# School of Physics

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Research Content for the School of Physics. Researchers in the School of the Physics.

For queries regarding content of Faculty of Science please contact Salome Potgieter by email : salome.potgieter@wits.ac.za or Tel : 011 717 1961

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Item The strains produced by precipitation in alloys(Royal Society of London, 1940) Nabarro, F.R.N.If the process in metals results simply in the interchange of pairs of atoms large strains must be set up when a new phase prcipitates in an alloy. The strain energy involved is calculated for particles of precipitate of various shapes, and is found to be least if the precipitate forms thin plates. The shape of the particle actually formed is influenced by this energy, by the degree of supersaturation of the solid solution, and by surface effects; the calculated shape is shown to agree reasonably well with that observed experimentally.Item The influence of elastic strain on the shape of particles segregating in an alloy(Physical Society, 1940) Nabarro, F.R.N.An estimate is given of the way in which the strain energy associated with a particle of precipitate depends upon the shape of the particle. It is shown that the energy can only be reduced if the precipitate forms in flat plates, and then only in the case of particles which have broken away from the lattice of the matrix. this dependence of strain energy on shape causes the precipitate to consist of plates of a shape similar to that observed in Widmanstatten structures.Item Orientation of nuclear spins in metals(Royal Society of London, 1940) Frohlich, H.; Nabarro, F.R.N.In connexion with the possible use of nuclear magnetism for the magnetic cooling method, the behaviour of the nuclear spins of monovalent metals at very low temperatures is investigated theoretically.Item An attempt to estimate the degree of precipitation hardening, with a simple model(Physical Society, 1940) Mott, N.F.; Nabarro, F.R.N.The process of diffusion being assumed to result simply in an interchange of atoms, an estimate is made of the internal strains produced by precipitation. According to the dislocation theory these strains are responsible for the hardness of the material, and so the degree of hardening can be predicted.Item The mechanical properties of metallic solid solutions(Physical Society, 1946) Nabarro, F.R.N.The theoretical relation between the lattice strains produced by precipitation in a mineral and the corresponding increase in hardness is extended to the case of lattice strains in metallic solid solutions. The elastic limit of a single crystal of a solid solution is calculated on the assumption that the crystal will slip when the applied external stress is equal to the mean value of the internal stress. This mean is taken over the length of a dislocation, assumed to be in numbers 1000 atoms. The estimate agrees in order of magnitude with the experimental observations. Similar considerations are applied to the hardness of polycrystalline solid solutions. The theory is extended to cases in which the increase of hardness produced by alloying is not large in comparison with the hardness of the pure solvent.Item Dislocations in a simple cubic lattice(Physical Society, 1947) Nabarro, F.R.N.The properties of dislocations are calculated by an approximate method due to Peierls. The width of a dislocation is small, displacements comparable with the interatomic distance being confined to a few atoms. The shear stress required to move a dislocation in an otherwise perfect latttice is of the order of a thousandth of the "theoretical" shear strength. The energy and effective mass of a single dislocation increase logarithmically with the size of the specimen.Item Diffusion and precipitation in alloys(Institute of Metals, 1947) Nabarro, F.R.N.A general review is given of recent work on diffusion and precipitation in alloys, with special emphasis on the theoretical mechanism of diffusion, and on the factors governing the shape and size of the precipitate. The effects of internal stresses on diffusion and precipitation, and of precipitation on the mechanical properties, are discussed.Item Nuclear ferromagnetism(Physical Society, 1947) Frohlich, H; Nabarro, F.R.N.It is shown that at sufficiently low temperatures metals become ferromagnetic owing to an orientation of the nuclear spins. The domain structure of such ferromagnetics is analogous to that of ordinary ferromagnetics.Item Laszlo's papers on tessellated stresses: a review(Institute of Metals, 1947-10) Nabarro, F.R.N.Four recent papers by Laszlo on "Tessellated Stresses, Parts I-IV" published in the Journal of the Iron and Steel Institute, are reviewed in detail. The present review contains no new results.Item Dislocation theory and transient creep(Physical Society, 1948) Mott, N.F.; Nabarro, F.R.N.The purpose of this paper is to put forward certain advances in the theory of dislocations, and in particular to discuss their application to the theory of transient creep, in the sense in which the term is used by Andrade (1911, 1914, 1932) and by Orowan (1947).Item Mechanical effects of carbon in iron(Physical Society, 1948) Nabarro, F.R.N.Snoek has shown that when carbon atoms move from one possible set of interstitial sites in the lattice of a-iron to another set they cause shear strains. Cottrell has shown that the stress around a dislocation may be relieved by the migration of solute atoms in its neighbourhood, and that the dislocation is then bound to its present position. By combining these theories with the usual theory of age-hardening it is possible to explain the existence of a yeield point, quench aging, strain aging, delayed yield and blue brittleness as consequences of the presence of carbon in iron. A rough quantitative theory of the time-aging effects is given, and shown to agree with experiment. The need for further experiment is