Members of the two-pore potassium-selective (K2P) ion channel superfamily control cell excitability by contributing to the resting membrane potential. Through this, K2P channels are involved in a variety of physiological processes and dysfunction of these channels has been linked to diseases such as epilepsy, depression and migraine. The aim of this study was to develop a greater understanding of how K2P channels, in particular TREK-1, are gated. In the initial stage of this study we hoped to identify mutations which alter the function of the TREK-1 channel by screening a random mutant library using a K+-auxotrophic strain of S. cerevisiae, SGY1528. From the assay we identified a number of gain-of-function (GoF) mutations, primarily distributed through the pore-lining transmembrane (TM) helices TM2 and TM4. The clustering of mutations in these regions suggested a major role for these helices in channel gating. Subsequent electrophysiological characterisation of these mutations revealed an increase in basal channel activity and altered sensitivity to modulation by extracellular pH, as well as by activators DEPC and BL-1249. The publication of two K2P channel crystal structures, TWIK-1 (pdb code: 3UKM) and TRAAK (3UM7), enabled us to build an accurate homology model of TREK-1 and more accurately interpret these functional studies. This approach revealed a number of interesting points, the most important being an interaction between TM4 and pore-helix 1 (PH1). Further mutagenesis studies of this region confirmed that this interaction is essential for normal channel function. Another interaction was identified involving a number of residues within the interface between TM helices TM2, TM3 and TM4 emphasised the importance of helical movements for gating TREK-1 channels. Based on our findings we therefore present a model for gating of the TREK-1 channel, which suggests that the movement of TM4 in particular, is transduced to the selectivity filter gate via PH1.

Given their unique structural properties, quantum rings (QRs) structures have recently been of particular interest for investigating quantum interference which is called the neutral charged exciton optical Aharonov-Bohm (AB) effect. A delocalized wavefunction around the rim is a prerequisite for the AB effect, but asymmetry and anisotropy seem to have been overlooked in the spectroscopy of QRs. In this thesis, the presence of a localized state in a single GaAs QR is presented.

<p> For a broad class of two-dimensional (2D) materials, the transition from isotropic fluid to crystalline solid is described by the theory of melting due to Kosterlitz, Thouless, Halperin, Nelson and Young (KTHNY). According to this theory, long-range order is achieved via elimination of the topological defects which proliferate in the fluid phase. However, many natural and man-made 2D systems posses spatial curvature and/or non-trivial topology, which require the presence of topological defects, even at T=0. In principle, the presence of these defects could profoundly affect the phase behavior of such a system. In this thesis, we develop and characterize an experimental system of charged colloidal particles that bind electrostatically to the interface between an oil and an aqueous phase. Depending on how we prepare the sample, this fluid interface may be flat, spherical, or have a more complicated geometry. Focusing on the cases where the interface is flat or spherical, we measure the interactions between the particles, and probe various aspects of their phase behavior. On flat interfaces, this phase behavior is well-described by KTHNY theory. In spherical geometries, however, we observe spatial structures and inhomogeneous dynamics that cannot be captured by the measures traditionally used to describe flat-space phase behavior. We show that, in the spherical system, ordering is achieved by a novel mechanism: sequestration of topological defects into freely-terminating grain boundaries (&ldquo;scars&rdquo;), and simultaneous spatial organization of the scars themselves on the vertices of an icosahedron. The emergence of icosahedral order coincides with the localization of mobility into isolated &ldquo;lakes&rdquo; of fluid or glassy particles, situated at the icosahedron vertices. These lakes are embedded in a rigid, connected &ldquo;continent&rdquo; of locally crystalline particles.</p><p>

