Dissertations / Theses on the topic 'Lattice spin'
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Berge, Siri Alva. "Phase diagrams of finite spin chains withcoupled spin and lattice vibrations." Thesis, Uppsala universitet, Materialteori, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-448790.
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The spin-lattice coupling is due to itinerant electrons interacting with both spins of ions and phonons, mediating a coupling between magnetic spin moments andlattice vibrations. In this project, the coupling is studied systematically for finitequantum spin chains of size 2 – 10. The coupling is included in a Hamiltonian model with the Heisenberg exchange interaction and an external magnetic fieldresulting in an eigenvalue problem which is solved numerically to find phase diagrams of the magnetic moment of the system depending on an external magneticfield and the lattice vibration parameter. The eigenvalue problem is also solved analytically for the 2-ion chain, dimer, and 3-ion chain, trimer, systems. Based on these phase diagrams two propositions are made: the effect of the coupling is larger than an external magnetic field and the behavior of the coupling converges to a common phase diagram for larger spin chains.Kopplade spinn- och vibrationsfrihetersgrader beror på kringvandrande elektroner som växelverkar med både spin och fononer, vilket förmedlar en koppling mellan magnetiskt spinmoment och gittervibrationer. I detta projekt studeras denna koppling systematiskt för ändliga spin-kedjor av 2–10 joner. Systemet beskrivs av en Hamiltonian med Heisenberg modellen som beskriver spin-spin kopplingen samt ett externt magnetiskt fält. Detta egenvärdesproblem löses analytiskt för dimer- och trimersystem samt numeriskt för längre kedjor. Lösningarna används för att ta fram fasdiagram av de magnetiska momenten av kedjorna beroende på externt magnetfält och spin- och vibrationsfrihetsgradsparametern . Baserat på dessa fasdiagram, framförs två propositioner: kopplingens effekt är större än ett externtmagnetfält och kopplingens beteende konvergerar till ett enhetligt fasdiagram för större spinnkedjor.
Subject reader/ämnesgranskare: Anders Bergman
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Benassi, Costanza. "On classical and quantum lattice spin systems." Thesis, University of Warwick, 2018. http://wrap.warwick.ac.uk/108566/.
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This thesis focuses on some results about quantum and classical lattice spin systems. We study a wide class of two-dimensional quantum models which enjoy a U(1) symmetry. Using the so called complex rotation method we show that the decay of the relevant correlation functions is at least algebraically fast. We provide some examples of relevant models which belong to our class. We review some results present in the literature concerning the so called GriffthsGinibre inequalities for the classical XY model and propose a generalisation to its quantum counterpart. Correlation inequalities indeed hold for the quantum XY model with spin- 1 2 and for the ground state of the spin-1 system. We propose some applications of these results, namely that the infinite volume limit of some correlation functions exists and that it is possible to compare quenched and annealed averages for a quantum XY model with random couplings. We investigate loop representations for O(n) classical spin systems. We propose a generalised random current representation and show its relationship with the Brydges-Fröhlich-Spencer one. We review some conjectures regarding the expected behaviour of these loop models { namely that macroscopic loops appear and their lengths are distributed according to a Poisson-Dirichlet distribution. We propose some arguments in favour of these conjectures for O(n) loop models, using a mix of exact results and heuristic considerations. In order to do so we de ne a stochastic process which is an effective split-merge process for macroscopic loops and we reformulate some correlation functions for the O(2) spin system in terms of loop properties.
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Tarnawski, Maciej. "Asymptotic phase diagrams for lattice spin systems." Diss., Virginia Polytechnic Institute and State University, 1985. http://hdl.handle.net/10919/53610.
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We present a method of constructing the phase diagram at low temperatures, using the low temperature expansions. We consider spin Iattice systems described by a Hamiltonian with a d-dimensional perturbation space. We prove that there is a one-one correspondence between subsets of the phase diagram and extremal elements of some family of convex sets. We also solve a linear programming problem of the phase diagram for a set of affine functionals.Ph. D.
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Collins, Alexander Rory Physics Faculty of Science UNSW. "Quantum lattice models." Publisher:University of New South Wales. Physics, 2008. http://handle.unsw.edu.au/1959.4/43408.
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This thesis presents studies of the low energy properties of nseveral frustrated spin-1/2 Heisenberg antiferromagnets using various analytic and computational methods. The models studied include the union jack model, the alternating Heisenberg chain, the Heisenberg bilayer model, and the spin-Peierls model. The union jack model is a Heisenberg antiferromagnetic spin model with frustration, and is analyzed using spin-wave theory. For small values of the frustrating coupling $\alpha$, the system is N{\' e}el ordered, while for large $\alpha$ the frustration is found to induce a canted phase. Spin wave theory with second order corrections finds the critical coupling at $\alpha \simeq 0.645$,which agrees quantitatively with series expansion results. No intermediate spin-liquid phase is found to exist between the two phases. The alternating Heisenberg chain is studied using an alternative triplet-wave expansion formalism for dimerized spin systems, modification of the ??bond operator?? formalism of Sachdev and Bhatt. Projection operators are used to confine the system to the physical subspace, rather than constraint equations. Comparisons are made with the results of dimer series expansions and exact diagonalization. The S=1/2 Heisenberg bilayer spin model at zero temperature is studied in the dimerized phase using analytic triplet-wave expansions and dimer series expansions. The occurrence of two-triplon bound states in the S=0 and S=1 channels, and antibound states in the S=2 channel, is predicted with triplet-wave theory and confirmed by series expansions. All bound states are found to vanish at or before the critical coupling separating the dimerized phase from the N{\' e}el phase. The critical behavior of the total and single-particle static transverse structure factors is also studied by series expansion methods and found to conform with theoretical expectations. The Heisenberg spin-Peierls model with dispersive, gapless phonons is studied with Density Matrix Renormalization Group methods. We investigate the zero temperature properties of the model using the crossover method. The calculations were found to converge poorly and no conclusive results could be found using this method. An analysis of the convergence problems and the discovery of an anomalous triplet ground state is presented in this chapter.
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Berge, Siri Alva. "Phase Diagrams of Finite Spin Chains with Coupled Spin and Lattice Vibrations." Thesis, Uppsala universitet, Materialteori, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-449117.
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The spin-lattice coupling is due to itinerant electrons interacting with both spins of ions and phonons, mediating a coupling between magnetic spin moments and lattice vibrations. In this project, the coupling is studied systematically for finite quantum spin chains of size 2 – 10. The coupling is included in a Hamiltonian model with the Heisenberg exchange interaction and an external magnetic field resulting in an eigenvalue problem which is solved numerically to find phase diagrams of the magnetic moment of the system depending on an external magneticfield and the lattice vibration parameter. The eigenvalue problem is also solved analytically for the 2-ion chain, dimer, and 3-ion chain, trimer, systems. Based on these phase diagrams two propositions are made: the effect of the coupling is larger than an external magnetic field and the behavior of the coupling converges to a common phase diagram for larger spin chains.Kopplade spinn- och vibrationsfrihetersgrader beror på kringvandrande elektroner som växelverkar med både spin och fononer, vilket förmedlar en koppling mellan magnetiskt spinmoment och gittervibrationer. I detta projekt studeras denna koppling systematiskt för ändliga spin-kedjor av 2–10 joner. Systemet beskrivs av en Hamiltonian med Heisenberg modellen som beskriver spin-spin kopplingen samt ett externt magnetiskt fält. Detta egenvärdesproblem löses analytiskt för dimer- och trimersystem samt numeriskt för längre kedjor. Lösningarna används för att ta fram fasdiagram av de magnetiska momenten av kedjorna beroende på externt magnetfält och spin- och vibrationsfrihetsgradsparametern. Baserat på dessa fasdiagram, framförs två propositioner: kopplingens effekt är större än ett externt magnetfält och kopplingens beteende konvergerar till ett enhetligt fasdiagram för större spinnkedjor.
Subject reader/ämnesgranskare: Anders Bergman
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Aßmann, Matthias [Verfasser]. "Atomistic simulation of spin-lattice dynamics / Matthias Aßmann." Konstanz : Bibliothek der Universität Konstanz, 2018. http://d-nb.info/1173616292/34.
