Dissertations / Theses: 'Theoretical Condensed matter Physics'

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Govind, Niranjan. "Theoretical study of models for driven interface dynamics." Thesis, McGill University, 1992. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=56667.

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In this dissertation, we review the physics associated with surfaces and interfaces in equilibrium and non-equilibrium. Our emphasis will be on interfaces that are driven far away from equilibrium with special interest in the phenomenon of kinetic roughening. Models which describe non-equilibrium interfaces will be introduced and analyzed using techniques such as the Renormalization Group, Monte Carlo simulations, and direct integration of the equation of motion. Different interface relaxation mechanisms will be discussed with a focus on surface diffusion, which is believed to be the dominant effect in Molecular Beam Epitaxy. These interface growth models generate self-affine structures with various correlations satisfying a dynamic scaling law. We compute the scaling exponents and functions. Finally, we study the effect of quenched impurities on the dynamics of a driven interface with a conservation law. The impurity effect leads to anomalous scaling exponents and qualitatively changes the interface dynamics. Our results are summarized in two articles to be published: Refs. (Govind and Guo, 1992; Govind, Guo and Grant, 1992).

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Pomorski, Pawel. "Theoretical study of conductance, capacitance and transport properties of nanostructures." Thesis, McGill University, 2002. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=84414.

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In the past two decades, significant progress in constructing physical systems of reduced dimensionality has been made. In these systems, quantum effects are observed in the electric current response to an applied bias voltage. Considerable theoretical effort has been made to understand conductance and capacitance which characterize this response. In addition to a fundamental science interest, there has been a significant technological need to build and understand small scale devices in order to maintain the current rate of progress in increasing computer performance.
In this thesis, we theoretically investigated the conductance and capacitance of mesoscopic and molecular scale systems. Our approach incorporated Landauer-Buttiker transport theory.
We developed a highly efficient method, based on the solution of the time-dependent Schrodinger equation incorporating a magnetic field, to solve the quantum scattering problem in mesoscopic nanostructures. We studied linear response capacitance in a two plate mesoscopic capacitor with one plate a quantum conductor in the ballistic scattering regime. By determining the scattering wavefunctions in the quantum plate, we were able to obtain relevant densities of states and use them to self-consistently calculate capacitance matrix coefficients for the system. We find the capacitance to be highly dependent on the external magnetic field and the number of probes attached to the quantum conductor.
To study molecular scale systems, our approach was based on Density Functional Theory within Local Density Approximation and non-equilibrium Green's functions, implemented in the simulation package McDcal. For the work in this thesis, we modified McDcal to run on parallel computer architectures. We studied the current-voltage characteristics of silicon cage nanowires sandwiched between aluminum electrodes. We successfully analyzed our results using the complex band structure of the nanowire. Finally, we studied the capacitance properties of carbon nanotube junctions. In junctions with tubes so far apart that their wavefunctions do not overlap, we studied the variation in capacitance for different relative tube positions and radii. We also studied junctions where the nanotubes are in contact but in which there is no current due to a conductance gap. In this system we find an enhancement in the value of capacitance.

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Hashi, Ryan. "Realistic effects on the electron Wigner crystal energy in the quantum Hall regime." Thesis, California State University, Long Beach, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=1591583.

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Electron systems in the quantum Hall regime change from a liquid state to a Wigner crystal state as the filling factor is lowered below approximately 1/5. This phase transition can be studied with theoretical methods by comparing the ground-state energies of the quantum liquid and the quantum Wigner crystal. Past studies have not included realistic effects such as finite thickness, Landau-level mixing, and disorder on the electron system. We expand upon the classic work by Maki and Zotos to calculate Wigner crystal energies that include a finite thickness of the two-dimensional electron lattice.

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Acharya, Pramod. "Charge regulation of a surface immersed in an electrolyte solution." Thesis, Florida Atlantic University, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10172666.

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In this thesis, we investigate theoretically a new model of charge regulation of a single charged planar surface immersed in an aqueous electrolyte solution. Assuming that the adsorbed ions are mobile in the charged plane, we formulate a field theory of charge regulation where the numbers of adsorbed ions can be determined consistently by equating the chemical potentials of the adsorbed ions to that of the ions in the bulk. We analyze the mean-field treatment of the model for electrolyte of arbitrary valences, and then beyond, where correlation effects are systematically taken into account in a loop expansion. In particular, we compute exactly various one-loop quantities, including electrostatic potentials, ion distributions, and chemical potentials, not only for symmetric (1; 1) electrolyte but also for asymmetric (2; 1) electrolyte, and make use of these quantities to address charge regulation at the one-loop level. We find that correlation effects give rise to various phase transitions in the adsorption of ions, and present phase diagrams for (1; 1) and (2; 1) electrolytes, whose distinct behaviors suggest that charge regulation, at the one-loop level, is no longer universal but depends crucially on the valency of the ions.

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Giomi, Luca. "Unordinary order a theoretical, computational and experimental investigation of crystalline order in curved space /." Related electronic resource: Current Research at SU : database of SU dissertations, recent titles available full text, 2009. http://wwwlib.umi.com/cr/syr/main.

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Ben-Shach, Gilad. "Theoretical Considerations for Experiments to Create and Detect Localised Majorana Modes in Electronic Systems." Thesis, Harvard University, 2015. http://nrs.harvard.edu/urn-3:HUL.InstRepos:14226093.

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This thesis presents our work on building a bridge between the theoretical proposals for the condensed matter realisation of peculiar localised excitations, known as Majorana modes, and experiments to search for them. The main focus in the first two sections is on charge sensing of localised Majorana modes in two distinct systems. First, we address the properties of charged quasiparticles in the $\nu=5/2$ fractional quantum Hall regime. In particular, we focus on the case where these particles are trapped by disorder, often in close proximity to one another. Next, we consider one-dimensional semi-conducting wires with strong spin-orbit coupling and proximity-induced superconductivity. The Majorana modes in this system are predicted to be charge-neutral. We show, however, that when the wire is short enough, there is a uniform charge distribution along the wire, and we show how the presence of this charge depends on system parameters. A third portion is related to HgTe quantum wells, another system predicted to host Majorana modes when coupled to a superconductor. Here we consider a HgTe well in the metallic regime, coupled to two superconducting strips. We compute the Josephson coupling in the presence of spin-orbit interactions and in-plane external magnetic fields.

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Moore, Christopher Paul. "Tunneling Transport Phenomena in Topological Systems." Thesis, Clemson University, 2019. http://pqdtopen.proquest.com/#viewpdf?dispub=13420479.

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Originally proposed in high energy physics as particles, which are their own anti-particles, Majorana fermions have never been observed in experiments. However, possible signatures of their condensed matter analog, zero energy, charge neutral, quasiparticle excitations, known as Majorana zero modes (MZMs), are beginning to emerge in experimental data. The primary method of engineering topological superconductors capable of supporting MZMs is through proximity-coupled semiconductor nanowires with strong Rashba spin-orbit coupling and an applied magnetic field. Recent tunneling transport experiments involving these materials, known as semiconductor-superconductor heterostructures, were capable for the first time of measuring quantized zero bias conductance plateaus, which are robust over a range of control parameters, long believed to be the smoking gun signature of the existence of MZMs. The possibility of observing Majorana zero modes has garnered great excitement within the field due to the fact that MZMs are predicted to obey non-Abelian quantum statistics and therefore are the leading candidates for the creation of qubits, the building blocks of a topological quantum computer. In this work, we first give a brief introduction to Majorana zero modes and topological quantum computing (TQC). We emphasize the importance that having a true topologically protected state, which is not dependent on local degrees of freedom, has with regard to non-Abelian braiding calculations. We then introduce the concept of partially separated Andreev bound states (ps-ABSs) as zero energy states whose constituent Majorana bound states (MBSs) are spatially separated on the order of the Majorana decay length. Next, through numerical calculation, we show that the robust 2 e²/h zero bias conductance plateaus recently measured and claimed by many in the community to be evidence of having observed MZMs for the first time, can be identically created due to the existence of ps-ABSs. We use these results to claim that all localized tunneling experiments, which have been until now the main way researchers have tried to measure MZMs, have ceased to be useful. Finally, we outline a two-terminal tunneling experiment, which we believe to be relatively straight forward to implement and fully capable of distinguishing between ps-ABSs and true topologically protected MZMs.

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Damewood, Liam James. "Theoretical Models of Spintronic Materials." Thesis, University of California, Davis, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3602035.

