4.90Ã102 m2/s,
Q=190.8 kJ/mol (1.978 eV), ÎG=134 kJ/mol (1.39 eV) and Î©Fe/S=20 kJ/mol (0.21 eV). The
segregation parameters of P obtained for the undoped 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=1.79Ã102 m2/s and Q=228.7 kJ/mol (2.370 eV), ÎG=145 kJ/mol (1.50 eV) and
Î©Fe/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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