We developed and implemented a first-principles based theory of the Landauer ballistic conductance, to determine the transport properties of nanostructures and molecular-electronics devices. Our approach starts from a quantum-mechanical description of the electronic structure of the system under consideration, performed at the density-functional theory level and using finite-temperature molecular dynamics simulations to obtain an ensemble of the most likely microscopic configurations. The extended Bloch states are then converted into maximally-localized Wannier functions to allow us to construct the Green’s function of the conductor, from which we obtain the density of states (confirming the reliability of our microscopic calculations) and the Landauer conductance. A first application is presented to the case of carbon nanotubes. / Singapore-MIT Alliance (SMA)

We determined the Landauer ballistic conductance of pristine nanotubes at finite temperature via a novel scheme that combines ab-initio molecular dynamics, maximally-localized Wannier functions, and a tight-binding formulation of electronic transport in nanostructures. Large-scale ab-initio molecular dynamics simulations are used to obtain efficiently accurate trajectories in phase space. The extended Bloch orbitals for states along these trajectories are converted into maximally-localized orbitals, providing an exact mapping of the ground-state electronic structure onto a short-ranged Hamiltonian. Green's functions, self-energies, and ballistic conductance can then be obtained for any given configuration, and averaged over the appropriate statistical ensemble. / Singapore-MIT Alliance (SMA)

The activity of glutamate transporters is essential for the temporal and spatial regulation of the neurotransmitter concentration in the synaptic cleft which is critical for proper neuronal signaling. Because of their role in controlling extracellular glutamate concentrations, dysfunctional glutamate transporters have been implicated in several neurodegenerative diseases and psychiatric disorders. Therefore, investigating the mechanism of substrate transport by these transporters is essential in understanding their behavior when they malfunction. A bacterial glutamate transporter homologue has been successfully crystallized revealing the molecular architecture of glutamate transporters. However, many important questions remain unanswered. In this thesis, I will address the role of D439 in the binding of Na+, and I will identify other electrogenic steps that contribute to the total electrogenicity of the transporter cycle. The role of D439 in the binding of Na+ to the transporter was explored previously in this lab. While it was proposed that the effect of D439 in Na+ binding is indirect, the results described in this thesis provides added support to this work. Here, I will show that the D439 mutation changed the pharmacology of EAAC1 such that THA was converted from a transported substrate to a competitive inhibitor. I will also show that Na+ binding to the substrate-bound mutant transporter occurred with the same affinity as that of Na+ to the substrate-bound wild-type transporter. Therefore, based on these results, D439 is not directly involved in the binding of Na+ to the substrate-bound transporter, but that its effect is rather indirect through changing the substrate binding properties. Na+ binding steps to the empty transporter and to the glutamate-bound EAAC1 contribute only 20% of the total electrogenicity of the glutamate transporter reactions cycle. While K+-induced relocation has been proposed to be electrogenic, there is no experimental evidence that supports it. In this work, I will show that the K+-induced relocation of the empty transporter is electrogenic. Moreover, the results in this work show that the K+-dependent steps are slower than the steps associated with the Na+/glutamate translocation suggesting that the K+-induced relocation determines the transporter?s properties at steady state.

This study is an attempt to conceptually replicate a study by Ramakers, Stevens and Morris (2005) using a measure of electrodermal activity skin conductance (EDA) to evaluate the possibility of telepathy occurring between biologically and/or emotionally related senders and receivers. Ten negatively valenced and highly arousing target pictures were mixed with 10 blank control pictures in 10 blocks, with one of each kind in each block. The order of presentation of the target and control pictures within the block was determined randomly by a computer program. The series of 20 pictures were shown for the sender on a computer screen. Relaxation for the receiver was facilitated by soft music. It was hypothesized that there would be significantly more variance in the receiver EDA when the sender was exposed to negative arousing pictures, than to blank pictures. The results failed to show a significant difference in EDA variance between negative arousing and blank pictures, and did thus not support the telepathy hypothesis. It was recommended that future replications allocate more time for relaxation for the receiver.