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In the past three decades, spintronic devices have played an important technological role. Half-metallic alloys have drawn much attention due to their special properties and promised spintronic applications. This dissertation describes some theoretical techniques used in first-principal calculations of alloys that may be useful for spintronic device applications with an emphasis on half-metallic ferromagnets. I consider three types of simple spintronic materials using a wide range of theoretical techniques. They are (a) transition metal based half-Heusler alloys, like CrMnSb, where the ordering of the two transition metal elements within the unit cell can cause the material to be ferromagnetic semiconductors or semiconductors with zero net magnetic moment, (b) half-Heusler alloys involving Li, like LiMnSi, where the Li stabilizes the structure and increases the magnetic moment of zinc blende half-metals by one Bohr magneton per formula unit, and (c) zinc blende alloys, like CrAs, where many-body techniques improve the fundamental gap by considering the physical effects of the local field. Also, I provide a survey of the theoretical models and numerical methods used to treat the above systems.

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Lu, Zijun. "Theoretical and Numerical Analysis of Phase Changes in Soft Condensed Matter." Case Western Reserve University School of Graduate Studies / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=case15620007885239.

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Green, Timothy Frederick Goldie. "Prediction of NMR J-coupling in condensed matter." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:39ee4a7c-58f9-49fa-b14c-16bc03141e53.

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Nuclear magnetic resonance (NMR) is a popular spectroscopic method and has widespread use in many fields. Recent developments in solid-state NMR have increased interest in experiment and, alongside simultaneous developments in computational theory, have led to the field dubbed 'NMR crystallography.' This is a suite of methodologies, complementing the capabilities of other crystallographic methods in the determination of atomic structure, especially when large crystals cannot be made and when exploring materials with phenomena such as compositional, positional and dynamic disorder. NMR J-coupling is the indirect coupling between nuclear spins, which, when measured, can reveal a wealth of information about structure and bonding. This thesis develops and applies the method of Joyce for the prediction of NMR J-coupling in condensed matter systems using plane-wave pseudopotential density-functional theory, an important requirement for efficient treatment of finite and infinite periodic systems. It describes the first-ever method for the use of ultrasoft pseudopotentials and inclusion of special relativistic effects in J-coupling prediction, allowing for the treatment of a wider range of materials systems and overall greater user friendliness, thus making the method more accessible and attractive to the wider scientific community.

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Marten, Tobias. "Theoretical Considerations of Local Environment Effects in Alloys." Doctoral thesis, Linköpings universitet, Teoretisk Fysik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-63443.

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This thesis is devoted to a theoretical study of local environment effects in alloys. A fundamental property of a disordered system is that all chemically equivalent atoms are different due to their different chemical environments, in contrast to an ideal periodic solid where all the atoms that occupy equivalent positions in the crystal have exactly the same physical properties. The local environment effects have been largely ignored in earlier theories of disordered systems, that is the system has been treated as a whole and average properties have been derived. Moreover, inhomogeneous systems, such as surfaces and interfaces, induce local environment effects that are not necessarily present in the bulk. The importance and presence of local environment effects are illustrated by calculating observable physical properties in various systems. In particular, by employing the complete screening picture the effects of local environments on the core-level binding energy shifts as well as Auger shifts in random alloys are in- vestigated. This so-called disorder broadening effect has recently been observed experimentally. It is shown that there are different contributions to the disorder broadening that vary with the local chemical environment. Furthermore, the influ- ence of inhomogeneous lattice distortions on the disorder broadening of the core- level photoemission spectra are considered for systems with large size-mismatch between the alloy components. The effects of local chemical environments on physical properties in magnetic systems are illuminated. A noticeable variation in the electronic structure, local magnetic moments and exchange parameters at different sites is obtained. This reflects the sensitivity to different chemical environments and it is shown to be of qualitative importance in the vicinity of magnetic instability. The local environment effects due to the presence of surfaces and interfaces are also considered. The effect is explicitly studied by considering the concentration profile of a thin Ag-Pd film deposited on a Ru substrate. Two computational approaches are utilized to calculate the relative composition in each layer of the thin film as a function of temperature in a theoretically consistent way. It is shown that, opposed to the situation in the bulk, where a complete solubility between Ag and Pd takes place, a non-uniform distribution of the alloy components across the film is observed. In another study it is investigated whether the presence of TiN interfaces changes the dynamical and thermodynamic stability of B1 SiN. Phonon calcula- tions show that TiN interfaces have a stabilization effect on the lattice dynamics. On the other hand, calculations of the Si vacancy formation energy show that the structures are unstable with respect to composition variations.

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Huang, Shuo. "Theoretical Investigations of High-Entropy Alloys." Licentiate thesis, KTH, Tillämpad materialfysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-218162.

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High-entropy alloys (HEAs) are composed of multi-principal elements with equal or near-equal concentrations, which open up a vast compositional space for alloy design. Based on first-principle theory, we focus on the fundamental characteristics of the reported HEAs, as well as on the optimization and prediction of alternative HEAs with promising technological applications. The ab initio calculations presented in the thesis confirm and predict the relatively structural stability of different HEAs, and discuss the composition and temperature-induced phase transformations. The elastic behavior of several HEAs are evaluated through the single-crystal and polycrystalline elastic moduli by making use of a series of phenomenological models. The competition between dislocation full slip, twinning, and martensitic transformation during plastic deformation of HEAs with face-centered cubic phase are analyzed by studying the generalized stacking fault energy. The magnetic moments and magnetic exchange interactions for selected HEAs are calculated, and then applied in the Heisenberg Hamiltonian model in connection with Monte-Carlo simulations to get further insight into the magnetic characteristics including Curie point. The Debye-Grüneisen model is used to estimate the temperature variation of the thermal expansion coefficient. This work provides specific theoretical points of view to try to understand the intrinsic physical mechanisms behind the observed complex behavior in multi-component systems, and reveals some opportunities for designing and optimizing the properties of materials

QC 20171127

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Sensarma, Rajdeep. "A theoretical study of strongly interacting superfluids and superconductors." Columbus, Ohio : Ohio State University, 2007. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1186581954.

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Mehrez, Hatem. "Theoretical study of nano structures and molecular electronic systems." Thesis, McGill University, 2001. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=38237.

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Research studies on systems with reduced size and dimensionality have attracted great attention for the past two decades. This is mainly driven by industrial development, which demands the fabrication of new, small, well-controlled devices as well as the desire to understand quantum effects which manifest in these small structures.
In this thesis we theoretically investigate quantum coherent transport properties of nano structures in the form of molecular electronic systems. Our approach is based on Landauer-Buttiker transport theory. However, the details of the method depend on the interaction complexity.
We have carried out detailed analysis on finite length carbon nanotubes based magnetic tunnel junction using tight binding atomic model and Green's function approach. This device shows clear spin valve effect even when contacted with the same ferro-magnetic material with a long spin scattering length. In addition to this, transport at the atomic level is highly affected by the molecular states resulting in conductance oscillation of finite size arm-chair carbon nanotube as a function of its length.
When short carbon nanotubes are weakly contacted to external leads, they act as quantum dots with strong interaction at the molecular scale. To analyse these systems, we have developed a many-body wave function formalism which include spin degeneracy. This approach clearly shows the strong dependence of the device electronic response on the number of electrons already inside the tube.
Finally, we have carried out ab initio analysis based on Density Functional Theory within Local Density Approximations to investigate the current-voltage (I-V) characteristics of various gold nanowires. Our results demonstrate that transport properties of these systems crucially depend on the electronic properties of the scattering region, the leads, and most importantly the interaction of the scattering region with the leads. For ideal, clean Au contacts, the theoretical results indicate a linear I-V behavior. However, when sulfur impurities exist at the contact junction, nonlinear I-V curves emerge due to a tunnelling barrier established in the presence of the S atom. The most striking observation is that even a single S atom can cause a qualitative change of the I-V curve from linear to nonlinear.
Our theoretical results were compared to experimental data, qualitative and sometimes quantitative understanding of the experiments are obtained.

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Hutzel, William D. "Particle-Hole Symmetry Breaking in the Fractional Quantum Hall Effect at nu = 5/2." Thesis, California State University, Long Beach, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10841528.

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The fractional quantum Hall effect (FQHE) in the half-filled second Landau level (filling factor ν = 5/2) offers new insights into the physics of exotic emergent quasi-particles. The FQHE is due to the collective interactions of electrons confined to two-dimensions, cooled to sub-Kelvin temperatures, and subjected to a strong perpendicular magnetic field. Under these conditions a quantum liquid forms displaying quantized plateaus in the Hall resistance and chiral edge flow. The leading candidate description for the FQHE at 5/2 is provided by the Moore-Read Pfaffian state which supports non-Abelian anyonic low-energy excitations with potential applications in fault-tolerant quantum computation schemes. The Moore-Read Pfaffian is the exact zero-energy ground state of a particular three-body Hamiltonian and explicitly breaks particle-hole symmetry. In this thesis we investigate the role of two and three body interaction terms in the Hamiltonian and the role of particle hole symmetry (PHS) breaking at ν = 5/2. We start with a PHS two body Hamiltonian (H₂) that produces an exact ground state that is nearly identical with the Moore-Read Pfaffian and construct a Hamiltonian H(α) = (1 – α)H₃ + α H₂ that tunes continuously between H₃ and H₂. We find that the ground states, and low-energy excitations, of H₂ and H₃ are in one-to-one correspondence and remain adiabatically connected indicating they are part of the same universality class and describe the same physics in the thermodynamic limit. In addition, evidently three body PHS breaking interactions are not a crucial ingredient to realize the FQHE at 5/2 and the non-Abelian quasiparticle excitations.