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Harper, R. E. "Anomalous nuclear spin-lattice relaxation peaks in InSb." Thesis, University of St Andrews, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.373036.
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Benassi, Costanza. "Su(3) lattice gauge theories and spin chains." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amslaurea.unibo.it/7171/.
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I modelli su reticolo con simmetrie SU(n) sono attualmente oggetto di studio sia dal punto di vista sperimentale, sia dal punto di vista teorico; particolare impulso alla ricerca in questo campo è stato dato dai recenti sviluppi in campo sperimentale per quanto riguarda la tecnica dell’intrappolamento di atomi ultrafreddi in un reticolo ottico. In questa tesi viene studiata, sia con tecniche analitiche sia con simulazioni numeriche, la generalizzazione del modello di Heisenberg su reticolo monodimensionale a simmetria SU(3). In particolare, viene proposto un mapping tra il modello di Heisenberg SU(3) e l’Hamiltoniana con simmetria SU(2) bilineare-biquadratica con spin 1. Vengono inoltre presentati nuovi risultati numerici ottenuti con l’algoritmo DMRG che confermano le previsioni teoriche in letteratura sul modello in esame. Infine è proposto un approccio per la formulazione della funzione di partizione dell’Hamiltoniana bilineare-biquadratica a spin-1 servendosi degli stati coerenti per SU(3).
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Magnusson, Einar B. "High-spin impurities and surface acoustic waves in piezoelectric crystals for spin-lattice coupling." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:09d23fb2-f501-4be2-a25f-b69ada0ce5b1.
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In this thesis we investigate various aspects of SAW devices and strain sensitive spin species in ZnO and LiNbO3 for coupling surface acoustic waves to spin ensembles. Firstly, we performed a series of ESR experiments exploring the potential of Fe3+ impurities in ZnO for spin-lattice coupling. This spin system has already been identified as a high potential quantum technology component due to its long coherence time. We show that the system also has good properties for spin-lattice coupling experiments, with a strain-coupling parameter G33 = 280 ± 5GHz/strain, which is about 16 times larger than the largest reported for NV centres in diamond. We found that the LEFE effect as well as the spin Hamiltonian parameter D have a linear temperature dependence. As the relative change in each coincide, this strongly supports the notion that the modification of D by an electric field is a multiplicative effect rather than an additive one, D = D0(1 + κΕ). The LEFE coefficient we measured is several times larger for Fe3+:ZnO than for Mn2+:ZnO. Secondly, we have fabricated and characterised SAW devices on bulk ZnO crystals and Fe doped lithium niobate. We found that the nominally pure ZnO was conductive at room temperature due to n-type intrinsic doping, and electrical losses inhibited any transmission through a SAW delay line above T = 200K. The one-port resonator measured down to milli-Kelvin temperatures showed excellent quality factors of up to Q ≃ 1.5 x 105 in its superconducting state. Finally, we performed a surface acoustic wave spin resonance (SAWSR) experiment using a one-port SAW resonator fabricated on Fe2+:LN. We observed a clear signal at T ≃ 25 K, at a field near the expected one for a Δms = 2 transition between the |â1⟩ and |+1⟩ states. We concluded it to be a transition induced by acoustic coupling since the signal intensity did not tend to zero when the magnetic field was parallel to the crystal anisotropy axis. Furthermore, this tells us that the coupling is due to a modulation of the E zero-field splitting parameter rather than D. We investigated the dependence on microwave power and found the saturation limit. We performed a measurement of Fe3+:LN as well to reassure ourselves that the resonance is not magnetically excited by the field around the IDT.
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Turner, Helen J. "Proton NMR spin lattice relaxation studies of solid polyethylenes." Thesis, University of Nottingham, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.243420.
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del, Valle-Inclán Redondo Yago Baltasar. "Spin and lattice properties of optically trapped exciton polaritons." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/284554.
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Exciton-polaritons are the fundamental excitations arising from the strong coupling of quantum well excitons and cavity photons in semiconductor microcavities. They are compound bosons for which stimulated scattering and macroscopic occupation of single quantum states can occur at sufficiently high densities. One way of creating such polariton condensates is with nonresonant optical pumping. Doing so creates a large density of free- carriers and excitons that strongly interact and blueshift the polariton energy levels. Using spatially patterned nonresonant fields, the polariton potential landscape can be tailored and optically trapped condensates can be created. This thesis shows that the spin properties of polariton condensates are strongly modified by such trapping. Under linearly polarised pumping, helicity can spontaneously develop at a critical occupation, breaking the parity symmetry. This formation of spin-up/spin-down condensates is explained within a Gross-Pitaevskii model which accurately reproduces the influence of electric fields and condensate density. Under elliptically polarised pumping, two phenomena are observed: the formation of condensates with the opposite handedness to the pump and hysteresis of both occupation and spin with respect to pump power. The spatial dependence of these effects highlights the limitations of commonly used models of polariton condensation. Finally, the suitability of patterned optical fields for the creation of polariton lattices is explored. For small chains of condensates, controllable coupling between adjacent spins is demonstrated, with the formation of antiferromagnetic and ferromagnetic domains. The extent of these domains is strongly affected by sample nonuniformity, fundamentally limiting the scalability of these lattices.
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Tomeno, Shinya. "Experimental Study of Organic Triangular Lattice Quantum Spin Liquids." Kyoto University, 2020. http://hdl.handle.net/2433/254505.
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Benton, John Owen. "Classical and quantum spin liquids on the pyrochlore lattice." Thesis, University of Bristol, 2015. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.681486.
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The study of frustrated magnetism sits at the frontier of modern condensed matter physics, offering the possibility to discover new states of matter with exotic excitations. A textbook example of this is seen in the rare earth pyrochlore oxides H02Ti20 7 and DY2Ti207, better known as "spin ice". These magnets exhibit a classical spin liquid state with emergent magnetic monopole excitations. Inspired by this, we explore in this thesis a wide landscape of possibilities for novel magnetic behaviour on the pyrochlore lattice. Starting from the most general model for anisotropic nearest neighbour exchange interactions we show that the full zero temperature classical phase diagram can be obtained simply by considering the symmetries of a tetralledron. In the process of doing this we obtain an exact rewriting of the model in terms of a set of local order parameter fields defined on tetrahedra. After classifying and describing the ordered phases of this model we turn to give consideration to the novel physics which may be found near the boundaries where phases with different symmetry meet. We find that interesting properties of several materials can be traced back to the "accidental" degeneracies which arise on these boundaries. These include the "order by disorder" in Er2Ti207, the "dimensional-reduction" observed in Yb2Ti207, and the suppression of The observation that the physics of the phase boundaries manifests itself throughout the nearby regions of the phase diagram leads us to a systematic analysis of all of the phase boundaries of the model. In several cases dipole order persists at the phase boundary, stabilised by fluctuations. By constructing the ground state manifolds in each case we identify those configurations which are favoured by fluctuations. We also identify two limits of the phase diagram in which dipolar order vanishes to be replaced by spin nematic order. At certain phase boundaries we find that magnetic order vanishes entirely. This leads to the discovery of a new classical spin liquid on the pyrochlore lattice- the anisotropic Coulomb liquid. This spin liquid, like the Coulomb phase observed in spin ice, possesses algebraic correlations and conserved fluxes but also presents several new features including an anisotropic Coulomb interaction between its excitations. A long-wavelength theory of this spin liquid is derived from the microscopic model by coarse graining the fluctuations of the local order-parameter fields. Homing in on a different limit of the phase diagram, we move on to consider the effects of quantum fluctuations on a spin ice material and in particular how these effects would manifest themselves in neutron scattering experiments. We explore the correlations of the U(I) spin liquid which arises in this limit by explicitly constructing a basis of photon operators on the pyrochlore lattice and calculating the associated dispersion relation and correlation functions. This enables us to quantitatively describe how these unusual excitations would manifest themselves in neutron scattering experiments. We find that "pinch points," which are the signal feature of a classical spin ice, fade away as a quantum ice is cooled to its quantum spin liquid ground state.
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Fong, Manson Cheuk-Man. "Heisenberg model with spin anisotropy on the Kagomé lattice /." View abstract or full-text, 2008. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202008%20ZFONG.
