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Electronic transport properties have been used to classify and characterize materials and describe their functionality. Recent surge in computational power has enabled computational modelling and accelerated theoretical studies to complement and accelerate experimental discovery of novel materials. This work looks at methods for theoretical calculations of electronic transport properties and addresses the limitations of a common approximation in the calculation of these properties, namely, the constant relaxation time approximation (CRTA). This work takes a look at the limitations of this approximation and introduces energy and temperature dependent relaxation times. This study is carried out on models and real systems and compared with experiments.

MSc, Faculty of Science, University of the Witswatersrand, 2011 / The objective of this project is to understand the details of the electronic transport in low dimensional carbon structures at low temperatures as well as high magnetic fields. The emphasis is on the quasi-2 dimensional thin grain boundary regions of nanodiamond films and one dimensional carbon nanotubes. As such nitrogen “doped” and undoped nanodiamond films were synthesized by the hot filament chemical vapor deposition method (HFCVD). The films were micro-structurally and electrically characterized using several techniques such as Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy and magnetoresistance (MR) measurements. The electronic transport properties were compared to the films deposited by microwave plasma enhanced chemical vapour deposition (MWCVD). The conductivity revealed a typical semiconducting and semi-metallic behavior for the HFCVD films depending on the nitrogen percentage in the chamber. The dephasing time of the electronic wave function was found to be weakly temperature dependant i.e. τ T-p with p < 1, a behavior reported in artificial superlattices. These results show potential application of these materials in novel nano-electronic devices. Previously the transport mechanism in nanodiamond films has been attributed to hopping conduction in the grain boundaries which is predominately disordered sp2 phases. Our studies on nanodiamond films have however shown different mechanisms in these films. We observed very little contribution from hopping and pronounced weak localization contributions in nanodiamond films. We thus establish the significance of tunneling transport in nanodiamond films. We also studied the electronic transport in films of metal filled multiwalled carbon nanotubes which show significant contribution from the hopping mechanism and a negative magnetoresistance at low fields that crosses over into positive MR at high magnetic fields.

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Even changing one atom in nanoscale materials is expected to alter their properties due to their small physical sizes. Such sensitivity can be utilized to modify materials' properties from bottom up and is essential for the utility of nanoscale materials. As such, the impact of extrinsic atomic adsorbates was measured on pristine graphene and a network of carbon nanotubes using atomic hydrogen, cesium atoms, and dye molecules. In order to further quantify such an atomic influence, the resistance induced by a single potassium atom on metallic and semiconducting carbon nanotubes was measured for the first time. Carbon nanotubes are sensitive to adsorbates due to their high surface-to-volume ratio. The resistance arising from the presence of extrinsic impurity atoms depends on the types of nanotubes. Metallic carbon nanotubes are resilient to a long-ranged, Coulomb-like potential, whereas semiconducting carbon nanotubes are susceptible to these impurities. The difference in the scattering strength originates from the chirality of carbon nanotubes, which defines their unique electronic properties. This difference had not directly measured experimentally because of the issue of contact resistance, the difficulty of chirality identification, and the uncertainty in the number of impurity atoms introduced on carbon nanotubes. We synthesized atomically clean, long ( > 100 ?m) carbon nanotubes, and their chirality was identified by Rayleigh scattering spectroscopy. We introduced potassium atoms on the nanotubes to impose a long-range, Coulomb potential and measured the change in resistivity, excluding the contact resistance, by plotting the resistance as a function of the carbon nanotube length. The flux of potassium atoms coming onto the nanotubes was monitored by quartz crystal microbalance, and the scattering strength of a single potassium atom was deduced from the change in resistivity and the density of potassium atoms on the nanotubes. We found that the scattering strength of potassium atoms on semiconducting nanotubes depends on the charge carrier type (holes or electrons). Metallic nanotubes were found to be less affected by the presence of potassium atoms than semiconducting nanotubes, but the scattering strength showed a large dependence on Fermi energy. These experimental results were compared to theoretical simulations, and we found a good agreement with the experiments. Our findings provide crucial information for the application of carbon nanotubes for electronic devices, such as transistors and sensors.