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Tan, Zhiming Darren. "Frustrated magnetism in the extended kagome lattice." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:151fb421-198b-44b5-9f0d-8b35333f6450.

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The extended kagome lattice, composed of alternating kagome and triangular layers, provides a novel geometry for frustrated magnetism. In this thesis, we study the properties of Heisenberg spins with nearest-neighbour antiferromagnetic interactions on this lattice. In common with many other models of frustrated magnets, this system has highly degenerate classical ground states. It is set apart from other examples, however, by the strong interlayer correlations between triangular layer spins. We study the implications of such correlations in both the statics and dynamics. We characterise classical ground states using a flux picture for a single layer of kagome spins, a theoretical description that sets geometrical bounds on correlations. We quantify the divergent but sub-extensive ground state degeneracy by a Maxwellian counting argument, and verify this calculation by analysing the energy eigenvalues of numerical ground states. We explore the ground state connectedness but do not reach firm conclusions on this issue. We use the self-consistent Gaussian approximation (SCGA) to calculate static spin correlations at finite temperature. The results of these calculations agree well with elastic neutron scattering experiments. We derive an expression for the effective interlayer interaction between kagome spins by integrating out the triangular lattice spins. We use linear spinwave theory to compute the spin excitation spectrum numerically. This shows encouraging similarity with inelastic neutron scattering data on a single-crystal YBaCo$_4$O$_7$ sample, for a wide range of wavevector and frequency. This agreement shows that our spin model is a reasonable description of the physics, and suggests that this numerical technique might be useful for other geometrically frustrated magnets. We study the dynamics analytically using the stochastic SCGA recently developed for the pyrochlore lattice. For technical reasons, we apply this technique on a related model, the stacked kagome lattice, rather than on the extended kagome lattice itself. From this we find slow relaxation at low temperature, with a rate ~ T² compared to the faster ~ T scaling for the pyrochlore. Strikingly, in simulations of the dynamics on the extended kagome lattice by numerical integration of the semiclassical equations of motion, we find two different relaxation rates. Kagome layer spins relax more quickly than the triangular layer spins, having ~ T.

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Paulose, Jayson Joseph. "Cooperativity, Fluctuations and Inhomogeneities in Soft Matter." Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:10854.

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This thesis presents four investigations into mechanical aspects of soft thin structures, focusing on the effects of stochastic and thermal fluctuations and of material inhomogeneities. First, we study the self-organization of arrays of high-aspect ratio elastic micropillars into highly regular patterns via capillary forces. We develop a model of capillary mediated clustering of the micropillars, characterize the model using computer simulations, and quantitatively compare it to experimental realizations of the self-organized patterns. The extent of spatial regularity of the patterns depends on the interplay between cooperative enhancement and history-dependent stochastic disruption of order during the clustering process. Next, we investigate the influence of thermal fluctuations on the mechanics of homogeneous, elastic spherical shells. We show that thermal fluctuations give rise to temperature- and size-dependent corrections to shell theory predictions for the mechanical response of spherical shells. These corrections diverge as the ratio of shell radius to shell thickness becomes large, pointing to a drastic breakdown of classical shell theory due to thermal fluctuations for extremely thin shells. Finally, we present two studies of the mechanical properties of thin spherical shells with structural inhomogeneities in their walls. The first study investigates the effect of a localized reduction in shell thickness—a soft spot—whereas the second studies shells with a smoothly varying thickness. In both cases, the inhomogeneity significantly alters the response of the shell to a uniform external pressure, revealing new ways to control the strength and shape of initially spherical elastic capsules.
Engineering and Applied Sciences

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Marnham, Lachlan Leslie. "Bi-electron bound states in single- and double-layer graphene nanostructures." Thesis, University of Exeter, 2016. http://hdl.handle.net/10871/23165.

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The prototypical exciton model of two interacting Dirac particles in graphene was analysed by Sabio et al, Phys. Rev. B 81, 045428 (2010), and it was found that in one of the electron-hole scattering channels the total kinetic energy vanishes, resulting in a singular behaviour. We show that this singularity can be removed by extending the quasiparticle dispersion, thus breaking the symmetry between upper and lower Dirac cones. The dynamics of an electron-electron pair are then mapped onto that of a single particle with negative mass and anisotropic dispersion. We show that the interplay between dispersion and repulsive interaction can result in the formation of bound, Cooper-pair-like, metastable states in double-layered hybrid structures. We analyse these states by calculating their binding energies, decay rates into the free- electron continuum and semiclassical trajectories. We also analyse the problem of bi-electron pairing with the inclusion of the two dominant many-body effects at zero temperature: screening of the Coulomb interaction by the Dirac sea, and reduction of the available phase space due to Pauli blocking of transitions into the states below the Fermi level. We show that these effects result in strong renormalization of the binding energy, but do not destroy the metastable states. Thus the binding energies are strongly dependent on the chemical potential owing to the combined effects of screening and Pauli blocking. Hence, the quasibound resonances can be tuned by electrostatic doping.

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Lee, Choon-Heung. "Theoretical study of diamond-like carbons and nucleation of diamond." Case Western Reserve University School of Graduate Studies / OhioLINK, 1993. http://rave.ohiolink.edu/etdc/view?acc_num=case1060349568.

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Miao, Jiayuan. "Theory and Simulation of the Responses of Polymers to Electric Fields, Stress, Irradiation, and Diffusive Solvents." Case Western Reserve University School of Graduate Studies / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=case1481279886096515.

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Pickles, Thomas Stanley. "Ordering transitions and localisation properties of frustrated systems." Thesis, University of Oxford, 2009. http://ora.ox.ac.uk/objects/uuid:2cc83b99-3733-4900-9ba5-d7b8476b62b2.

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In this work we investigate themes related to many-body systems in which multiple ground states are accessible, a condition known as frustration. Frustration can arise in a number of contexts, and we consider the consequences of this situation with some examples from condensed-matter physics. In some magnetic materials interactions between spins are such that no single spin configuration provides a unique ground state. In the class of frustrated magnets where the number of ground states is extensive, thermal fluctuations are strong even at temperatures significantly below the interaction strength. At such temperatures spins are highly correlated, and small perturbations may have profound consequences. In this thesis we provide an example of this. By considering classical n-component spins with nearest-neigbour exchange on a frustrated octahedral lattice we show that – in the limit where exchange interactions are large – the system is in a disordered, correlated phase where correlations have the form of a dipole field. This is termed a Coulomb phase. From this phase we induce an ordering transition, lifting the degeneracy with weak, additional short-range interactions. By studying the transition in the solvable limit of n → ∞, we discover that the transition has identical thermodynamics to that of a magnetic system interacting through long-range, dipolar forces. Finally, we provide a more apposite characterisation of the transition, where the high-temperature side of the transition is described through the fluctuations of solenoidal fields, and the ordering corresponds to a condensation of these fields. In a separate part of the thesis, we investigate the influence of disorder on frustrated lattices. We study a two-dimensional tight-binding model with nearest-neighbour hopping and on-site disorder. Restricting the allowed states to being those from the low-lying manifold of ground states, the disorder feeds through to act as effective disorder in the hopping terms, which decay algebraically with distance. The quasi-long range nature of this effective hopping leads to a situation in which the resultant single-particle eigenstates are critical, and we probe their behaviour numerically with a transfer matrix calculation.

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Jarrold, Thomas Furnley. "Single channel Kondo physics in triple quantum dots." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:e2772c4e-6c76-44b8-9c02-401d9f90b27f.

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In this thesis we investigate a system of three tunnel-coupled quantum dots, arranged in a triangular geometry and attached to a single metallic conduction band, using both analytic and semi-analytic methods and the numerical renormalisation group technique. This is the simplest coupled quantum dot system to exhibit frustration. We study three different models of the triple quantum dot device: a mirror symmetric arrangement of dots in which only one dot is connected to the conduction band, a triple quantum dot system in which only one dot is connected to the conduction band without a plane of mirror symmetry and a mirror symmetric arrangement of quantum dots in which all three dots are coupled to the conduction band. We study these models over a wide range of parameter space, and in both the presence and absence of a magnetic field. Both antiferromagnetic and ferromagnetic Kondo effects are observed, and in all three models we find that the system contains at least two phases, and so a number of quantum phase transitions may be observed, associated in some cases with significant changes in the low temperature conductance through the triple quantum dot device. In addition to zero-field Kondo physics, a number of field induced Kondo effects are also observed. Both first order quantum phase transitions and Kosterlitz-Thouless phase transitions are observed. We use both symmetry arguments and low energy effective models which we derive to explain and understand both the position of and type of phase boundary that is observed in each case, and perturbative methods are used to accurately predict Kondo temperatures for a wide range of systems.