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Allalen, Mohammed. "Magnetic properties and proton spin-lattice relaxation in molecular clusters." Doctoral thesis, [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=979984777.
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Cochran, Christopher S. "Even-number spin correlations on two-dimensional Ising lattice structures." Virtual Press, 2002. http://liblink.bsu.edu/uhtbin/catkey/1237760.
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Many physical systems can be represented by a regular arrangement of molecules in a lattice structure. Knowing how neighboring molecules in the lattice interact with one another can give great insight into a material's macroscopic behavior. A very popular and effective means of investigating these microscopic interactions is the Ising Model. This model, suggested first by Wilhelm Lenz in 1920 and later expanded by Ernst Ising in 1925, is based on the assumptions that each molecule in a lattice structure can be represented by its spin value (+l or -1) and that only nearest neighbors contribute to the total interaction energy. The Ising Model, which was initially used in the study of ferromagnetic systems, can now be used to study a variety of physical systems. Some of these include antiferromagnetic crystals, binary alloys, DNA, and lattice gasses.Department of Physics and Astronomy
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Huang, Yang. "Analysis of NMR Spin-lattice Relaxation Dispersion on Complex Systems." Doctoral thesis, Umeå universitet, Kemiska institutionen, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-110721.
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This thesis focus on the analysis of spin-lattice NMRD relaxation profilesmeasured in various complex systems such as proteins, zeolites and ionicliquids. Proton, deuterium and fluoride T1-NMRD relaxation profiles wereobtained from a fast-field cycling (FFC) instrument. It is found that alsopossible to obtain NMRD profiles from the molecular dynamics (MD)simulation trajectories. NMRD Profiles were analyzed by using differentrelaxation models, such as the Solomon-Bloembergen-Morgan (SBM) theoryand the Stochastic Liouville (SL) theory. Paper I described the hydration of protein PrxV obtained from a MDsimulation, and compared with the picture emerges from an analysis byusing a generally accepted relaxation model [appendix C]. The result showsthat the information from NMRD analysis is an averaged picture of watermolecules with similar relaxation times; and the MD simulations containsinformation of all types of interested water molecules with differentresidence times. In paper II NMRD profiles have been used to characterize the hydration ofthe oxygen-evolving complex in state S1 of photosystem II. NMRDexperiments were performed on both intact protein samples and Mndepletedsamples, and characteristic dispersion difference were foundbetween 0.03 MHz to 1 MHz; approximately. Both the SBM theory and theSL theory have been used to explain this dispersion difference, and it isfound that this is due to a paramagnetic enhancement of 1-2 water moleculesnearby ~10 Å from the spin center of the Mn4CaO5 cluster. The result showsthe reorientation of the molecular cluster is in μs time interval. Whencompare these two theories, the SL theory presented a better interpretationbecause parameters obtained from the SBM theory shows they didn’t fulfilthe presupposed perturbation criterion (the Kubo term). Paper III deals with the water dynamics in the restricted/confined spaces inthe zeolite samples (H-ZSM-5 and NH4-ZSM-5) and obtained by proton anddeuterium spin-lattice NMRD profiles. The results show that the spin-latticeNMRD can be used to characterize various zeolites. The temperature has aweak effect on the relaxation rate R1, but the change of different counter ionsmay change the hydration and the translational diffusion pores and givedifferent R1. Proton and fluoride NMRD profiles and MD simulations were both used tostudy the dynamics of BMIM[PF6] in paper IV. Results indicate the reorientation of the molecules are in the ns time regime, and the effectivecorrelation time obtained from 1H and 19F are the same. From the MDsimulation it is found the reorientation of [PF6]- ions is much faster (in ps)compare with BMIM+ ion which moves in the ns time range. With previous results, the FFC NMRD profiles are indeed very informativetools to study the molecular dynamics of complex systems. The MDsimulation can be used as a complementary method to obtain detailedinformation. By combine these two methods, it provide a more colorfulpicture in the study of protein hydration and liquid molecular dynamics.
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Dutreix, Clément. "Impurity and boundary modes in the honeycomb lattice." Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112217/document.
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La présente thèse s’articule autour de deux sujets. Le premier concerne la localisation des électrons en présence d’impuretés ou d’interfaces dans le réseau hexagonal. Le deuxième, en revanche, traite de l’accumulation de spin dans un supraconducteur hors-Équilibre de type s.Le graphène est la principale motivation de la première partie. Ce matériau bidimensionnel consiste en un feuillet d’atomes de carbones et peut être décrit comme un réseau hexagonal, c’est-à-dire un réseau de Bravais triangulaire avec un motif diatomique. La structure de bande électronique révèle alors l’existence d’électrons de Dirac sans masse et chiraux à basse énergie.D’une part, il est possible d’annihiler ces fermions chiraux en étirant de façon uni-Axiale le matériau. Pour une valeur seuil de l’étirement, les électrons deviennent massiques et non-Relativistes, ce qui définit une transition de phase dite de Lifshitz. Afin de caractériser cette transition, nous étudions la diffusion des électrons sur des impuretés en fonction de l’étirement. Une impureté localisée induit des interférences quantiques dans la densité électronique, connues sous le nom d’oscillations de Friedel. Etant sensibles à la nature chirale des électrons, nous montrons que ces oscillations décroissent selon des lois de puissances qui permettent de caractériser chacune des phases de la transition. La même étude est réalisée dans le cas limite où le diffuseur est une lacune.D’autre part, le motif diatomique du réseau hexagonal propose aussi une incursion dans le monde des isolants et supraconducteurs topologiques. Pour ces systèmes, la caractérisation topologique de la structure de bande électronique permet de prédire l’existence d’états de bord aux interfaces. Nous développons notamment un modèle de supraconducteur topologique basé sur le réseau hexagonal du graphène, en présence de supraconductivité de type singulet (s ou d). Lorsque la symétrie par renversement du temps est brisée par un champ Zeeman, et en présence de couplage spin-Orbit Rashba, nous donnons une prescription qui permet de caractériser les différentes phases topologiques possibles et de prédire l’apparition d’états de bord (états de Majorana) dans des nano-Rubans de graphène.La seconde partie discute l’accumulation de spin dans un supraconducteur hors-Équilibre, joint à un ferromagnétique. Lorsqu’il est à l’équilibre, le supraconducteur est composé de quasiparticules et d’un condensat. L’injection de particules polarisées en charge et en spin, à savoir des électrons polarisés en spin, induit une accumulation de spin et de charge à l’intérieur du supraconducteur. Si l’injection cesse, les populations de spin et de charge vont relaxer vers l’équilibre, mais pas nécessairement sur des échelles de temps identiques. Récemment, la réalisation d’une expérience a mis en évidence que le la charge pouvait relaxer bien plus rapidement que le spin. Afin de confirmer cet effet, une nouvelle expérience a été réalisée grâce à des mesures établies dans le domaine fréquentiel. Ici, nous adressons un model relatif à cette dernière expérience, dans le but d’extraire le temps caractéristique de relaxation du spin qui s’avère être de l’ordre de quelques nanosecondesTwo fields of research define the framework in which the present thesis can be apprehended. The first one deals with impurity and boundary modes in the hexagonal lattice. The second one concerns a spin accumulation in an out-Of-Equilibrium superconductor.Two fields of research define the framework in which the present thesis can be apprehended. The first one deals with impurity and boundary modes in the hexagonal lattice. The second one concerns a spin accumulation in an out-Of-Equilibrium superconductor.Graphene is the main motivation of the first part. From a crystallographic perspective, the carbon atoms in graphene, a graphite layer, design a triangular Bravais lattice with a diatomic pattern. This gives rise to an extra degree of freedom in the electronic band structure that crucially reveals chiral massless Dirac electrons at low-Energy. First of all, it is possible to make these chiral fermions annihilate when a uniaxial strain stretches the graphene layer. For a critical value of the strain, all the fermions become massive and nonrelativistic, which defines a Lifshitz transition. We study the impurity scattering as a function of the strain magnitude. A localised impurity yields quantum interferences in the local density of states that are known as Friedel oscillations. Because they are affected by the chiral nature of the electrons, we show that the decaying laws of these oscillations are specific to the phase the system belongs to. Thus, the impurity scattering offers the possibility to fully characterise the transition.Second, the diatomic pattern of the graphene lattice can also be considered as an invitation to the world of topological insulators and superconductors. The existence of edge states in such systems relies on the topological characterization of the band structure. Here we especially introduce a model of topological superconductor based on the honeycomb lattice with induces spin-Singlet superconductivity. When a Zeeman field breaks the time-Reversal invariance, and in the presence of Rashba spin-Orbit interactions, we give a prescription to describe the topological phases of the system and predict the emergence of Majorana modes (edge states) in strained and doped nanoribbons.The second part discusses the study of a spin accumulation in an out-Of-Equilibrium s-Wave superconductor. At the equilibrium, the superconductor is made of particles coupled by a s-Wave pairing, as well as unpaired quasiparticles. Injecting spin-Polarised electrons into the superconductor induces charge and spin imbalances. When the injection stops, it may happen that charge and spin do not relax over the same time-Scale. The first experiment that points out such a spin-Charge decoupling has recently been realised. In order to confirm this chargeless spin-Relaxation time, a new experiment has been developed [96], based on measurements in the frequency domain. Here, we address a model that fits the experimental data and thus enables the extraction of this characteristic time that is of the order of a few nanoseconds
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Kenneway, Debra A. "An Investigation of the Two-Dimensional Ising Spin Glass Using Information Theoretic Measures." Fogler Library, University of Maine, 2005. http://www.library.umaine.edu/theses/pdf/KennewayDA2005.pdf.