In the first part of this dissertation, we study mainly novel boron structures and their electronic and mechanical properties, using ab initio calculations. The electronic structure and construction of the boron buckyball B80, and boron nanotubes as the α-sheet wrapped around a cylinder are studied. The α-sheet is considered so far to be the most stable structure energetically out of the two dimensional boron assemblies. We will argue however that there are other sheets close in energy, using cluster expansion method. The boron buckyball is shown to have different possible isomers. Characterization of these isomers according to their geometry and electronic structure is studied in detail. Since the B80 structure is made of interwoven double-ring clusters, we also investigate double-rings with various diameters. We investigate the properties of nanotubes obtained from α-sheet. Computations confirm their high stability and identify mechanical stiffness parameters. Careful relaxation reveals the curvature-induced buckling of certain atoms off the original plane. This distortion opens up the gap in narrow tubes, rendering them semi-conducting. Wider tubes with the diameter d  1.7 nm retain original metallic character of the α-sheet. We conclude this part by investigation into hydrogen storage capacity of boron-rich compounds, namely the metallacarboranes. In the second part of dissertation, we switch our focus to electronic and transport properties of carbon nano-structures. We study the application of carbon nanotubes as electro-chemical gas sensors. The effect of physisorption of NO2 gas molecules on electron transport properties of semi-conducting carbon nanotubes is studied using ab initio calculations and Green’s function formalism. It is shown that upon exposure of nanotube to different concentrations of gas, the common feature is the shift in conductance towards lower energies. This suggests that physisorption of NO2 will result in a decrease (increase) in conductance of p-type (n-type) nanotubes with Fermi energies close to the edge of valence and conduction band. Finally we study the effect of torsion on electronic properties of carbon nano-ribbons, using helical symmetry of the structures.

Recent research has suggested that boredom is a construct that can be distinguished from similar affective states including apathy, anhedonia, and depression, using self-reports. The current study investigated whether boredom and sadness (an analogue for depression) are distinct in terms of their physiological signatures. State boredom and sadness were induced in a group of healthy participants while their physiological parameters of heart rate (HR), skin conductance (SCL), and cortisol levels were monitored. Results indicated that the autonomic nervous system response for both states can be characterized by directional fractionation, with boredom resulting in increased HR but decreased SCL relative to sadness. Cortisol levels were higher after the boring induction than the sad induction, indicating increased hypothalamic-pituitary-adrenal axis activation for boredom. Overall, boredom appears to have a physiological signature that is distinguishable from a primary symptom of depression.

The metal/graphite interface is interesting due to the typically large disparity in the characteristics of the electronic structure (e.g. Fermi wavelength and Fermi energy) and dimensionality (3D in the metal versus quasi-2D in graphite). The goal of this work is to determine how the contact conductance to graphite depends on the metal contact area for nanometer-scale contacts. From this we deduce the effect of electronic screening in the graphite. Three different metals were chosen for this work: Solid Cu and Al, and liquid Ga. Liquid Ga provided a unique opportunity to reduce the effect of mechanical interactions to near zero, while Cu and Al were chosen for their different electronic structures. At the interface between the metal and graphite, the large Fermi wavevector of Al should allow phase matching of Al states to those in graphite, while the Cu Fermi surface lies inside of all available graphite wavevector states.