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Eriksson, Hjalmar. "From the quantum Hall effect to topological insulators : A theoretical overview of recent fundamental developments in condensed matter physics." Thesis, Uppsala University, Theoretical Physics, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-126860.

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In this overview I describe the simplest models for the quantum Hall and quantum spin Hall effects, and give some general indications as to the description of topological insulators. As a background to the theoretical models I will first trace the development leading up to the description of topological insulators . Then I will present Laughlin's original model for the quantum Hall effect and briefly discuss its limitations. After that I will describe the Kane and Mele model for the quantum spin Hall effect in graphene and discuss its relation to a general quantum spin Hall system. I will conclude by giving a conceptual description of topological insulators and mention some potential applications of such states.

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Mithen, James Patrick. "Molecular dynamics simulations of the equilibrium dynamics of non-ideal plasmas." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:3bae84f9-530d-43da-ad7e-bb9a1784cd1d.

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Molecular dynamics (MD) simulations are used to compute the equilibrium dynamics of a single component fluid with Yukawa interaction potential v(r) = (Ze)^2 exp(−r/λs )/4π eps_0 r. This system, which is known as the Yukawa one-component plasma (YOCP), represents a simplified description of a non-ideal plasma consisting of ions, charge Ze, and electrons. For finite screening lengths λs, the MD results are used to investigate the domain of validity of the hydrodynamic description, i.e., the description given by the Navier-Stokes equations. The way in which this domain depends on the thermodynamic conditions of the YOCP, as well as the strength and range of the interactions, is determined. Remarkably, it is found that the domain of validity is completely determined by the range of the interactions (i.e., λs); this alone determines the maximum wave number k_max at which the hydrodynamic description is applicable. The dynamics of the YOCP at wavevectors beyond k_max are then investigated; these are shown to be in striking agreement with a simple and well known generalisation of the Navier-Stokes equations. In the extreme case of the Coulomb interaction potential (λs = ∞), the very existence of a hydrodynamic description is a known but unsolved problem [Baus & Hansen, 1980]. For this important special case, known as the one-component plasma (OCP), it is shown that the ordinary hydrodynamic description is never valid. Since the OCP is the prototypical system representing a non-ideal plasma, a number of different approaches for modelling its dynamics have been formulated previously. By computing the relevant quantities with MD, the applicability of a number of models proposed in the literature is examined for the first time.

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Willans, Adam J. "Disorder in an exactly solvable quantum spin liquid." Thesis, University of Oxford, 2010. http://ora.ox.ac.uk/objects/uuid:8ea5b2cc-4843-44ef-aa0a-8535f00c6dc8.

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We investigate the properties of the Kitaev honeycomb model with both site dilution and exchange randomness. Embarking on this work, we review disorder in some strongly correlated electron systems, including spin-½ and spin-1 Heisenberg antiferromagnetic chains, two dimensional Heisenberg antiferromagnets, the cuprates and graphene. We outline some aspects of resonating valence bond phases, valence bond solids, spin liquids and quantum computation that are pertinent to an understanding of the Kitaev model. The properties of the Kitaev model without disorder are discussed and it is found to be a critical spin liquid, with algebraic correlations in two spin operators sigma^{alpha}_{i}sigma^{alpha}_{j}, where i and j,/em> are either end of a link of type alpha = x, y or z on the honeycomb lattice. The Kitaev model is exactly solvable and we show that this remains so in the presence of site dilution and exchange randomness. We find that vacancies bind a flux. In the gapped phase, a vacancy forms an effective paramagnetic moment. With two or more vacancies we describe the interaction of their effective moments and show that a finite density of vacancies leads to a divergent macroscopic susceptibility at small fields. In the gapless phase the effective moment has a susceptibility that is, to leading order at small fields, chi(h)~log(1/h). Interaction between the moments from two vacancies on opposite sublattices cuts off this divergence in susceptibility at a large but finite constant. Two vacancies on the same sublattice behave quite differently and we find the combined susceptibility is parametrically larger than that of an isolated vacancy, chi(h)sim [h(log(1/h))^{3/2}]^{-1}. We also investigate the effects of slowly varying, quenched disorder in exchange coupling. We demonstrate that this does not qualitatively affect the susceptibility but show that the heat capacity C ~ T^{2/z}, where z is a measure of the disorder and increases from one with increasing disorder strength.

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Bennett, Rachel R. "Physics of microorganism behaviour : motility, synchronisation, run-and-tumble, phototaxis." Thesis, University of Oxford, 2015. http://ora.ox.ac.uk/objects/uuid:accc7f3c-b472-4bb9-b821-59725a54ccb7.

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Microorganisms have evolved in a low Reynolds number environment and have adapted their behaviour to its viscosity. Here, we consider some features of behaviour observed in microorganisms and use hydrodynamic models to show that these behaviours emerge from physical interactions, including hydrodynamic friction, hydrodynamic interactions and mechanical constraints. Swimming behaviour is affected by surfaces and observations of Vibrio cholerae show that it swims near a surface with two distinct motility modes. We develop a model which shows that friction between pili and the surface gives the two motility modes. The model is extended to study the behaviour of bacteria which are partially attached to a surface. Observations of Shewanella constrained by a surface show several different behaviours. The model shows that different degrees of surface constraint lead to different types of behaviour; the flexibility of the flagellar hook and the torque exerted by the flagellar motor also cause different behaviours. Near surface behaviour is important for understanding the initial stages of biofilm formation. Chlamydomonas swims using synchronous beating of its two flagella. A simple model of Chlamydomonas is developed to study motility and synchronisation. This model shows that the stability of synchronisation is sensitive to the beat pattern. Run-and-tumble behaviour emerges when we include intrinsic noise, without the need for biochemical signalling. The model is also used to show how observed responses of the flagella to light stimuli produce phototaxis. Finally we study hydrodynamic synchronisation of many cilia and consider the stability of metachronal waves in arrays of hydrodynamically coupled cilia. This thesis shows that physical interactions are responsible for many behavioural features and that physical models provide a useful technique for exploring open questions in biology.

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27

Wright, Christopher James. "Theoretical studies of underscreened Kondo physics in quantum dots." Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:62207edb-af3a-4340-a6f2-5264b1374a41.

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We study correlated two-level quantum impurity models coupled to a metallic conduction band in the hope of gaining insight into the physics of nanoscale quantum dot systems. We focus on the possibility of formation of a spin-1 impurity local moment which, on coupling to the band, generates an underscreened (USC) singular Fermi liquid state. By employing physical arguments and the numerical renormalization group (NRG) technique, we analyse such systems in detail examining in particular both the thermodynamic and dynamic properties, including the differential conductance. The quantum phase transitions occurring between the USC phase and a more ordinary Fermi liquid (FL) phase are analysed in detail. They are generically found to be of Kosterlitz-Thouless type; exceptions occur along lines of high symmetry where first-order transitions are found. A `Friedel-Luttinger sum rule' is derived and, together with a generalization of Luttinger's theorem to the USC phase, is used to obtain general results for the $T=0$ zero-bias conductance --- it is expressed solely in terms of the number of electrons present on the impurity and applicable in both the USC and FL phases. Relatedly, dynamical signatures of the quantum phase transition show two broad classes of behaviour corresponding to the collapse of either a resonance or antiresonance in the single-particle density of states. Evidence of both of these behaviours is seen in experimental devices. We study also the effect of a local magnetic field on both single- and two-level quantum impurities. In the former case we attempt to resolve some points of contention that remain in the literature. Specifically we show that the position of the maximum in the spin resolved density of states (and related peaks in the differential conductance) is not linear in the applied field, showing a more complicated form than a simple `Zeeman splitting'. The analytic result for the low-field asymptote is recovered. For two-level impurities we illustrate the manner in which the USC state is destroyed: due to two cancelling effects an abrupt change in the zero-bias conductance does not occur as one might expect. Comparison with experiment is made in both cases and used to interpret experimental findings in a manner contrary to previous suggestions. We find that experiments are very rarely in the limit of strong impurity-host coupling. Further, features in the differential conductance as a function of bias voltage should not be simply interpreted in terms of isolated quantum dot states. The many-body nature of such systems is crucially important to their observed properties.