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Ono, Koki. "Two-Orbital Quantum Gases in an Optical Lattice: Interorbital Spin-Exchange Dynamics and Spin-Space Quantum Transport." Doctoral thesis, Kyoto University, 2021. http://hdl.handle.net/2433/263444.
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Taie, Shintaro. "Ultracold Fermi Gases with High Spin Symmetry in an Optical Lattice." 京都大学 (Kyoto University), 2013. http://hdl.handle.net/2433/175106.
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Dimitrova, Ivana Ljubomirova. "Realizing quantum spin models with ⁷Li atoms in an optical lattice." Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/128327.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, February, 2020Cataloged from PDF version of thesis.
Includes bibliographical references (pages 279-292).
Quantum spin Hamiltonians are paradigmatic models, which display different kinds of quantum phase transitions, strongly-correlated and topological ground states, and various regimes of transport. Expanding their significance, many mappings exist between quantum spin models and other systems in different areas of physics, mathematics, and beyond. Even though quantum spin models have been studied extensively, there are still many open questions. Simulating these Hamiltonians with the system of ultracold atoms in optical lattices provides a new perspective with the wide tunability of parameters and the minimal coupling to the environment. The mapping involves using the Mott insulating state of ultracold atoms in optical lattices, where the energy of a second-order tunneling process (superexchange) maps to the parameters of a Heisenberg model. This thesis provides a detailed roadmap for the design and building of such a quantum simulator with ⁷Li atoms in optical lattices.
Each step of the process is described, together with the methods and techniques used for the building and the characterization of the physical system. A focus is placed on using the Mott insulator as a starting point for spin physics experiments and, in particular, on the characterization and improvements of the mapping from a density sector description to a spin sector description of the system. Several schemes for implementing and studying spin systems are presented. In particular, the feasibility of implementing the Heisenberg spin-1/2 and spin-1 models in this system is described. The tilted lattice is presented as a tool for studying pure superexchange-driven dynamics and for increasing their timescale by suppressing first order tunneling and the role of number defects. The first measurements and the tuning with this machine of superexchange-driven dynamics over a wide range in the anisotropic Heisenberg spin-1/2 models are presented.
Finally, the versatility of the BEC 5 machine is showcased by a study which does not involve an optical lattice. It explores the realization of an exotic quantum phase, a supersolid, in a new way. After many years of building and improvements, the BEC 5 machine emerges as a repeatable and reliable quantum simulator which has a clear scientific agenda of exploring many-body ground states and non-equilibrium dynamics.
by Ivana Ljubomirova Dimitrova.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Physics
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Liu, Yuzhi. "Renormalization group and phase transitions in spin, gauge, and QCD like theories." Diss., University of Iowa, 2013. https://ir.uiowa.edu/etd/4872.
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In this thesis, we study several different renormalization group (RG) methods, including the conventional Wilson renormalization group, Monte Carlo renormalization group (MCRG), exact renormalization group (ERG, or sometimes called functional RG), and tensor renormalization group (TRG).
We use the two dimensional nearest neighbor Ising model to introduce many conventional yet important concepts. We then generalize the model to Dyson's hierarchical model (HM), which has rich phase properties depending on the strength of the interaction. The partition function zeros (Fisher zeros) of the HM model in the complex temperature plane is calculated and their connection with the complex RG flows is discussed. The two lattice matching method is used to construct both the complex RG flows and calculate the discrete β functions. The motivation of calculating the discrete β functions for various HM models is to test the matching method and to show how physically relevant fixed points emerge from the complex domain.
We notice that the critical exponents calculated from the HM depend on the blocking parameter b. This motivated us to analyze the connection between the discrete and continuous RG transformation. We demonstrate numerical calculations of the ERG equations. We discuss the relation between Litim and Wilson-Polchinski equation and the effect of the cut-off functions in the ERG calculation.
We then apply methods developed in the spin models to more complicated and more physically relevant lattice gauge theories and lattice quantum chromodynamics (QCD) like theories. Finite size scaling (FSS) technique is used to analyze the Binder cumulant of the SU(2) lattice gauge model. We calculate the critical exponent nu and omega of the model and show that it is in the same universality class as the three dimensional Ising model. Motivated by the walking technicolor theory, we study the strongly coupled gauge theories with conformal or near conformal properties. We compare the distribution of Fisher zeros for lattice gauge models with four and twelve light fermion flavors. We also briefly discuss the scaling of the zeros and its connection with the infrared fixed point (IRFP) and the mass anomalous dimension.
Conventional numerical simulations suffer from the critical slowing down at the critical region, which prevents one from simulating large system. In order to reach the continuum limit in the lattice gauge theories, one needs either large volume or clever extrapolations. TRG is a new computational method that may calculate exponentially large system and works well even at the critical region. We formulate the TRG blocking procedure for the two dimensional O(2) (or XY ) and O(3) spin models and discuss possible applications and generalizations of the method to other spin and lattice gauge models.
We start the thesis with the introduction and historical background of the RG in general.
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Platt, William N. "Proton NMR relaxation studies of water in pulp fibers." Diss., Georgia Institute of Technology, 1985. http://hdl.handle.net/1853/7045.
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Green, R. M. "NMR and neutron scattering investigations of molecular motion in the solid state." Thesis, University of Nottingham, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233682.
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Heine, Matthew. "Ab Initio Theory of Thermal Spin-Lattice Disorder in Iron and Invar:." Thesis, Boston College, 2020. http://hdl.handle.net/2345/bc-ir:108985.
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Thesis advisor: David BroidoDespite its deceptive simplicity and because of its scientific and technological importance, bcc Fe is still the subject of research and debate. We develop an ab initio theoretical framework and apply it to calculate temperature-dependent phonon modes and magnetic interaction parameters in bcc Fe. This framework incorporates realistic thermal disorder in a coupled spin-lattice system. Thermal spin-lattice coupling is found to significantly renormalize the phonon modes and magnetic interaction strength, resulting in significant temperature-dependencies. A method for treating magnetic systems of unknown entropy is developed and applied to calculate phonon modes and investigate the anomalous thermal expansion of the classical invar alloy, Fe0.65Ni0.35. Results over the temperature range 50K to room temperature are consistent with the observed low thermal expansion of this material. Excellent agreement with measured data is achieved for calculated phonon modes in both bcc Fe and the invar alloy
Thesis (PhD) — Boston College, 2020
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics
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Shinjo, Kazuya. "Density-matrix renormalization group study of quantum spin systems with Kitaev-type anisotropic interaction." 京都大学 (Kyoto University), 2016. http://hdl.handle.net/2433/215292.
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Solomon, Lazarus. "Study of spin-lattice relaxation rates in solids lattice-frame method compared with quantum density-matrix method, and Glauber dynamic /." Master's thesis, Mississippi State : Mississippi State University, 2006. http://library.msstate.edu/etd/show.asp?etd=etd-11062006-114635.