<p> The theoretical formulation of the electronic transport properties (in the absence of a magnetic field) of pure single crystals of simple metals is extended to incorporate the effect of a non~spherical Fermi surface, using a multiple orthogonalized plane wave description of the conduction electrons. Two approaches are considered, one using a variational principle, and the other employing a scattering time approximation. </p> <p> Formal results for the electrical resistivity and the electronic contribution to the thermal resistivity are expressed in terms of effective phonon frequency distributions. These distributions are particularly convenient for numerical computations and are generalization: of those previously used for the case of a spherical Fermi surface. </p> <p> The generalization of the scattering time method to dilute nonmagnetic substitutional alloys is applied to hexagonal close~packed metalsc It is shown that the addition of small amounts of impurities to pure Zn leads to measurable changes in the temperature dependence of the electrical resistivity ratio (see text for ratio with symbols). The corresponding deviations front Matthiessen's rule for polycrystalline samples are also calculated. </p> / Thesis / Master of Science (MSc)

The resistivity-size effect has emerged as an obstacle in our pursuit of ever shrinking electronic devices. Interconnects and vias are the nanoscale copper conductors connecting within and between layers of a CPU, respectively. New materials and methods are required to address this problem. In particular, there is a critical need for a theoretical framework which can evaluate the properties of new materials in a way that reflects real-world performance. To this end, a computational methodology is developed by introducing an ab initio parameterized tight-binding model to accurately calculate electronic structure and simulating electronic transport via the calculation of the Kubo-Greenwood conductivity tensor. Transport properties are computed using the kernel polynomial method, a highly scalable approach wherein physical quantities can be represented as a weighted sum of Chebyshev polynomials. Using this combined approach, it is possible to simulate mesoscale electronic transport for systems with over 10^6 sites containing various forms of realistic disorder. Through the use of ensemble calculations, an examination of resistivity due to surface disorder and disorder due to realistic phonon fields is presented.

Metals with excitonic instabilities are multiband systems with significant electron-electron interaction. The electronic transport in such systems is affected by collective fluctuations of the electrons, leading to anomalous features in the measured transport coefficients. Many of these anomalies have not been well understood because the transport mechanisms in these systems tend to be rather complex. The complexity arises, on the one hand, from the multiband nature and, on the other, from the anisotropic scattering of electrons accompanied by emitting or absorbing collective fluctuations. Previous works considering scattering due to collective fluctuations have mainly focused on single-band systems, for example in the context of the normal-state transport in cuprates. The recent discovery of high-temperature superconductivity in iron pnictides has renewed the interest in multiband systems. Exploring the transport mechanisms in multiband systems, I find some interesting new aspects, which do not occur in single-band systems. In particular, anisotropic scattering in a model with electronlike and holelike Fermi surfaces can lead to a negative conductivity contribution of the minority carriers, i.e., in an electric field, the minority carriers drift in the direction opposite of what one would expect based on their charge. I show that this effect can explain a reduced magnetoresistance in connection with an enhanced Hall coefficient, which has been measured in pnictides. Of particular interest are multiband models with hot spots on the Fermi surface, in part because of their relevance for the iron pnictides. Hot spots are states with enhanced scattering and therefore reduced excitation lifetimes. In single-band systems, the hot spots are found to have a much lower contribution to the total conductivity than other parts of the Fermi surface, which leads to the so-called hot-spot structure. I show that in the multiband case, the conductivity contributions are much more isotropic around the Fermi surface so that hot spots contribute to transport with a similar strength as other parts of the Fermi surface. I discuss this effect on the basis of an approximate analytical solution of the transport problem and numerically calculate the temperature dependence of several transport coefficients. It turns out that in the nematic phase of iron pnictides, the unexpectedly strong conductivity contribution of hot spots can explain the puzzling behavior of the resistive anisotropy. I show that the experimental observations can be explained within a scenario in which the anisotropy is mainly due to the broken symmetry of the spin-fluctuation spectrum in the nematic phase. In the spin-density-wave state, strongly anisotropic scattering can arise due to the propagating magnons. Using a two-band model relevant for iron pnictides, I find that this scattering can lead to an unusual interruption of the orbital motion of electrons in the magnetic field. As a consequence, the low-field magnetoresistance is linear with an alternating sign of the slope as a function of the direction of the current. In strong magnetic fields, the interrupted orbital motion makes the system unstable, which is characterized by a drop of the resistivity to zero.