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Jayatilaka, Frederic William. "Theoretical studies of tunnel-coupled double quantum dots." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:756add23-aae6-4a71-a22b-087695bc600a.

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We study the low-temperature physics arising in models of a strongly correlated, tunnel-coupled double quantum dot (DQD), particularly the two-impurity Anderson model (2AIM) and the two-impurity Kondo model (2IKM), employing a combination of physical arguments and the Numerical Renormalisation Group. These models exhibit a rich range of Kondo physics. In the regime with essentially one electron on each dot, there is a competition between the Kondo effect and the interdot exchange interaction. This competition gives rise to a quantum phase transition (QPT) between local singlet and Kondo singlet phases in the 2IKM, which becomes a continuous crossover in the 2AIM as a result of the interlead charge transfer present. The 2IKM is known to exhibit two-channel Kondo (2CK) physics at the QPT, and we investigate whether this is also the case for the 2AIM at the crossover. We find that while in principle 2CK physics can be observed in the 2AIM, extremely low temperatures are required, such that it is unlikely that 2CK physics will be observed in an experimental DQD system in the near future. We have studied the effect of a magnetic field on the 2AIM and the 2IKM, finding that both the zero-field QPT in the 2IKM and the zero-field crossover in the 2AIM, persist to finite field. This presents the possibility of observing 2CK physics in an experimental DQD at finite field, but we find that the temperatures required to do so are extremely low. We show that longer even-numbered chains of spins also exhibit QPTs at finite field, and argue that a 2N-spin chain should undergo N QPTs as field is increased (starting deep in the local singlet phase at zero field). We have also carried out a joint theoretical-experimental study of a carbon nanotube based DQD, in collaboration with Dr. Mark Buitelaar et al. The agreement between experimental and theoretical results is good, and the experiments are able to access the crossover present in the 2AIM at finite field. Furthermore, the experiments show the wide range of physics exhibited by DQD systems, and illustrate the utility of such systems in probing correlated electron physics.

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29

Weston, Leigh. "First-Principles Theoretical Studies of Bulk, Defect and Interface properties of Oxide Semiconductors." Thesis, The University of Sydney, 2015. http://hdl.handle.net/2123/13806.

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Oxide semiconductors have been shown to exhibit rich physics related to their bulk, defect and interface properties. First-principles calculations have and will continue to play a major role in developing an understanding of the microscopic origins of these phenomena. In this thesis, first-principles studies are presented for several oxide semiconductors, with a view to understand how their microscopic properties ultimately determine device functionality. In Chapter 3, a detailed study of bulk SrZrO3 and Sr(Ti,Zr)O3 alloys is performed. For Sr(Ti,Zr)O3 alloys with 50% Ti concentration, we find that arranging the Ti and Zr atoms into a 1×1 SrZrO3/SrTiO3 superlattice along the [001] direction leads to breaking of the conduction band t2g orbital degeneracy, which could suppress scattering due to electron-phonon interactions. In Chapter 4, we present an investigation into the properties of native defects and hydrogen in SrZrO3. It is found that oxygen and strontium vacancies are the dominant defects in the absence of impurity doping, and will form deep donor and deep acceptor states, respectively. Hydrogen is found to be amphoteric in this material at different lattice sites; additionally, this impurity forms a stable complex with oxygen vacancies. In Chapter 5, the tendency for ABO3 perovskite oxides with 3dn B-cations to exhibit ferroelectricity and multiferroicity is investigated. Using the LaBO3 series as a model, we find that initially, as electrons are added to the B-cation d orbital, the tendency for the system to exhibit a ferroelectric distortion disappears - however, for high spin d5 - d7 and d8 cations a strong ferroelectric instability is recovered, and this effect is explained within the pseudo Jahn-Teller theory for ferroelectricity. This finding provides a new route for the design of strongly coupled magnetoelectric materials. In Chapters 6 and 7 the fundamental properties of the technologically important oxide heterostructure systems ZnO/MgZnO and SrTiO3/LaAlO3 are characterized. For the latter, we identify a previously unreported mechanism for interface induced magnetism based on surface aluminium vacancies, which will aid in interpreting experimental results for this system and other polar/non-polar oxide heterostructures.

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30

Nakhmanson, Serge M. "Theoretical Studies of Amorphous and Paracrystalline Silicon." Ohio University / OhioLINK, 2001. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou984426180.

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Sulc, Petr. "Coarse-grained modelling of nucleic acids." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:3e1573ec-033c-4971-85e1-ccecd57e7f70.

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This thesis considers coarse-grained models of DNA and RNA, developed in particular to study nanotechnological applications as well as some important biophysical processes. We first introduce sequence-dependent thermodynamics into a previously developed coarse-grained rigid base-pair model of DNA. This model is then used to study sequence-dependent effects in multiple DNA systems including: the heterogeneous stacking transition of single strands, the fraying of a duplex, the effects of stacking strength in the loop on the melting temperature of hairpins, the force-extension curve of single strands, and the structure of a kissing-loop complex. We further apply the DNA model to study in detail the properties of an autonomous unidirectionally propagating DNA nanotechnological device, called the ``burnt bridges motor''. We then apply the coarse-graining methods developed for the DNA model to construct a new sequence-dependent coarse-grained model of RNA, which aims to capture basic thermodynamic, structural and mechanical properties of RNA molecules. We test the model by studying its thermodynamics for a variety of secondary structure motifs and also consider the force-extension properties of an RNA duplex. This RNA model allows for efficient simulations of a variety of RNA systems up to hundreds or even thousands of base-pairs. Its versatility is further demonstrated by studying the thermodynamics of a pseudoknot folding, the formation of a kissing loop complex, the structure of a hexagonal RNA nanoring, and the unzipping of a hairpin.

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Khosravi, Khorashad Larousse. "Theoretical and Computational Study of Optical Properties of Complex Plasmonic Structures." Ohio University / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou150834414639462.

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33

Radha, Santosh Kumar. "Knitting quantum knots-Topological phase transitions in Two-Dimensional systems." Case Western Reserve University School of Graduate Studies / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=case1595870012750826.

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34

Mitchell, Andrew Keith. "Two-channel Kondo phases in coupled quantum dots." Thesis, University of Oxford, 2009. http://ora.ox.ac.uk/objects/uuid:3d4e9d86-794c-441c-9d4b-20e6f1bd1de1.

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We investigate systems comprising chains and rings of quantum dots, coupled to two metallic leads. Such systems allow to study the competition between orbital and spin degrees of freedom in a nanodevice, and the effect this subtle interplay has on two-channel Kondo (2CK) physics. We demonstrate that a rich range of strongly correlated electron behaviour results, with non-Fermi liquid 2CK phases and non-trivial phase transitions accessible. We employ physical arguments and the numerical renormalization group (NRG) technique to analyse these systems in detail, examining in particular both thermodynamic and dynamical properties. When leads are coupled to either end of a chain of dots, we show that the resulting behaviour on low temperature/energy scales can be understood in terms of simpler paradigmatic quantum `impurity' models. An effective low-energy single-spin 2CK model is derived for all odd-length chains, while the behaviour of even-length chains is related fundamentally to that of the classic `two-impurity Kondo' model. In particular, for small interdot coupling, we show that an effective coupling mediated though incipient single-channel Kondo states drives all odd chains to the 2CK fixed point (FP) on the lowest temperature/energy scales. A theory is also developed to describe a phase transition in even chains. We derive an effective channel-anisotropic 2CK model, which indicates that the critical FP of such models must be the 2CK FP. This physical picture is confirmed using NRG for various chain systems. We also examine the effect of local frustration on 2CK physics in mirror-symmetric ring systems. The importance of geometry and symmetry is demonstrated clearly in the markedly different physical behaviour that arises in systems where two leads are either connected to the same dot, or to neighbouring dots. In the latter case, we show for all odd-membered rings that two distinct 2CK phases, with different ground state parities, arise on tuning the interdot couplings. A frustration-induced phase transition thus occurs, the 2CK phases being separated by a novel critical point for which an effective low-energy model is derived. Precisely at the transition, parity mixing of the quasidegenerate local trimer states acts to destabilise the 2CK FPs, and the critical FP is shown to consist of a free pseudospin together with effective single-channel spin quenching. While connecting both leads to the same dot again results in two parity-distinct phases, a simple level-crossing transition now results due to the symmetry of the setup. The proposed geometry also allows access to a novel ferromagnetically-coupled two-channel local moment phase. Driven by varying the interdot couplings and occurring at the point of inherent magnetic frustration, such transitions in ring structures provide a striking example of the subtle interplay between internal spin and orbital degrees of freedom in coupled quantum dot systems, and the resulting effect on Kondo physics.