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Zou, Haiyuan. "Tensor renormalization group methods for spin and gauge models." Diss., University of Iowa, 2014. https://ir.uiowa.edu/etd/1420.
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The analysis of the error of perturbative series by comparing it to the exact solution is an important tool to understand the non-perturbative physics of statistical models. For some toy models, a new method can be used to calculate higher order weak coupling expansion and modified perturbation theory can be constructed. However, it is nontrivial to generalize the new method to understand the critical behavior of high dimensional spin and gauge models. Actually, it is a big challenge in both high energy physics and condensed matter physics to develop accurate and efficient numerical algorithms to solve these problems. In this thesis, one systematic way named tensor renormalization group method is discussed. The applications of the method to several spin and gauge models on a lattice are investigated. theoretically, the new method allows one to write an exact representation of the partition function of models with local interactions. E.g. O(N) models, Z2 gauge models and U(1) gauge models. Practically, by using controllable approximations, results in both finite volume and the thermodynamic limit can be obtained. Another advantage of the new method is that it is insensitive to sign problems for models with complex coupling and chemical potential. Through the new approach, the Fisher's zeros of the 2D O(2) model in the complex coupling plane can be calculated and the finite size scaling of the results agrees well with the Kosterlitz-Thouless assumption. Applying the method to the O(2) model with a chemical potential, new phase diagram of the models can be obtained. The structure of the tensor language may provide a new tool to understand phase transition properties in general.
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Flynn, James S. "A study of two quantum spin models on the body-centred cubic lattice." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0028/MQ36440.pdf.
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Chilukuri, Raghu N. "Spin Diffusion Associated with a Quantum Random Walk on a One-Dimensional Lattice." University of Cincinnati / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1406820159.
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Groves, Ronald William. "O17 spin-lattice relaxation solid state NMR studies of pure and doped ices." Columbus, Ohio : Ohio State University, 2002. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1021903674.
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Thesis (Ph. D.)--Ohio State University, 2002.Title from first page of PDF file. Document formatted into pages; contains xx, 128 p.; also contains graphics. Includes abstract and vita. Co-advisors: Charles H. Pennington and James V. Coe, Dept. of Chemistry. Includes bibliographical references (p. 124-128).
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Wiese, Christian. "Investigating new lattice approaches to the momentum and spin structure of the nucleon." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2016. http://dx.doi.org/10.18452/17522.
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Diese Arbeit beschäftigt sich mit der Berechnung von für die Struktur des Nukleons relevanten Observablen, die experimentell durch inklusive und semi-inklusive Streuexperimente bestimmt werden können. Es werden zwei Pilotstudien erörtert, welche die Spin- und Impulsstruktur des Nukleons mithilfe von Gitter-QCD untersuchen. Hierfür wird der Twisted-Mass-Formalismus mit dynamischen Fermionen verwendet, um sicherzustellen, dass die untersuchten Größen einen verbesserten Kontinuumslimes aufweisen. Der erste Teil dieser Arbeit untersucht die Umsetzbarkeit einer Rechnung, die sich mit dem durchschnittlichen Impulsanteil der Gluonen im Nukleon auseinandersetzt. Diese Größe wurde bisher kaum im Rahmen der Gitter-QCD behandelt. In diesem Zusammenhang werden zwei verschiedene Gittermethoden untersucht: das Feynman-Hellman-Theorem, sowie die direkte Berechnung der relevanten Formfaktoren. Mithilfe der zweiten Methode und mehreren Iterationen des Schmierens der Eichlinks ist es möglich, statistisch aussagekräftige Resultate zu erhalten. Die zweite Studie beschäftigt sich mit der direkten Berechnung der vollständigen Impuls- und Spinverteilung von Quarks und Antiquarks im Nukleon. Hierfür wird untersucht, ob eine kürzlich publizierte Methode praktikabel ist, nach der eine räumliche Quasiverteilung zu berechnen und aus dieser die physikalische Verteilung abzuleiten ist. In diesem Zusammenhang wird der Einfluß des Schmierens der Eichlinks und unterschiedlicher Impulsboosts des Nukleons erprobt. Die anschließend berechneten Isovektor-Quarkverteilungen (unpolarisiert und polarisiert) weisen eine gute qualitative Übereinstimmung mit Verteilungen auf, die mithilfe von phänomenologischen Analysen bestimmt wurden. Zentrale Erkenntnis dieser Arbeit ist der Nachweis, dass es auf dem Gitter prinzipiell möglich ist, beide Observablen zu berechnen. Trotzdem muss noch erheblich mehr Arbeit aufgewendet werden, um verlässliche Resultate für diese Größen zu erhalten.This thesis deals with the theoretical computation of nucleon structure observables as they can be experimentally obtained from inclusive and semi-inclusive scattering experiments. I present two exploratory studies on spin and momentum structure observables of the nucleon in the framework of lattice QCD. Throughout this work, I use the twisted mass formalism with dynamical fermions at maximal twist, which ensures an improved continuum limit scaling for the relevant quantities. In the first part, I investigate the feasibility of a lattice calculation of the gluons’ average momentum fraction in the nucleon, a quantity that is rarely studied in lattice QCD. For this purpose, I study two different methods, namely the Feynman-Hellman theorem and the direct computation of the relevant form factor. Applying the latter method and combining it with several steps of stout gauge link smearing, I obtain a statistically significant results for the gluon content. The second study is concerned with the direct computation of the full momentum and spin distribution of quarks and antiquarks within the nucleon. I investigate the feasibility of a recently published approach proposing the computation of a purely spatial quasi-distribution that can be related to the physical distribution. I test the influence of gauge link smearing and different nucleon momentum boosts on the lattice data. Ultimately, I obtain iso-vector quark distributions for the unpolarized and polarize case that featuring a decent qualitative agreement to quark distributions acquired from phenomenological fits. As a key result of this work, I demonstrate that the demanding calculation of gluon content and the novel approach of computing quark distributions directly within lattice QCD are feasible in principle, although significantly more effort has to be invested into obtaining accurate results with reliable uncertainties.
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Andersson, Jonatan. "A lattice model for topological phases." Thesis, Karlstads universitet, Institutionen för ingenjörsvetenskap och fysik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-28281.
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Matter exists in many different phases, for example in solid state or in liquid phase. There are also phases in which the ordering of atoms is the same, but which differ in some other respect, for example ferromagnetic and paramagnetic states. According to Landau's symmetry breaking theory every phase transition is connected to a symmetry breaking process. A solid material has discrete translational symmetry, while liquid phase has continuous translational symmetry. But it has turned out that there also exist phase transitions that can occur without a symmetry breaking. This phenomenon is called topological order. In this thesis we consider one example of a theoretical model constructed on a two dimensional lattice in which one obtains topological order.
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Bunn, Barbara B. "Studies on the preparation and characterization of novel water-soluble catalysts." Diss., This resource online, 1993. http://scholar.lib.vt.edu/theses/available/etd-06062008-170743/.
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Padilla, Pantoja Jessica. "Spin‐lattice coupling in strongly correlated cobalt oxides investigated by synchrotron and neutron techniques." Doctoral thesis, Universitat Autònoma de Barcelona, 2016. http://hdl.handle.net/10803/381262.
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El trabajo que se presenta en esta tesis has sido realizado entre Enero del 2011 y Diciembre del 2015 en el Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), con la colaboración del Sincrotrón ALBA (Cerdanyola del Vallès).
Los óxidos de cobalto presentan numerosas propiedades funcionales y fenómenos físicos de interés como magnetorresistencia gigante, doble intercambio, separación de fases, cambios de estado de espín, transiciones metal-aislante (TMI), alto poder termoeléctrico, difusión de oxígeno, conducción mixta, orden de carga, orbital, de estado de espín o superconductividad, entre otras propiedades. Presentan interés desde un punto de vista fundamental y debido a su potencial aplicación en campos como la espintrónica, los dispositivos termoeléctricos, como materiales para SOFC, catálisis, sensores, etc..
Destaca en algunas cobaltitas la singular habilidad de los iones del cobalto para adoptar diferentes estados de espín. Ello proporciona un grado de libertad adicional, que además favorece el acoplamiento espín-red, y tiene efectos importantes sobre la movilidad electrónica, la respuesta termoeléctrica y catalítica, o la estabilidad estructural y magnética.