Carbon nanostructures have emerged as a key material in nanotechnology and continuously find new areas of applications. Particularly, they are attractive due to their excellent properties as support for catalyst nanostructures leading to highly efficient composite materials for various electrochemical applications. The interest in these structures is further increased by the possibility to alter their electronic and structural properties by various methods. Heteroatom doping of carbon nanostructures is one of the approaches which may induce intrinsic catalytic activity in these materials. In addition, such introduction of guest elements into the hexagonal carbon skeleton provides strong nucleation sites which facilitate the stabilization of nanostructures on their surface. In this thesis we present detailed studies on the nitrogen incorporation into carbon nanostructures, particularly carbon nanotubes and reduced graphene oxide. Due to the high impact of nitrogen configuration on the intrinsic electrocatalytic properties of carbon nanostructures, we investigated the nitrogen functionalities using X-ray photoelectron spectroscopy and Raman spectroscopy. Based on our achievements we could assign the most electrocatalytic active nitrogen site in nitrogen-doped carbon nanotubes (NCNTs) for catalytic oxygen reduction reaction (ORR) which is an important reaction in energy conversion systems such as fuel cells. We then used nitrogen-doped carbon nanostructures as a key component to manufacture hybrid material, where the nitrogen doped nanostructures has a role of both stabilizing the nanostructures and to work as conductive additive to assist the charge transfer from the other constituents suffering from inherently poor conductivity. Our hybrid material comprising transition metal oxides (Fe2O3 and Co3O4) anchored on nitrogen-doped carbon nanostructure were used to both manufacture an exotic type of graphene nanoscrolls, as well as studied and evaluated as an electrocatalyst in various electrochemical reactions. We show that the self-assembled electrodes exhibited better performance and higher stability compared to when the same material was loaded on common current collectors such as fluorine tin oxide (FTO) coated glass and glassy carbon electrode, with both higher current densities, more efficient charge transfer and lower overpotentials for oxygen evolution and hydrogen evolution reactions, the two important processes in a water splitting device. Our NCNTs-based electrodes showed further excellent performance in lithium ion batteries with high cyclability and capacity. The thesis gives insight into processes, materials, and methods that can be utilized to manufacture an efficient water splitting device, based on earth-abundant self-assembled materials. It further represents a significant advancement of the role of nitrogen in heteroatom-doped nanostructures, both regarding their intrinsic catalytic activity, as well as their role for stabilizing nanostructures.

No description available.

The theoretical studies of several magnetic materials are presented in this thesis. To each of them, it was investigated the electronic structure, by means of density functional theory calculations, and/or magnetization dynamics, in the context of atomistic spin dynamics (ASD).  For bulk properties, we evaluate the magnon spectra of the heavy rare earths (Gd, Tb, Dy, Ho, Er, and Tm), using the exchange parameters and magnetic moments from first-principles calculations in ASD simulations. Additionally, we performed Monte Carlo simulations that nicely reproduced the qualitative trend of lowering of the critical temperatures across the series. Next, we discuss about the microscopic mechanism of the vanishingly low magnetic anisotropy of Permalloy using the concept of the orbital moment anisotropy for Fe and Ni atoms in the alloy.  Turning to surface magnetism, we discuss the use of exchange parameters computed by a noncollinear formalism for 6 monolayers of Fe on the Ir(001) substrate, in order to have a more accurate description of magnons at finite temperature and to obtain good comparison with experimental data. Besides that, we also studied surface magnons on 3 and 9 Ni monolayers on Cu(001) and Cu(111) in order to track the significant surface and/or interface effects and contrast it to properties that are fcc Ni bulk-like. Likewise, we used the Monte Carlo method to estimate the critical temperatures of Ni surfaces and compared with experimental data.  Finally, in the field of low dimensional magnetism, we present the ab-initio calculations for the electronic structure of Cr nanostructures of diverse geometries adsorbed on the Pd(111) surface, with focus on the formation of non-collinear spin configurations, either due to geometric frustration or the spin-orbit coupling provided by the substrate.

In this thesis the spark plasma sintering process (SPS) was used to press Mg2Si powder with Ni and Cu slices into alternating layer stacks. These stacks, once cut at an angle, are an artificially anisotropic material. This anisotropy provides transverse thermoelectric properties to the sample. The transverse transport properties were measured along with the individual component transport properties. The SPS process provided malleable samples that gave a power factors of for the Ni/Mg2Si stack and for the Cu/Mg2Si stack. These fall short of the theoretical calculations which would give the power factors as .0254 for the Ni/Mg2Si stack and .211 for the Cu/Mg2Si stack. It is theorized that eddy currents and interface resistances between the layers are the causes for these discrepancies.