emphasized.Item Professor F.R.N. Nabarro - curriculum vitae(2004) Nabarro, Professor FrankCurriculum vitae of Professor F.R.N. Nabarro, compiled in 2004Item Professor F.R.N. Nabarro - Obituary(2006-07-25) Every, Professor ArthurObituary on Professor Nabarro written by Professor Arthur Every, School of PhysicsItem Prof A.G. Every - List of Publications(2006-08-28) Every, Arthur GeorgeList of Publications of Prof A.G. EveryItem Prof A.G. Every - Resume(Professor A.G. Every, 2006-08-28) Every, Arthur GeorgeResume of Professor A.G Every, Emeritus Professor, School of Physics, University of the Witwatersrand, JohannesburgItem Higher loop nonplanar anomalous dimensions from symmetry(Springer, 2014-02-28) Koch, Robert de Mello; Graham, Stuart; Messamah, IliesIn this article we study the action of the one loop dilatation operator on operators with a classical dimension of order N. These operators belong to the su(2) sector and are constructed using two complex fields Y and Z. For these operators non-planar diagrams contribute already at the leading order in N and the planar and large N limits are distinct. The action of the one loop and the two loop dilatation operator reduces to a set of decoupled oscillators and factorizes into an action on the Z fields and an action on the Y fields. Direct computation has shown that the action on the Y fields is the same at one and two loops. In this article, using the su(2) symmetry algebra as well as structural features of field theory, we give compelling evidence that the factor in the dilatation operator that acts on the Y s is given by the one loop expression, at any loop order.Item Light third-generation squarks from flavour gauge messengers(Springer, 2014-04-10) Brummer, Felix; McGarrie, Moritz; Weiler, AndreasWe study models of gauge-mediated supersymmetry breaking with a gauged horizontal SU(3)F symmetry acting on the quark superfields. If SU(3)F is broken nonsupersymmetrically by F -term vacuum expectation values, the massive gauge bosons and gauginos become messengers for SUSY breaking mediation. These gauge messenger fields induce a flavour-dependent, negative contribution to the soft masses of the squarks at one loop. In combination with the soft terms from standard gauge mediation, one obtains large and degenerate first- and second-generation squark masses, while the stops and sbottoms are light. We discuss the implications of this mechanism for the superparticle spectrum and for flavour precision observables. We also provide an explicit realization in a model with simultaneous SUSY and SU(3)F breaking.Item Heavy quark diffusion in strongly coupled anisotropic plasmas(Springer, 2014-06-09) Giataganasa, Dimitrios; Soltanpanahib, HesamWe study the Langevin diffusion of a relativistic heavy quark in anisotropic strongly coupled theories in the local limit. Firstly, we use the axion space-dependent deformed anisotropic N = 4 sYM, where the geometry anisotropy is always prolate, while the pressure anisotropy may be prolate or oblate. For motion along the anisotropic direction we find that the effective temperature for the quark can be larger than the heat bath temperature, in contrast to what happens in the isotropic theory. The longitudinal and transverse Langevin diffusion coefficients depend strongly on the anisotropy, the direction of motion and the transverse direction considered. We analyze the anisotropy effects to the coefficients and compare them to each other and to them of the isotropic theory. To examine the dependence of the coefficients on the type of the geometry, we consider another bottom-up anisotropic model. Changing the geometry from prolate to oblate, certain diffusion coefficients interchange their behaviors. In both anisotropic backgrounds we find cases that the transverse diffusion coefficient is larger than the longitudinal, but we find no negative excess noise.Item Natural supersymmetry and dynamical flavour with meta-stable vacua(Springer, 2014-07-29) Abela, Steven; McGarrie, MoritzWe show how gauged flavour breaking and N = 1 supersymmetry breaking can be dynamically aligned to produce natural models. Supersymmetry is broken in a metastable vacuum, while a weakly gauged flavour symmetry is identified with an SU (3) F subgroup of the global symmetry of the SQCD model. We find that alignment can easily occur either through strongly coupled flavour models, with SQCD bound states playing the role of flavons, or entirely weakly coupled and renormalizable models with elementary SU (3) F adjoint flavons. In both cases it is essential that the SU (3) F breaking, and hence all the flavour structure, is driven dynamically by the SUSY breaking. The resulting flavour gauge mediation in conjunction with the usual gauge mediation contribution leads to naturally light third generations squarks, with first and second generations above.Item Large A without the desert(Springer, 2014-07-31) Abdalgabar, Ammar; Cornell, Alan S.; Deandrea, Aldo; McGarrie, MoritzEven if the unification and supersymmetry breaking scales are around 106 to 109 TeV, a large A t coupling may be entirely generated at low energies through RGE evolution in the 5D MSSM. Independent of the precise details of supersymmetry breaking, we take advantage of power law running in five dimensions and a compactification scale in the 10 − 103 TeV range to show how the gluino mass may drive a large enough A t to achieve the required 125.5 GeV Higgs mass. This also allows for sub-TeV stops, possibly observable at the LHC, and preserving GUT unification, thereby resulting in improved naturalness properties with respect to the four dimensional MSSM. The results apply also to models of “split families” in which the first and second generation matter fields are in the bulk and the third is on the boundary, which may assist in the generation of light stops whilst satisfying collider constraints on the first two generations of squarks.

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