Pages 6 and 7 are labelled as the same page. / <p> The purpose of this work is to calculate phonon dispersion curves in metals paying particular attention to the evaluation of a new electron-ion matrix element by use of orthogonalized plane waves (OPW). The dynamic role of the electrons in screening the electron-ion interaction has been studied. Our formalism makes use of recent developments in the theory of the many-body problem. Applications of our theory have been made to aluminum. The pseudopotential part of the OPW electron-ion matrix element produced an overscreening of the frequency modes. Comparison is made to the use of the Bardeen matrix element. Our results strongly suggest that this calculation applied to lead would explain the magnitude of Kohn kinks observed by Brockhouse et al. (B 62a).</p> / Thesis / Doctor of Philosophy (PhD)

<p> The phonon dispersion relation in the principal symmetry directions of a crystal of Cu-Ni-Zn was measured at 298°K by means of inelastic scattering of thermal neutrons. The specimen had an electronic concentration per atom very close to that of pure copper and it came as no surprise that no significant shift in the phonon spectrum relative to that of pure copper could be observed. </p> <p> The particular specimen used had a poor mosaic distribution; this contribution was taken into account in the calculation of the line shape and it was found that the widths of the neutron groups could be well accounted for in this way. </p> <p> The appendix is divided into six parts, four of which contain a description of projects of secondary importance that were realized. </p> / Thesis / Master of Science (MSc)

Orientador: Ricardo Paupitz Barbosa dos Santos / Resumo: Neste trabalho, tivemos como objetivo obter a dispersão de fônons de materiais bidimensionais, por meio de simulações de dinâmica molecular clássica, a fim de examinar propriedades mecânicas e elásticas, como a velocidade do som, módulo volumétrico, módulo de cisalhamento, coeficiente de Poisson e módulo de Young. Apresentamos as principais características da ferramenta utilizada na investigação, o método de dinâmica molecular clássica. Abordamos o Algoritmo Velocity Verlet, empregado para a integração das equações de movimento; o Ensemble estatístico, utilizado para realizar as simulações; o termostato de Nosé-Hoover, responsável por regular a temperatura do sistema; e os potenciais que descrevem as interações atômicas. Utilizamos potenciais reativos, sendo eles Tersoff, Tersoff-2010, AIREBO e ReaxFF. As simulações computacionais foram realizadas através do software LAMMPS. Além disso, discorreremos sobre a dinâmica de rede, a obtenção das curvas de dispersão a partir da construção da matriz dinâmica, por meio da matriz dos coeficientes de rigidez baseado nos deslocamentos dos átomos. Os materiais de baixa dimensionalidade investigados nesta dissertação são derivados do carbono como o grafeno, grafeno bifenileno – BPC e nanotubos. As propriedades vibracionais e elásticas calculadas, para o grafeno foram comparadas com resultados experimentais para o grafite no plano e resultados de simulações de dinâmica molecular. O grafeno bifenileno e os nanotubos de carbono foram compara... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In this work, we aimed to obtain the phonon dispersion of two-dimensional materials by classical molecular dynamics simulations, in order to explore mechanical and elastical properties, such as velocity of sound, volumetric modulus, shear modulus, Poisson's ratio, and Young's modulus. We present the main features of the tool used in the research, the classical molecular dynamics method. We approach the Velocity Verlet Algorithm, used for the integration of the equations of motion; the statistical Ensemble, used to perform the simulations; Nosé-Hoover’s thermostat, responsible for regulate the system temperature; and the potentials that describe the atomic interactions. We used reactive potentials, being Tersoff, Tersoff-2010, AIREBO and ReaxFF. The computational simulations were performed through LAMMPS software. In addition, we will discuss lattice dynamics, the obtaining the dispersion curves from the dynamic matrix construction, through the matrix of stiffness coefficients based on the displacements of the atoms. The low-dimensional materials investigated in this dissertation are derived from carbon such as graphene, graphene biphenylene - BPC and nanotubes. The calculated vibrational and elastical properties for graphene were compared with experimental results for graphite in-plane and results of molecular dynamics simulations. Graphene biphenylene and carbon nanotubes were compared with graphene values. The other materials, biphenylene nanotubes, were compared with carbo... (Complete abstract click electronic access below) / Mestre