Esta tesis investiga la influencia de las inestabilidades del estado de espín en cobaltitas con acoplamiento espín-red. Aborda cuatro clases principales de cobaltitas correlacionadas, con diferentes estructuras, preparadas o fabricadas principalmente (pero no sólo) en forma másica: LnCoO3, Ln0.50Sr0.50CoO3, LnBaCoO5.50 and Ba2Co9O14. Distintas transiciones estructurales, electrónicas, magnéticas, magnetostructurales y metal-aislante han sido investigadas mediante experimentos en fuentes de radiación sincrotrón y de neutrones. Entre ellas, los sincrotrones ALBA (Barcelona), ESRF (Grenoble), SLS ((PSI, Switzerland)) y Helmholtz-Zentrum Berlin-BESSY II (HZB, Berlín), el reactor de neutrones ILL (Grenoble) y la fuente de espalación SINQ (PSI, Switzerland). A partir de experimentos de difracción de polvo mediante neutrones (NPD) y radiación sincrotrón (SXRPD), espectroscopías de absorción y emisión de rayos-X (XAS y XES) y dicroísmo magnético circular (XMCD).
El interés por los sistemas con fórmula general LnCoO3 reside en la naturaleza de las transiciones de estado de espín (SS) y los dos posibles escenarios propuestos para el compuesto de referencia LaCoO3: LS→LS+HS→HS o LS→IS→HS. Nosotros hemos combinado el análisis de datos de absorción XAS con difracción de neutrones para comprender la influencia de la configuración de espín sobre la evolución de las estructuras cristalinas, y su variación en el diagrama de fases aumentando la distorsión de las cobaltitas LnCoO3.
En el caso de la inesperada segunda transición magnética que muestra la cobaltita metálica Pr0.50Sr0.50CoO3 , la resolución de las estructuras cristalinas y magnéticas que aquí se presenta, junto a la characterización del momento orbital, han sido cruciales para entender el mecanismo de la transición magnetoestructural que presenta este compuesto. La síntesis de otros compuestos similares nos ha permitido trazar las propiedades magnéticas y electrónicas de las cobaltitas semi-dopadas ricas en estroncio de la familia Ln0.50(Sr, A)0.50CoO3 (A=Ba, Ca).
Se han examinado cuidadosamente las complejas propiedades de algunas perovskitas dobles laminares con fórmula LnBaCoO5.50 que presentan una transición metal-aislante cerca de temperatura ambiente. YBaCoO5.50 muestra sucesivas transiciones estructurales y magnéticas que están relacionadas con cambios en el estado de espín. A partir de técnicas de difracción y espectroscopías de rayos-X hemos examinado el papel de los cambios en el estado de espín de los iones Co3+ (en coordinación octaédrica y piramidal) como mecanismo para la transición metal-aislante.
La cobaltita de fórmula Ba2Co9O14 presenta una cierta complejidad estructural y propiedades catalíticas y termoeléctricas prometedoras. Hemos encontrado y descrito una fuerte correlación entre el orden magnético singular, los cambios estructurales y la transición aislante-aislante de alta temperatura con cambios del estado del espín en posiciones específicas de los átomos de cobalto durante dicha transición.The work presented in this thesis has been carried out between January of 2011 and December of 2015 at the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), with the collaboration of the ALBA Synchrotron Light Source (Cerdanyola del Vallès). Cobalt oxides present a plethora of remarkable properties and physical phenomena like giant magnetoresistance, double-exchange, phase separation, spin-state changes, metal-insulator transitions (MIT), high thermoelectric power, oxygen diffusivity, mixed-conduction, charge, spin-state and orbital ordering or superconductivity among others. These properties are interesting not only from a fundamental point of view but also due to their potential applicability in different fields (spintronics, thermoelectric devices, materials for SOFCs, catalyst systems, sensors, etc..). A very remarkable characteristic of cobaltites is the almost unic ability of Co ions to adopt different spin states. This makes that Co oxides have, in comparison with other transition metal oxides, an extra degree of freedom, which favors the spin-lattice coupling and plays a prominent role in the electron mobility, the thermoelectric and catalytic responses or the structural and magnetic stability. In this thesis, we have investigated the influence of spin state instabilities on the structural, electrical and magnetic properties of cobalt oxides with spin-lattice coupling. Four main classes of strongly correlated cobaltites having different crystallographic structures were prepared or fabricated (mainly but not solely) in bulk form: LnCoO3, Ln0.50Sr0.50CoO3, LnBaCoO5.50 and Ba2Co9O14. Structural, electronic, magnetic, magnetostructural and metal-insulator transitions were addressed by means of experiments using synchrotron X-rays and neutron sources. Among them, the ALBA (Barcelona), ESRF (Grenoble), SLS (PSI, Switzerland) and Helmholtz-Zentrum Berlin-BESSY II (HZB, Berlín) synchrotrons, the ILL neutron reactor (Grenoble) and the SINQ (PSI, Switzerland) spallation source, performing Neutron Powder Diffraction (NPD), Synchrotron X-Ray Powder Diffraction (SXRPD), X-ray Absorption and Emission Spectroscopies (XAS and XES), and X-ray Magnetic Circular Dichroism (XMCD). The interest for LnCoO3 arises from the nature of the two stages spin-state (SS) transition and the possible scenarios proposed for the LaCoO3 reference compound: LS→LS+HS→HS or LS→IS→HS. We have combined the analysis of XAS and NPD to understand the influence of the espín configuration on the crystalline structure evolution, and its variation in the phase diagram by increasing the distortion of LnCoO3 compounds. In the case of the low temperature unexpected second magnetic transition in the FM metal Pr0.50Sr0.50CoO3, the resolution of the crystal and magnetic structures we here provide together with the orbital moment characterization have been crucial to understand the mechanism of the magnetostructural transition in this compound. The synthesis of compounds with similar composition has allowed us to draw an overview of the magnetic and electronic properties of the half-doped Ln0.50(Sr, A)0.50CoO3 cobaltites family (A=Ba, Ca). The complex properties of some double-layered LnBaCoO5.50 perovskites presenting a metal-insulator transition close to room temperature were carefully examined. YBaCoO5.50 exhibits successive concomitant structural and magnetic transitions coupled to spin-state changes. The influence of the changes in the spin state of Co3+ ions as the mechanism for the metal-insulator transition has been described for Co atoms in octahedral and pyramidal coordination using X-ray spectroscopies and diffraction techniques. The charge-ordered Ba2Co9O14 cobaltite presents structural complexity and promising catalytic and thermoelectric properties. We have described a close correlation between its singular magnetic order, structural changes and the insulator-insulator transition at high temperature, and the change in the spin-state of specific Co sites at this transition.
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Riegger, Luis [Verfasser], and Immanuel [Akademischer Betreuer] Bloch. "Interorbital spin exchange in a state-dependent optical lattice / Luis Riegger ; Betreuer: Immanuel Bloch." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2019. http://d-nb.info/1184202729/34.
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Shen, Lingjia. "Spin and lattice structures in materials with competing interactions investigated by neutron scattering techniques." Thesis, University of Birmingham, 2016. http://etheses.bham.ac.uk//id/eprint/7045/.