Since the early 1960s, Abrikosov-Gorkov theory has been used to describe superconductors with paramagnetic impurities. Interestingly, the density of states resulting from the theoretical framework has to date only been known approximately, as a numeric solution of a complex polynomial. An analytical solution to the theory was discovered and applied to extract the spin polarization from the tunneling conductance of superconducting aluminium with 3-dimensional (3-D) amorphous (a-) gadoliniumxsilicon1-x (GdxSi1-x) as a counter electrode (Al/Al2O3/a-GdxSi1-x planar tunnel junction measured at T = 25 mK and H less than or equal to 3.0 T) in the quantum critical regime (QCR). The analytical solution is valid in the whole regime of Abrikosov-Gorkov theory independent of the presence of an energy gap. Applying the spin polarized Abrikosov-Gorkov theory to describe aluminium gives a larger spin polarization in GdxSi1-x than the spin polarized Bardeen-Cooper-Schrieffer (BCS) theory. The purpose of this study is to extract polarization at various applied magnetic fields, but no specific relationship between the two could be determined. Results obtained shows a transition from a superconductor with a gap to a gapless superconductor in varying external magnetic fields was observed. To improve understanding of GdxSi1-x near the metal-insulator transition (MIT) and compare it with prior work, the initial experimental attempts to investigate the transport property of GdxSi1-x near the MIT in the 2-dimensional limit are presented. A low temperature ultra high vacuum quench condensation system was used to make thin films of GdxSi1-x and in-situ measurements were performed. The transport properties for different values of x and thicknesses were measured for T = 4.2 K to ~10 K. In addition to other possible causes, the uncertainty in the electron impact emission spectroscopy (EIES) appeared to be a major reason behind the observed error in x when gadolinium and silicon are co-evaporated. The problems faced during the co-evaporation are also discussed.

The design, fabrication, and in-vivo testing of an implantable device to measure the pressure-volume (PV) relationship in the hearts of conscious, untethered rats is presented. Volume is measured using a tetrapolar catheter positioned in the left-ventricle which emits a 20kHz current field across the LV blood pool and parallel heart tissue and measures the resulting voltage. The admittance method is used to instantaneously remove the contribution of the parallel heart muscle and Wei’s non-linear blood conductance-to-volume equation is used to calculate volume. Pressure is measured with a strain gauge sensor at the tip of the catheter. The implant was designed to be small, light, and low-power. An average implant occupies 5 cm3, weighs 8g, and on a single charge collects data for 2 months taking 43 samples per day. Collected data is transmitted wirelessly via RF to a base station where it is recorded. The functionality of the implant and measurement system was verified in six rat experiments. In all experiments, ambulatory PV loops were measured on implantation day. Viable pressure data was recorded for 11 days in one rat; in another rat viable admittance data was collected for 10 days. Changing catheter position and non-constant blood resistivity are considered as sources of error in the volume measurement. Pressure drift due to changing atmospheric pressure is considered as a source of error in the pressure measurement. Lastly, alternative uses for the implant and directions for future improvement are considered. / text

Radiation Efficiency improvement of miniaturized microstrip antenna is studied in this thesis. It is shown that, the loss reduction in miniaturized Microstrip Antenna can be achieved through two possible ways. The first is by modifying the materials used for building the antenna, and the second method is by increasing the radiation conductance of the antenna. Material modification at nano/micro scale by replacing conductors with Metallo-Dielectric one dimensional medium for applications in loss reduction is investigated first. It is shown by the Transfer Matrix Method and using simulations that, for a one dimensional medium replacing very thin conductors (less than skin depth) by laminated multilayered conductors reduce losses. However, the improvement does not exceed the case of single conductor which is a few times thicker than skin depth. Secondly, the efficiency improvement of a small H-Shaped patch antenna by using closely coupled stacked parasitic resonators is studied. It is shown that significant improvement in efficiency can be achieved with minimal changes in the foot print, radiation pattern and cross polarization levels of the antenna. The effect of the overall thickness and superstrate dielectric constant on the efficiency improvement is studied parametrically. It is shown that by using 5 radiating resonators and appropriate choice of inter-conductor dielectric constant, for a small increase in thickness of 0.127mm (5mil), the radiation efficiency can be increased from 2.34% to 6.3%. This efficiency improvement can be made very significant from 2.4% to 33%, by increasing the height to 1.27mm (50mil). These translate to a gain improvement of 4dB and 13dB, respectively. This technique is also demonstrated experimentally in H-Shaped antennas with two different levels of miniaturizations. / October 2015