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35

Phou, Pierre. "Creating a Bose-Einstein condensate of stable molecules using photoassociation and Feshbach resonance." Diss., Temple University Libraries, 2014. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/269211.

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Physics
Ph.D.
Quantum degenerate molecular gases are of interest for the unique level of control they offer over chemical interactions and processes. To reach the quantum degenerate regime, these molecular gases must be cooled to ultracold temperatures, typically on the order of 100 nanoKelvins. Unlike atoms, with a few-level system that facilitates cooling, molecules represent a many-level system, which makes these temperatures experimentally difficult to achieve. As a result, experiments have turned to photoassociation and Feshbach resonance as shortcuts to form ultracold molecules from already ultracold atoms. Photoassociation and Feshbach resonance have been utilized to successfully create stable quantum degenerate molecules, but not on a routine basis, and only for a small range of molecular species. The primary focus of this thesis will be to study photoassociation and Feshbach resonance, and investigate possible routes to more efficient long-lived quantum degenerate molecule formation. We will also investigate realistic limiting conditions to open the possibility to more routine molecules, and to molecular species that are currently inaccessible. Overall, we find combined photoassociation and Feshbach resonance are viable schemes for efficiently creating quantum degenerate molecules, under realistic restrictions such as low laser intensity, narrow Feshbach resonance, and strong elastic collisions. As the techniques to create quantum degenerate molecules become more robust and experimentally available, the creation of colder, larger, and more long-lived samples will facilitate study of these molecules, and spur development into new applications.
Temple University--Theses

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36

Robinson, Neil Joe. "Pairing, paramagnetism and prethermalization in strongly correlated low-dimensional quantum systems." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:167d164c-e318-49b3-83ea-69b54ec531e0.

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Quasi-one-dimensional quantum models are ideal for theoretically exploring the physical phenomena associated with strong correlations. In this thesis we study three examples where strong correlations play an important role in the static or dynamic properties of the system. Firstly, we examine the behaviour of a doped fermionic two-leg ladder in which umklapp interactions are present. Such interactions arise at special band fillings and can be induced by the formation of charge density wave order in an array of two-leg ladders with long-range (three-dimensional) interactions. For the umklapp which arises from the half-filling of one of the bands, we show that the low-energy theory has a number of phases, including a strong coupling regime in which the dominant fluctuations are superconducting in nature. These superconducting fluctuations carry a finite wave vector – they are the one-dimensional analogue of Fulde-Ferrell-Larkin-Ovchinnikov superconductivity. In a second example, we consider a quantum spin model which captures the essential one-dimensional physics of CoNb₂O₆, a quasi-one-dimensional Ising ferromagnet. Motivated by high-resolution inelastic neutron scattering experiments, we calculate the dynamical structure in the paramagnetic phase and show that a small misalignment of the transverse field can lead to quasi-particle breakdown – a surprising broadening in the single particle mode observed in experiment. Finally, we study the out-of-equilibrium dynamics of a model with tuneable integrability breaking. When integrability is broken by the presence of weak interactions, we show that the system relaxes to a non-thermal state on intermediate time scales, the so-called “prethermalization plateau”. We describe the approximately stationary behaviour in this regime by constructing a generalised Gibbs ensemble with charges deformed to leading order in perturbation theory. Expectation values of these charges are time-independent, but interestingly the charges do not commute with the Hamiltonian to leading order in perturbation theory. Increasing the strength of the integrability breaking interactions leads to behaviour compatible with thermalisation. In each case we use a combination of perturbative analytical calculations and non-perturbative numerical computations to study the problem at hand.

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Juniper, Michael P. N. "Dynamics of driven colloidal systems in one-dimensional potential energy landscapes." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:4319222a-72bf-4390-a134-fc9fb8fd2515.

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The dynamics of colloidal particles driven over optical potential energy landscapes is studied. Experiments are conducted using colloids driven by solvent flow or piezo-stage, optical tweezers, magnetic fields, and video-microscopy. Firstly, the properties of optical traps and potential energy landscapes are determined using driven colloidal particles and clusters. The trap stiffness and potential depth of single Gaussian traps are measured directly. It is shown that the nature of optical potential energy landscapes may be fully engineered and predicted using a sum of single Gaussian potentials. Next, the motion of colloidal particles driven by a constant force over a periodic optical potential energy landscape is considered. The average particle velocity is found as a function of the driving velocity, and the wavelength of the optical potential energy landscape, which is found to be sinusoidal at small trap spacings. The critical driving velocity required for a particle to move across the landscape is determined as a function of the wavelength. Brownian motion is found to have a significant effect on the critical driving velocity, but a negligible effect at high driving velocity. Subsequently, the dynamic mode locking caused by adding a modulation to the driving force is studied. This synchronisation manifests as a `Devil's staircase' in the average particle velocity as a function of driving velocity. The amplitude and frequency dependence of the mode locked steps are studied. Furthermore, particle trajectories are examined, and phase portraits show locked (unlocked) states as closed (open) loops in phase space. A state diagram of mode locked steps is constructed. Finally, driven systems of magnetically interacting colloidal particles are examined in potential energy landscapes. The critical driving velocity of a chain of coupled particles driven by a constant force is found to depend strongly on the chain length and the magnetic field. Secondly, a mobile density wave (kink) in an optically pinned chain of coupled particles is exposed to a constant and modulated drive. The kink is found to behave as a quasi-particle, exhibiting analogous dynamic mode locking behaviour to the single particle case. Finally, the mode locking of a finite mobile chain is considered, and found to be affected by the chain flexibility, which is a function of the magnetic field.

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Szczesniak, Dominik. "Theoretical and numerical modelling of electronic transport in nanostructures." Phd thesis, Université du Maine, 2013. http://tel.archives-ouvertes.fr/tel-00830908.

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The aim of this thesis in the nanoelectronics domain is to present a contribution to the analysis of the quantum electronic transport phenomena in nanostructures. For this purpose, we specifically develop the phase field matching theory (PFMT). Within this algebraic approach the electronic properties of the system are described by the tight-binding formalism, whereas the analysis of the transport properties based on the phase matching of the electronic states of the leads to the states of the molecular nanojunctions. By comparing some of our results with those of the first principles methods, we have shown the correctness and fonctionality of our approach. Moreover, our method can be considered as a practical and general alternative to the Green's function-based techniques, and is applied in this work to model the electronic transport across mono and diatomic nanojunctions, consisting of mono and multivalent Na, Cu, Co, C, Si, Ga and As elements.

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Derry, Philip. "Quasiparticle interference in strongly correlated electronic systems." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:f487c821-dbbb-4ebe-8b05-c13807379c2c.

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We investigate the manifestation of strong electronic correlations in the quasiparticle interference (QPI), arising from the scattering of conduction electrons from defects and impurities in an otherwise translationally-invariant host. The QPI may be measured experimentally as the Fourier transform of the spatial modulations in the host surface density of states that result, which are mapped using a scanning tunnelling microscope. We calculate the QPI for a range of physically relevant models, demonstrating the effect of strong local electronic correlations arising in systems of magnetic impurities adsorbed on the surface of non-interacting host systems. In the first instance the effect of these magnetic impurities is modelled via the single Anderson impurity model, treated via numerical renormalization group (NRG) calculations. The scattering of conduction electrons, and hence the QPI, demonstrate an array of characteristic signatures of the many-body state formed by the impurity, for example due to the Kondo effect. The effect of multiple impurities on the QPI is also investigated, with a numerically-exact treatment of the system of two Anderson impurities via state-of-the-art NRG calculations. Inter-impurity interactions are found to result in additional scattering channels and additional features in the QPI. The QPI is then investigated for the layered transition metal oxide Sr2RuO4, for which strong interactions in the host conduction electrons give rise to an unconventional triplet superconducting state at T_c ∼ 1.5K. The detailed mechanism for this superconductivity is still unknown, but electron-electron or electron-phonon interactions are believed to play a central role. We simulate the QPI in Sr₂RuO₄, employing an effective parametrized model consisting of three conduction bands derived from the Ru 4d t2g orbitals that takes into account spin orbit coupling and the anisotropy of the Ru t2g orbitals. Signatures of such interactions in the normal state are investigated by comparing these model calculations to experimental results. We also calculate the QPI in the superconducting state, and propose how experimental measurements may provide direct evidence of the anisotropy and symmetry of the superconducting gap, and thus offer insight into the pairing mechanism and the superconducting state.

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Prabha, FNU Sweta. "Theoretical Studies of the Structure and Stability of Metal Chalcogenide CrnTem (1≤n≤6, 1≤m≤8) clusters." VCU Scholars Compass, 2019. https://scholarscompass.vcu.edu/etd/5774.