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Three magnetic materials, including γ-CoV\(_2\)O\(_6\),Pr\(_0\)\(_.\)\(_5\)Ca\(_0\)\(_.\)\(_5\)Mn\(_0\)\(_.\)\(_9\)\(_7\)Ga\(_0\)\(_.\)\(_0\)\(_3\)O\(_3\), and Y\(_2\)(Cr\(_1\)\(_-\)\(_χ\)Ga\(_χ\)\(_-\)\(_0\)\(_.\)\(_5\)Sb\(_0\)\(_.\)\(_5\))\(_2\)O\(_7\)(χ = 0.5, 0.6, 0.7, 0.8, 0.9), in which the magnetic or crystallographic structure shows strong correlations with the competing interactions (e.g. electron-lattice coupling, spin exchange interactions, single-ion anisotropy), have been explored by the neutron diffraction and small angle neutron scattering techniques. For the quasi-one-dimensional magnet γ-CoV\(_2\)O\(_6\), our neutron diffraction measurements have revealed a spatially inhomogeneous magnetic ground state, i.e. phase separation, below T\(_N\) = 6.6 K. For the strained manganese perovskite Pr\(_0\)\(_.\)\(_5\)Ca\(_0\)\(_.\)\(_5\)Mn\(_0\)\(_.\)\(_9\)\(_7\)Ga\(_0\)\(_.\)\(_0\)\(_3\)O\(_3\), we have observed the decoupling of magnetic field induced carrier delocalization and para-ferromagnetic transitions. Concomitantly, distinct responses of the crystallographic structure to these two transitions have also been established. For the diluted pyrochlores Y\(_2\)(Cr\(_1\)\(_-\)\(_χ\)Ga\(_χ\)\(_-\)\(_0\)\(_.\)\(_5\)Sb\(_0\)\(_.\)\(_5\))\(_2\)O\(_7\), we cannot detect any magnetic long range order in Y\(_2\)CrSbO\(_7\) (χ = 0.5), although it is well below the nonmagnetic site percolation limit (χ\(_c\) = 0.61). We propose that the magnetism of Y\(_2\)CrSbO\(_7\) is governed by the bond disorder, caused by the weak ionic size mismatch between Cr\(^3\)\(^+\) and Sb\(^5\)\(^+\) and percolates at χ = 0.24, so that a spin-freezing transition is expected at very low temperatures according to the latest spin glass theories.
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Mitchell, Chad A. "Besign-directed measurements of B1 heterogeneity and spin-lattice relaxation for 8 Tesla MRI." The Ohio State University, 2004. http://rave.ohiolink.edu/etdc/view?acc_num=osu1095684374.
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Mitchell, Chad Alan. "Design-directed measurements of B1 heterogeneity and spin-lattice relaxation for 8 Tesla MRI." Connect to this title online, 2004. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1095684374.
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Thesis (Ph. D.)--Ohio State University, 2004.Title from first page of PDF file. Document formatted into pages; contains xxx, 256 p.; also includes graphics. Includes bibliographical references (p. 186-193).
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Nakamura, Yasuyuki. "Existence of a compensation temperature of a mixed spin-2 and spin-5/2 Ising ferrimagnetic system on a layered honeycomb lattice." The American Physical Society, 2000. http://hdl.handle.net/2237/7150.
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Halász, Gábor B. "Doping a topological quantum spin liquid : slow holes in the Kitaev honeycomb model." Thesis, University of Oxford, 2015. https://ora.ox.ac.uk/objects/uuid:928ba58d-c69c-4e85-8d49-677d7e9c0fdc.
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We present a controlled microscopic study of hole dynamics in both a gapped and a gapless quantum spin liquid. Our approach is complementary to previous phenomenological works on lightly doped quantum spin liquids as we introduce mobile holes into the ground state of the exactly solvable Kitaev honeycomb model. In the spatially anisotropic (Abelian) gapped phase of the model, we address the properties of a single hole [its internal degrees of freedom as well as its hopping properties], a pair of holes [their absolute and relative particle statistics as well as their interactions], and the collective state for a finite density of holes. Our main result is that the holes in the doped model possess internal degrees of freedom as they can bind the fractional excitations of the undoped model and that the resulting composite holes with different excitations bound are distinct fractional particles with fundamentally different single-particle properties and different experimental signatures in the multi-particle ground state at finite doping. For example, some hole types are free to hop in two dimensions, while others are confined to hop in one dimension only. Also, distinct hole types have different particle statistics and, in particular, some of them exhibit non-trivial (anyonic) relative statistics. At finite doping, the respective hopping dimensionalities manifest themselves in an electrical conductivity that is either approximately isotropic or extremely anisotropic. In the gapless phase of the model, we consider a single hole and address the possibility of a coherent quasiparticle description. Our main result is that a mobile hole has a finite quasiparticle weight which vanishes in the stationary limit. Although this result is obtained in terms of an approximate variational state, we argue that it is also applicable for the exact ground state of the doped model.
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Le, Gros Mark. "NMRON studies of insulating magnetic materials." Thesis, University of British Columbia, 1990. http://hdl.handle.net/2429/30569.
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Selective excitation pulsed NMRON, CW-NMRON and Thermal NMR methods have been used to study the low temperature ⁵⁴Mn nuclear spin-lattice relaxation mechanisms in magnetic insulators.
The selective single and double quantum excitation sequences have been used for the first time in NMRON to obtain single and double quantum rotation patterns, Free Induction Decays, Hahn spin echoes and pulsed T₁ measurements.
Two insulating magnets have been studied; MnCl₂.4H₂O and Mn(COOCH₃ )₂ .4H₂O. In the ⁵⁴Mn-MnCl₂ .4H₂O system the temperature dependence of the ⁵⁴Mn spin-lattice relaxation time at zero field was measured between 35 mK and 90 mK and it was found that the dominant relaxation process between 65 mK and 90 mK is an electronic magnon Raman process and below 65 mK a direct relaxation process dominates.
Single and double quantum Free Induction Decays and Hahn spin echoes have been used to determine the magnitude and nature of the spin-spin relaxation mechanism for ⁵⁴Mn oriented in MnCl₂.4H₂O at zero applied field.
NMRON was observed for the first time in the paramagnetic phase of MnCl₂.4H₂O. The resonance lines are inhomogeneously broadened and 300 kHz wide. A value of <⁵⁴AS>/h=-513.6(3) MHz has been determined for the paramagnetic phase hyperfine coupling constant, and this value has been used to determine the zero point spin deviation of the antiferromagnetic phase.
The field and temperature dependence of the ⁵⁴Mn T₁ was measured for values of field above the spin flop paramagnetic phase transition and a field dependent T₁ minimum was discovered at Ba=2.64 T.
For the ⁵⁴Mn-Mn(COOCH₃) .4H₂O system two ⁵⁴Mn resonances have been observed and the value of the hyper fine coupling constants for the two sites were found to be <⁵⁴AS>/h=-435 (1) MHz for the ⁵⁴Mn1 site and <⁵⁴AS>/h=-478(1) MHz for the ⁵⁴Mn2 site. The high field spin-lattice relaxation behavior has also been investigated and a T₁ minimum at Ba =2.74 T analogous to that observed in MnCl₂ .4H₂O was discovered.
A Hahn echo study of the low field single quantum spin-spin relaxation processes has been performed and anomalous behavior of the spin echo amplitude revealed.Science, Faculty of
Physics and Astronomy, Department of
Graduate
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Bookjans, Eva M. "Relative number squeezing in a Spin-1 Bose-Einstein condensate." Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/37148.
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The quantum properties of matter waves, in particular quantum correlations and entanglement are an important frontier in atom optics with applications in quantum metrology and quantum information. In this thesis, we report the first observation of sub-Poissonian fluctuations in the magnetization of a spinor 87Rb condensate. The fluctuations in the magnetization are reduced up to 10 dB below the classical shot noise limit. This relative number squeezing is indicative of the predicted pair-correlations in a spinor condensate and lay the foundation for future experiments involving spin-squeezing and entanglement measurements. We have investigated the limits of the imaging techniques used in our lab, absorption and fluorescence imaging, and have developed the capability to measure atoms numbers with an uncertainly < 10 atoms. Condensates as small as ≈ 10 atoms were imaged and the measured fluctuations agree well with the theoretical predictions. Furthermore, we implement a reliable calibration method of our imaging system based on quantum projection noise measurements. We have resolved the individual lattice sites of a standing-wave potential created by a CO2 laser, which has a lattice spacing of 5.3 µm. Using microwaves, we site-selectively address and manipulate the condensate and therefore demonstrate the ability to perturb the lattice condensate of a local level. Interference between condensates in adjacent lattice sites and lattice sites separated by a lattice site are observed.
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Schweizer, Christian [Verfasser], and Immanuel [Akademischer Betreuer] Bloch. "Minimal instances for ℤ₂ lattice gauge theories and spin pumps / Christian Schweizer ; Betreuer: Immanuel Bloch." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2019. http://d-nb.info/1196529140/34.
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Yu, QinQin. "Characterization of a two-color magneto-optical trap for a spin-squeezed optical lattice clock." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/100323.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2015.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 57-58).