Thermoelectric (TE) materials, which can convert temperature gradients directly into electricity and vice versa, have received renewed interest for waste heat recovery and refrigeration applications. Higher manganese silicides (HMS) are promising p-type TE materials due to the abundance of the constituent elements, environmental friendliness, and good chemical stability. The objective of this dissertation is to establish a better understanding of the structure-TE properties relationship of HMS with a complex Nowotny chimney ladder structure. The focus of this work is on the investigations of the phonon dispersion of HMS crystals and the effects of chemical doping and nanostructuring on the TE properties of polycrystalline HMS. HMS crystals have been synthesized by the Bridgeman method for inelastic neutron scattering measurements of the phonon dispersion. In conjunction with density functional theory calculations, the results clearly show the presence of numerous low-lying optical phonon branches, especially an unusually low-energy optical phonon polarization associated with the twisting motions of the Si helical ladders in the Mn chimneys. The obtained phonon dispersion can be used to explain the low and anisotropic thermal conductivity of HMS crystals. (Al,Ge) double doping was found to be effective in modifying the electrical properties of HMS polycrystals. The peak thermoelectric power factor occurs at an optimized hole concentration of 1.8~2.2×10²¹ cm⁻³ at room temperature. On the other hand, Re substitution can suppress the lattice thermal conductivity to approach the calculated minimum value corresponding to the amorphous limit. Meanwhile, the thermoelectric power factor does not markedly change at low Re content of x ≤ 0.04 although it drops considerably with increasing Re content. Hence, the peak ZT has been improved to ~0.6 in both systems. The effects of nanostructuring on the TE properties have been studied in the cold-pressed samples and ball-milled samples. The thermal conductivity was reduced remarkably by decreasing the grain size. It is found that the grain size effects are more significant at low temperature. However, it is difficult to reduce the grain size to less than 50 nm without the formation of impurity phases by ball milling. These facts limit the ZT enhancement of the nanostructured HMS at high temperatures in this study. / text

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Doctor of Philosophy / Department of Physics / Talat S. Rahman / In this work we present a study of the geometric, electronic, vibrational and magnetic properties of several nanostructured systems for which experimental data call for a theoretical understanding. In order to investigate the effect of magnetic dipolar interactions on the magnetization of nanomagnets arranged in finite lattices, we utilize a phenomenological classical approach, which is based on the Landau-Lifshitz equation. Dipolar interactions lead to hysteretic behavior of the magnetization curves and established that the external field sweep rate, sample temperature, and shape anisotropy play a role in determining the specifics. Our results (derived from a classical approach) for magnets arranged in a square lattice suggest that stepped hysteresis curves do not have necessarily a quantum origin (quantum tunneling of the magnetization). We also find that in the square lattice small changes in the dipolar strength introduce sudden transitions in the magnetic hysteresis. For the examination of geometric vibrational and electronic structure of systems of interest, we turn to density functional theory (DFT), which is the leading technique for modeling nanoscale systems from first principles. We have applied DFT to either address some old queries of surface science, such as the dynamics of the CO-chemisorbed Cu(001) surface, or to contribute to the forefront of hydrogen-based economy through the comprehension of the growth and diffusion of Pt islets on Ru(0001), or to predict the geometric and electronic properties of materials to-be-created, as in the case of core-shell bimetallic nanoclusters. In the case of CO on Cu(001), although the bond has been considered to be weak enough so as to treat the adsorbate and substrate separately, our calculations are able to reproduce measurements and provide evidence that the dynamics of the molecule is influenced by the substrate and vice versa, as well as by intermolecular interactions. Taking into account the adsorbate-substrate interplay, has furthermore clarified issues that were pending for the clean surface and led to the correct interpretation of some features in the phonon dispersion of the chemisorbed surface. DFT has also directed us to the conclusion that the catalytic properties of few-atom Pt islets on Ru nanoclusters are preserved by the low probability of these islets to diffuse through the edges of the Ru nanoclusters. Moreover, the analysis of the Ag_{27}Cu_7 nanoalloy from ab initio methods has opened a wide panorama in terms of the geometry, coordination, energetics, and electronic structure of alloyed phases, in general,that may aid in the assembling on new materials.