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In the presented work, first principle studies on electronic structure, stability, and magnetic properties of metal chalcogenide, CrnTem clusters have been carried out within a density functional framework using generalized gradient functions to incorporate the exchange and correlation effects. The energetic and electronic stability was investigated, and it was found that they are not always correlated as seen in the cluster Cr6Te8 which has smaller gap between its HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) and a high electron affinity of 3.39 eV indicating lower electronic stability whereas higher fragmentation energy indicating energetic stability. The high electron affinity shows that the stability of Cr6Te8 cluster can be enhanced by adding charge donating ligands including PEt3 to form stable Cr6Te8(PEt3)6 clusters as seen in experiments. The other cluster of interest was Cr4Te6 in which energetic stability was accompanied with electronic inertness marked by its large HOMO-LUMO gap, non-magnetic ground state and high fragmentation energy.

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41

Hu, Zhonghan. "Transport properties, optical response and slow dynamics of ionic liquids." Diss., University of Iowa, 2007. http://ir.uiowa.edu/etd/160.

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42

Barranco, López Alejandro. "Applications of Supersymmetry: Exact Results, Gauge/Gravity Duality and Condensed Matter." Doctoral thesis, Universitat de Barcelona, 2014. http://hdl.handle.net/10803/284195.

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The study of supersymmetry has led us to a better understanding of field theories, specially in the strong coupling regime. In this thesis we have tried to show this through several examples. These are: - The first of these examples has been the application of localization techniques in supersymmetric theories. Specifically, we have used the partition function of N=2 supersymmetric Chern-Simons theory with gauge group U(N) and 2Nf flavors. To regularize the theory, it is necessary to make the computation in a three sphere whose radius, R, serves as an IR regulator which can be taken to infinity at the end of the computation. Once we have the exact partition function in terms of a matrix integral, we can solve the integral by means of a saddle-point approximation. This approximation becomes exact in the large N limit. The saddle-point equation can be solved exactly and in the decompactification limit it shows different phases depending on the value of the 't Hooft coupling. We have also computed the free energy and the vacuum expectation value of a Wilson loop for a big circle of the three sphere. Both of them show discontinuities in their derivatives, in particular, the discontinuity in the free energy appears in the third derivative and thus, both phase transitions are third order. - Other application that we have seen consists of the use of the gauge/gravity duality. In particular, starting from the gravity dual to N=1 super Yang-Mills, proposed by Maldacena and Núñez, we have reviewed how to add flavors (quarks) to this theory, without mass first and with mass later. We have also seen how to extract information about the field theory from these gravity duals, we have paid special attention to how the beta-function of the field theory dual is obtained from the gravity background proposed by Conde, Gaillard and Ramallo, dual to N=1 super Yang-Mills field theory with Nf massive flavors and a quartic superpotential. The main result from the point of view of the field theory is that, in the case Nf=2N, the beta-function shows a non-trivial UV fixed point, which hints on possible IR fixed point as proposed by Seiberg in the conformal window picture. No evidence of non-trivial fixed points is found for Nf different from 2N. - Again, in the context of the gauge/gravity duality, we have studied how to generate new supergravity solutions applying T-duality and how this affects the G-structures that describe the supersymmetry of these solutions. We have applied T-duality to the IIB supergravity solution of Klebanov and Witten with flavors. The supersymmetry of these backgrounds can be described by an SU(3)-structure and an SU(2)-structure before and after T-dualizing, respectively. - Finally, we have presented an N=1 supersymmetric model that exhibits a superconducting phase transition. This model is based on a quartic Kähler potential for a chiral multiplet and no superpotential. The main difference with standard superconductivity is that the phase transition becomes first order rather than second order. Another difference is that, as it is typical in supersymmetric theories, the dependence on the cut-off is softened in our model.
El estudio de supersimetría nos ha permitido un mejor entendimiento de las teorías de campos, especialmente en el régimen de acoplamiento fuerte. En esta tesis hemos tratado de mostrar esto a través de varios ejemplos. A saber: - Primero hemos mostrado cómo calcular de manera exacta con técnicas de localización la función de partición de la teoría de Chern-Simons supersimétrica N=2 con grupo gauge U(N) y 2Nf sabores en una tres esfera. Una vez que tenemos la función de partición exacta en términos de una integral de matrices, podemos resolverla por medio de la aproximación de punto silla. Esta aproximación se vuelve exacta cuando tomamos el límite de N grande. En el límite de descompactificación calculamos la energía libre y el valor de expectación de un lazo de Wilson correspondiente a un círculo máximo de la tres-esfera, mostrando transiciones de fase de tercer orden. - Otra aplicación ha consistido en el uso de la dualidad gravedad/gauge. Hemos revisado como construir una solución de supergravedad dual a la teoría N=1 super Yang-Mills con Nf sabores masivos y un superpotencial cuártico. El principal resultado desde el punto de vista de la teoría de campos que corresponde a dicha solución, consiste en que, en el caso Nf=2N, la función beta exhibe un punto fijo UV no trivial que da lugar a indicios acerca de un punto fijo IR adicional tal y como propone Seiberg en su propuesta de la ventana conforme. - También en el contexto de la dualidad gravedad/gauge hemos estudiado cómo generar nuevas soluciones de supergravedad por medio de la aplicación de T-dualidad a la solución de supergravedad de Klebanov y Witten con sabores. La supersimetría de estas soluciones pueden ser descritas en términos de una SU(3) estructura antes de T-dualizar, o una SU(2)-estructura, después de T-dualizar. - Finalmente, hemos presentado un modelo supersimétrico que exhibe una transición de fase superconductora basado en un potencial de Kähler cuártico para un multiplete quiral y sin superpotencial. Encontramos dos diferencias con superconductores usuales: nuestro modelo exhibe una transición de fase de primer orden, en lugar de ser de segundo orden, y la dependencia con la energía de corte es más suave.

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Gustafsson, Alexander. "Theoretical modeling of scanning tunneling microscopy." Doctoral thesis, Linnéuniversitetet, Institutionen för fysik och elektroteknik (IFE), 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-69012.

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The main body of this thesis describes how to calculate scanning tunneling microscopy (STM) images from first-principles methods. The theory is based on localized orbital density functional theory (DFT), whose limitations for large-vacuum STM models are resolved by propagating localized-basis wave functions close to the surface into the vacuum region in real space. A finite difference approximation is used to define the vacuum Hamiltonian, from which accurate vacuum wave functions are calculated using equations based on standard single-particle Green’s function techniques, and ultimately used to compute the conductance. By averaging over the lateral reciprocal space, the theory is compared to a series of high-quality experiments in the low- bias limit, concerning copper surfaces with adsorbed carbon monoxide (CO) species and adsorbate atoms, scanned by pure and CO-functionalized copper tips. The theory compares well to the experiments, and allows for further insights into the elastic tunneling regime. A second significant project in this thesis concerns first-principles calculations of a simple chemical reaction of a hydroxyl (oxygen-deuterium) monomer adsorbed on a copper surface. The reaction mechanism is provided by tunneling electrons that, via a finite electron-vibration coupling, trigger the deuterium atom to flip between two nearly identical configurational states along a frustrated rotational motion. The theory suggests that the reaction primarily occurs via nuclear tunneling for the deuterium atom through the estimated reaction barrier, and that over-barrier ladder climbing processes are unlikely.

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Bowler, David Robert. "A theoretical investigation of gas source growth of the Si(001) surface." Thesis, University of Oxford, 1997. http://ora.ox.ac.uk/objects/uuid:a817986f-114d-4a8a-8001-767f795d0e55.

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The growth of the Si(001) surface from gas sources such as disilane is technologically important, as well as scientifically interesting. The aspects of growth covered are: the clean surface, its defects and steps; the action of bismuth, a surfactant; the diffusion behaviour of hydrogen in different environments; and the entire pathway for formation of a new layer of silicon from adsorption of fragments of disilane to nucleation of dimer strings. The theoretical methods used, density functional theory and tight binding, are described. Four linear scaling tight binding methods are compared. The construction of the tight binding parameterisations used is also explained. The structure of the most common defect on the Si(001) surface is identified by comparison of the electronic structure with scanning tunneling microscopy (STM) images. The energy and structure of steps is calculated, and their kinking behaviour is modelled, achieving good agreement with experimental results. Two unusual features which form when bismuth is placed on the surface and annealed are investigated. The first has possible applications as a quantum wire, and its structure and growth are described. The second relates to a controversial area in the field; a structure is proposed which fits all available experimental evidence. The behaviour of hydrogen is vital to understanding growth, as large amounts are deposited during disilane growth. After validating the tight binding parameterisation against DFT and experiment for the system of a single hydrogen diffusing on the clean Si(001) surface, the barriers for diffusion on the saturated surface, down a step and away from a defect are found, and prove to be in good agreement with available experimental data. The pathway for the formation of a new layer of silicon from disilane is described step by step, giving barriers and structures for all events. The interaction with experiment is highlighted, and demonstrates that great benefit accrues from such close work, and that the atomistic modelling techniques used in the thesis produce results in close agreement with reality.