We experimentally realize a magneto-optical trap (MOT) for ytterbium that simultaneously scatters light from two atomic transitions, in order to reduce the minimum magnetic field gradient required for trapping atoms. From the experimental data, we propose a model of the MOT where one transition is responsible for trapping the atoms the other transition is responsible for cooling the atoms. In order to test this model and to better understand the dynamics of this two-color MOT, we perform numerical simulations of the contributions of each transition to the steady-state number of trapped atoms. We then compare the results of the simulations with the experimental results and conclude that the model of the separated trapping and cooling functions is a good model for the two-color MOT. Finally, we examine other elements for which the two-color MOT is possible and derive a condition for the linewidths of the two transitions such that the atoms are trapped.
by QinQin Yu.
S.B.
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Ko, Wing-Ho. "Novel magnetic and superconducting states in spin-1/2 kagome lattice and its doped variant." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/68976.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 121-128).
Quantum spin liquids, which are quantum ground states of interacting spin systems that break no symmetries, have long been a fascination among the theoretical condensed matter community. After years of experimental searches, several promising candidates finally emerged, including herbertsinithite ZnCu3 (OH)6 Cl2 , which can be modeled as a spin-1/2 kagome lattice. Theoretically, the U(1) Dirac spin liquid (U(1) DSL) state is shown to be a plausible description of the system, and previous works have indicate that this particular quantum spin liquid state may enjoy a host of interesting properties, such as the power-law decay of correlation functions, the existence of spin-1/2 excitations known as the spinon, and the existence of an emergent U(1) gauge field. In this thesis, after the relevant motivation and background information are discussed, I shall present my work on the spin-1/2 kagome lattice that built upon the U(1) DSL state. First, I shall present the theoretical study of Raman scattering in the U(1) DSL state, which shows that in all symmetry channels the Raman intensity profiles contain broad continua that display power-law behaviors at low energy, which can be attributed to the excitations of spinon-antispinon pairs. In, addition, for the A2g channel, the Raman profile also contains a characteristic 1/o singularity, which arise from an excitation of the emergent U(1) gauge field. The possibility of more clearly observing the signature of this U(1) emergent gauge field in resonant inelastic X-ray scattering (RIXS) is also discussed. Next, I shall consider the case when the spin-1/2 kagome lattice is subjected to an external magnetic field, in which a state with an additional uniform amount of gauge flux of top of the U(1) DSL ansatz, which results in the formation of Landau levels in the spinon spectrum, is shown to be energetically favorable. Unlike the usual quantum Hall system, the Landau level state is shown to contain a gapless S2 density mode, which in turns indicate that system is XY ordered in the plane perpendicular to the applied magnetic field. Third, I shall consider the case in which the spin-1/2 kagome lattice is hole-doped. Similar to the B-field case, a Landau-level state is shown to be energetically favorable, in which a gapless charge density mode is shown to exists, and which through the Anderson-Higgs mechanism causes the system to become a superconductor. This resulting superconductor is then shown to be exotic, in the sense that it contains minimal vortices having a flux of hc/4e, as well as bosonic quasiparticles that have semionic mutual statistics. The thesis concludes with a short summary and outlook.
by Wing-Ho Ko.
Ph.D.
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Morais, Alysson Ferreira. "Uma abordagem tensorial para o estudo de dualidades entre modelos de spin na rede." Universidade de São Paulo, 2014. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-14112014-130241/.
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Neste trabalho, estudamos as dualidades entre modelos de spin em redes bidimensionais a partir de uma abordagem tensorial. Nessa abordagem, componentes de tensores são associadas aos vértices e arestas da rede de forma que a função de partição Z é construída a partir da contração dos índices dessas componentes e é, portanto, um escalar por mudanças de base da álgebra de grupo C[G] utilizada para a definição dos tensores. A partir daí, e observando que a forma das componentes fixam o modelo estudado, obtemos um modelo diferente para cada mudança de base proposta. Esses diferentes modelos possuirão, no entanto, a mesma função de partição, já que esta é um invariante sob tais transformações. De fato, haverá uma infinidade de modelos todos duais entre si. Neste ponto, fixamos nossa atenção nos modelos com spin Zn, nos quais estão incluídos o modelo de Ising, o modelo de Potts e o modelo de Ashkin-Teller-Potts. Explorando uma transformação de base específica, fomos capazes de rederivar a dualidade de Kramers e Wanniers para o modelo de Ising. Usando argumentos análogos, mostramos também que os modelos de Potts com n = 3 e 4 são autoduais e que não existe autodualidade para este modelo com n _ 5. O modelo de Ashkin-Teller-Potts foi mostrado ser autodual para todo n 2 N.In this work, we study the dualities between spin models in two-dimensional lattices from a tensorial approach. In this approach, we associate tensor components to the vertices and links so that the partition function Z is constructed by a contraction of the indices of the tensor components thereby making Z a scalar under change of basis of the group algebra C[G] used to de_ne the tensors. Having obtained this, and noting that the values of the components _x the studied model, we obtain a di_erent model for each basis transformation proposed. These di_erent models, however, have the same partition function since Z is invariant under these transformations. In fact we can obtain several models all dual to each other in this manner. We then focus on Zn spin models, which include the Ising model, the Potts model and Ashkin- Teller-Potts model. Exploring a speci_c basis transformation, we are able to rederive Kramers and Wanniers\' duality for the Ising model. With analogous arguments, we also show that Potts models with n = 3 and n = 4 are self-dual whereas this property is lost for n _ 5. The Ashkin-Teller-Potts model is shown to be self-dual for all n 2 N.
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Merdan, Mohammad Ghanim Merdan. "Study of the excited states of the quantum antiferromagnets." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/study-of-the-excited-states-of-the-quantum-antiferromagnets(dfd01ec6-806c-464a-9b68-910217679c3c).html.
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We investigate the quantum dynamics of the spins on different Heisenberg antiferromagnetic spin lattice systems. Firstly, we applied the coupled-cluster method to the spin-1/2 antiferromagnetic XXZ model on a square lattice by employing an approximation which contains two-body long-range correlations and high-order four-body local correlations. Improvement is found for the ground-state energy, sublattice magnetization, and the critical anisotropy when comparing with the approximation including the two-body correlations alone. We also obtain the full excitation spectrum which is in good agreement with the quantum Monte Carlo results and the high-order spin-wave theory. Secondly, we study the longitudinal excitations of quantum antiferromagnets on a triangular lattice by a recently proposed microscopic many-body approach based on magnon-density waves. We calculate the full longitudinal excitation spectra of the antiferromagnetic Heisenberg model for a general spin quantum number in the isotropic limit. Similar to the square lattice model, we find that, at the center of the first hexagonal Brillouin zone Γ(q=0) and at the magnetic ordering wavevectors ±[Q= (4π/3,0)], the excitation spectra become gapless in the thermodynamic limit, due to the slow, logarithmic divergence of the structure factor. However, these longitudinal modes on two-dimensional models may be considered as quasi-gapped, as any finite-size effect or small anisotropy will induce a large energy gap, when compared with the counterpart of the transverse spin-wave excitations. We have also investigated the excited states of the quasi-one-dimensional quantum antiferromagnets on hexagonal lattices, including the longitudinal modes based on the magnon-density waves. A model Hamiltonian with a uniaxial single-ion anisotropy is first studied by a spin-wave theory based on the one-boson method; the ground state thus obtained is employed for the study of the longitudinal modes. The full energy spectra of both the transverse modes (i.e., magnons) and the longitudinal modes are obtained as functions of the nearest-neighbor coupling and the anisotropy constants. We have found two longitudinal modes due to the non-collinear nature of the triangular antiferromagnetic order, similar to that of the phenomenological field theory approach by Affleck. The excitation energy gaps due to the anisotropy and the energy gaps of the longitudinal modes without anisotropy are then investigated. We then compares our results for the longitudinal energy gaps at the magnetic wavevectors with the experimental results for several antiferromagnetic compounds with both integer and non-integer spin quantum numbers, and we find good agreements after the higher-order contributions are included in our calculations.
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Foster, Martyn Stuart. "Adjoint sources, disconnected loops and other fruit of lattice QCD." Thesis, University of Liverpool, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.366408.
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