The Sutton Chen and quantum Sutton Chen potentials are used in molecular dynamics simulations to describe the structural, thermodynamical, and transport properties of Pd, Ni and their binary alloys in solid, liquid, and glass phases. Static properties including elastic constants, pair distribution function, static structure factor, and dynamical properties consisting of phonon dispersion relation, diffusion coefficient, and viscosity are computed at various temperatures. The melting temperatures for Pd-Ni system are obtained. The transferability of the potentials is tested by simulating the solid and liquid states. The eutectic concentration Pd0.45Ni0.55 is quenched at four different cooling rates. The system goes into glass formation at fast cooling rates, while it evolves to crystal at slow cooling rate. Comparison of calculated structural and dynamical properties with the available experiments and other calculations shows satisfactory consistency.

<p dir="ltr">Electronic structure calculations, specifically plane wave Density functional theory (DFT) in conjunction with data science resources on the nanoHUB platform, are a powerful tool for the study of materials at extreme conditions. nanoHUB’s resources for FAIR (Findable, Accessible, Interoperable and Reproducible) data and workflows accelerate research and sharing of results. With these combined resources we studied the high-pressure properties and stability of various ceramic materials, such as the polytypes of silicon carbides (SiC).</p><p dir="ltr">Silicon carbides are of interest for their exceptional mechanical strength, thermal stability and chemical resistance making them attractive for applications at extreme conditions. To understand the temperature- and pressure-induced phase transitions we studied the electronic structure, phonon dispersion and elastic constants of major synthesizable SiC polytypes as a function of pressure. In addition, we report on the elastic constants as a function of pressure for the main polymorphs. This DFT workflows are published online, not only enhances the reproducibility of findings, but can also accelerates the discovery and development of new material properties.</p>

We employ the empirical potential function Airebo to computationally model free-standing Carbon-12 graphene in a classical setting. Our objective is to measure the mean square displacement (MSD) of atoms in the system for different average temperatures and Carbon-13 isotope concentrations. From results of the MSD we aim to develop a technique that employs Transmission Electron Microscopy (TEM), using high-angle annular dark filed (HAADF) detection, to obtain atomic-resolution images. From the thermally diffusive images, produced by the vibrations of atoms, we intent to resolve isotopes types in graphene. For this, we establish a relationship between the full width half maximum (FWHM) of real-space intensity images and MSD for temperature and isotope concentration changes. For the case of changes in the temperature of the system, simulation results show a linear relationship between the MSD as a function of increased temperature in the system, with a slope of 7.858×10-6 Å2/K. We also note a power dependency for the MSD in units of [Å2] with respect to the FWHM in units of [Å] given by FWHM(MSD)=0.20MSD0.53+0.67. For the case of increasing isotope concentration, no statistically significant changes to the MSD of 12C and 13C are noted for graphene systems with 2,000 atoms or more. We note that for the experimental replication of results, noticeable differences in the MSD for systems with approximately 320,000 atoms must be observable. For this, we conclude that isotopes in free-standing graphene cannot be distinguished using TEM.

The majority of structural studies on DNA have been carried out using fibre diffraction, while studies of its dynamics and thermal behaviour have been mainly performed in solution. When the DNA double helix is heated, it exhibits local separation of the two strands that grow in size with temperature and lead to their complete separation. This work has investigated various aspects of this phenomenon. The experiments reported in this thesis were carried out on films of oriented fibres of DNA prepared with the Wet Spinning Apparatus. Thus, sample preparation and characterisation are essential parts of the research. The structures of two forms of DNA, A and B, have been explored as a function of relative humidity at fixed ionic conditions. A method to eliminate traces of ever-present B-form contamination in A-form samples was established. The high orientation of the DNA molecules within the samples allowed us to investigate dynamical fluctuations and the melting transition of DNA using neutron scattering, which can provide the spatial information crucial to understand a phase transition, probing the static correlation length along the molecule as a function of temperature. The transition has been investigated for A and B-forms in order to understand its dependence on molecular configuration.Furthermore, after the first melting, denatured DNA films show typical glass behaviour. Their thermal relaxation has been explored using calorimetry.Neutron and X-ray inelastic scattering (INS and IXS) were used in the past to measure longitudinal phonons in fibre DNA, and the results shown disagreement. Recent INS measurements supported with phonon simulations have been crucial to understand the different dispersion curves reported to date. Experiments using INS and IXS have been carried out to continue with this investigation. Attempts to observe the transverse fluctuations associated to the thermal denaturing of DNA, never experimentally investigated before, have been made.