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Kitson, Adrian Robert. "On the zero-point energy of elliptic-cyliindrical and spheroidal boundaries : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Theoretical Physics at Massey University, New Zealand." Massey University, 2009. http://hdl.handle.net/10179/1016.

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Zero-point energy is the energy of the vacuum. Disturbing the vacuum results in a change in the zero-point energy. In 1948, Casimir considered the change in the zeropoint energy when the vacuumis disturbed by two parallelmetal plates. The plates disturb the vacuum by restricting the quantum fluctuations of the electromagnetic field. Casimir found that the change in the zero-point energy implies that the plates are attracted to each other. With the recent advances made in the experimental verification of this remarkable result, theoretical interest has been rekindled. In addition to the original parallel plate configuration, several other boundaries have been studied. In this thesis, two novel boundaries are considered: elliptic-cylindrical and spheroidal. The results for these boundaries lead to the conjecture that zero-point energy does not change for small deformations of the boundary that preserve volume. Assuming the conjecture, it is shown that zero-point energy plays a stabilizing role in quantum chromodynamics, the leading theory of the strong interaction.

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Lima, Filipe Camargo Dalmatti Alves. "Interação de moléculas e superfície Au(111)." Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-08072015-182719/.

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O estudo de sistemas híbridos compostos por interfaces orgânico/inorgânico, ou sólido/líquido, tem apresentado crescente interesse nas áreas de eletroquímica e nanotecnologia. Além de objetos de pesquisas básicas, estes sistemas apresentam um potencial para inúmeras aplicações, dentre elas: línguas eletrônicas, \\emph{self-assembled monolayers} (SAMs), dispositivos fotovoltáicos, baterias alternativas, carreadores de drogas, entre outras. Em especial, o uso de superfícies de ouro em pesquisas ocorre principalmente por causa da natureza inerte deste material, permitindo explorar uma ampla quantidade de potenciais eletrostáticos que induzem a eletrólise ou reações eletroquímicas em outras superfícies. O estudo de voltametria cíclica em SAMs formadas por cadeias polipeptídicas funcionalizadas com ferroceno vem sendo realizado durante as duas últimas décadas. Recentemente, iniciou-se uma controvertida discussão a respeito do mecanismo de transporte eletrônico entre o centro oxidativo e o eletrodo destes sistemas. Alguns grupos argumentam a favor de tunelamento eletrônico do centro oxidativo, enquanto outros grupos têm proposto uma interação de pares \\emph{elétron-buraco} dos grupos amida dos peptídeos. Além disto, interfaces com sistemas primitivos, como por exemplo a água, ainda são temas de pesquisa correntes de diversos grupos de pesquisa, devido à complexidade dos resultados experimentais reportados. De forma a contribuir com estas discussões correntes na literatura, selecionamos dois problemas distintos utilizando a superfície Au(111) como base comum: i) estudo do mecanismo de transferência de carga de um peptídeo funcionalizado com ferroceno; ii) estudo das propriedades eletrônicas e estruturais da água interagindo com NaCl. Para realizarmos a investigação das propriedades eletrônicas, empregamos a Teoria do Funcional da Densidade no esquema de Kohn-Sham (KS). Para analisar as propriedades dinâmicas e estruturais, foi utilizada também a técnica de dinâmica molecular clássica (MD). A partir de diversos modelos da interação do peptídeo sobre Au(111), investigamos as densidades de estados, cargas de Löwdin e funções de onda de KS. Notamos a presença de estados eletrônicos localizados tanto sobre o ferroceno quanto sobre o ouro, ambos sempre próximos da região da energia de Fermi, em todos os casos propostos. Estes resultados sugerem um tunelamento eletrônico entre sítio do ferroceno e a superfície Au(111) como o mecanismo de transferência eletrônica. Para o caso do sal dissociado em água, investigamos e discutimos a estrutura eletrônica em diversas situações e configurações. Além disto, realizamos um estudo MD, onde observamos que o ordenamento das moléculas de água é bastante sensível à presença da superfície Au(111). Os resultados obtidos apresentam uma visão ampla dos comportamentos eletrônicos e dinâmicos de sistemas envolvendo a superficie Au(111) que discutem questões correntes na literatura.
The study of hybrid interfaces, e.g. organic/inorganic or solid/liquid, have been showing an increasing interest in electrochemistry and nanotechnology. Within this subject, there are basic and applied studies, such as electronic tongues, self-assembled monolayers (SAMs), photovoltaic devices, alternative batteries, drug carriers and others. In special, the preference for gold surface occurs due to its inert nature, allowing the exploration of a wide range of electrostatic potentials which induces electrolysis and chemical reactions in other surfaces. The Cyclic Voltammetry study in Peptide-SAMs modified by ferrocene has been investigated in the literature. In recent years, a controversy on the charge transfer mechanism in biological materials started: at one hand, there are arguments in favor of an electronic tunneling process from the oxidative center to the eletrode; on the other hand, some authors suggest electronic hopping from the amide groups of the peptides, generating an electron-hole pair that ``walks\'\' from the ferrocene to the eletrode. Furthermore, systems with primitive interfaces, such as water, are also subject of current research due to the complexity of the experimental results reported in the literature. Within this scenario, we selected two distinct problems using the surface Au(111): i) the study of charge transfer mechanism using a peptide modified by ferrocene; ii) the study of electronic and structural properties of water interacting with NaCl. In order to obtain the electronic properties, we employed the Density Functional Theory in the Kohn-Sham (KS) scheme. For the structural and dynamics properties, we also used classical molecular dynamics (MD). Based on different models for the ferrocene-peptide/Au(111) interface, we investigate the density of states, Löwdin charges and KS wavefunctions. We notice the presence of localized electronic states on the ferrocene and gold which are close to the Fermi energy in all studied cases. These results suggest an electronic tunneling from the ferrocene site to the surface Au(111) as the mechanism for the charge transfer. In the case of salt dissociated in water, we investigated the electronic properties in several different configurations. Furthermore, in a MD perspective, the orientation of the water molecules presented a high sensitivity for the Au(111) interface. These results represent a wide view of the electronic and dynamic behavior of systems using the surface Au(111) as a common subject.

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Rowland, James R. IV. "Theoretical Investigations of Skyrmions in Chiral Magnets." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1565920470204146.

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Haigh, Tania J. "Bose-Einstein condensates in coupled co-planar double-ring traps : a thesis presented in partial fulfillment of the requirements for the degree of Masterate of Science in Physics at Massey University, Palmerston North, New Zealand." Massey University, 2008. http://hdl.handle.net/10179/975.

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This thesis presents a theoretical study of Bose-Einstein condensates in a doublering trap. In particular, we determine the ground states of the condensate in the double-ring trap that arise from the interplay of quantum tunnelling and the trap’s rotation. The trap geometry is a concentric ring system, where the inner ring is of smaller radius than the outer ring and both lie in the same two-dimensional plane. Due to the difference in radii between the inner and outer rings, the angular momentum that minimises the kinetic energy of a condensate when confined in the individual rings is different at most frequencies. This preference is in direct competition with the tunnel coupling of the rings which favours the same angular momentum states being occupied in both rings. Our calculations show that at low tunnel coupling ground state solutions exist where the expectation value of angular momentum per atom in each ring differs by approximately an integer multiple. The energy of these solutions is minimised by maintaining a uniform phase difference around most of the ring, and introducing a Josephson vortex between the inner and outer rings. A Josephson vortex is identified by a 2p step in the relative phase between the two rings, and accounts for one quantum of circulation. We discuss similarities and differences between Josephson vortices in cold-atom systems and in superconducting Josephson junctions. Josephson vortices are actuated by a sudden change in the trapping potential. After this change Josephson vortices rotate around the double-ring system at a different frequency to the rotation of the double-ring potential. Numerical studies of the dependence of the velocity on the ground state tunnel coupling and interaction strength are presented. An analytical theory of the Josephson vortex dynamics is also presented which is consistent with our numerical results.

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Nawarange, Amruta V. "Optical Emission Spectroscopy during Sputter Deposition of CdTe Solar Cells and CuTe-Based Back Contacts." University of Toledo / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1321656598.

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Trinkle, Dallas Rhea III. "A theoretical study of the HCP to omega martensitic phase transition in titanium." Connect to this title online, 2003. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1070481734.

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Thesis (Ph. D.)--Ohio State University, 2003.
Title from first page of PDF file. Document formatted into pages; contains xviii, 201 p.; also includes graphics. Includes bibliographical references (p. 195-201).

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Dissertations / Theses on the topic 'Theoretical Condensed matter Physics'

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