<div>Achievements in the growth of ultra-pure III-V semiconductor materials using state of the art molecular beam epitaxy (MBE) machine has led to the discovery of new physics and technological innovations. High mobility two-dimensional electron gas (2DEG) embedded in GaAs/Al_xGa_1−xAs heterostructures provides an unparalleled platform for many-body physics including fractional quantum Hall effect. On the other hand, single electron devices fabricated on modulation doped GaAs/Al_xGa_1−xAs heterostructures have been extensively used for fabrication of quantum devices such as spin qubit with application in quantum computing. Furthermore, epitaxial hybrid superconductor-semiconductor heterostructures with ultra clean superconductor-semiconductor interface have been grown using MBE technique to explore rare physical quantum state of the matter namely Majorana zero modes with non-abelian exchange statistics.</div><div><br></div><div><div>Chapter 1 in the manuscript starts with description of GaAs MBE system at Purdue University and continues with the modifications have been made to MBE hardware and growth conditions for growing heterostrcutures with 2DEG mobility exceeding 35 × 10⁶ cm⁻²/V s. Utilizing an ultra-high pure Ga source material and its further purification by thermal evaporation in the vacuum are determined to have major impact on growth of high mobility GaAs/Al_xGa_1−xAs heterostructures.</div></div><div><br></div><div>Chapter 2 reports a systematic study on the effect of silicon doping density on low frequency charge noise and conductance drift in laterally gated nanostructures fabricated on modulation doped GaAs/Al_xGa_1−xAs heterostructures grown by Molecular Beam Epitaxy (MBE). The primary result of this study is that both charge noise and conductance drift are strongly impacted by the silicon doping used to create the two-dimensional electron gas. These findings shed light on the physical origin of the defect states responsible for charge noise and conductance drift. This is especially significant for spin qubit devices, which require minimization of conductance drift and charge noise for stable operation and good coherence. <br></div><div><br></div><div>Chapter 3 demonstrates measurements of the induced superconducting gap in 2D hybrid Al/Al_0.15In_0.85As/InAs heterostructures which is a promising platform for scaling topological qubits based on Majorana zero modes. The 2DEG lies in an InAs quantum well and is separated from the epitaxial Al layer by a barrier of Al_0.15In_0.85As with thickness d. Due to hybridization between the wave functions of 2DEG and superconductor, the strength of induced gap in the 2DEG largely depends on the barrier thickness. This chapter presents a systematic study of the strength of the induced gap in hybrid Al/Al_0.15In_0.85As/InAs superconductor/semiconductor heterostructures as a function of barrier thickness.<br></div><div><br></div><div><br></div>

Disorder in crystalline materials can take different forms and originate from different sources. In particular, temperature introduces disorder in any kind of material. This can be observed as the appearance of vacant lattice sites in an otherwise perfect crystal, or as a random distribution of different elements on the same lattice in an alloy; at the same time, if the material is magnetic, temperature induces disorder also on the magnetic degrees of freedom. In this thesis, different levels of disorder associated to structure and magnetism are investigated by means of density functional theory and thermodynamic models. I start with diffusion of Ti vacancies in TiN, which is studied by means of nonequilibrium ab initio molecular dynamics using the color diffusion algorithm at different temperatures. The result is an Arrhenius behavior of Ti vacancy jump rates. A method to perform structural relaxations in magnetic materials in their hightemperature paramagnetic phase is then developed based on the disordered local moments approach in order to study vacancies, interstitial atoms, and combinations of defects in paramagnetic bcc Fe and B1 CrN, as well as the mixing enthalpy of bcc Fe1−xCrx random alloys. A correction to the energetics of every system due to the relaxation in the disordered magnetic state is observed in all cases. Not related to temperature and disorder, but very important for an accurate description of magnetic materials, is the choice of the exchange and correlation functional to be employed in the first principles calculations. We have investigated the performance of a recently developed meta-GGA functional, the strongly constrained and appropriately normed (SCAN) functional, in comparison with the more commonly used LDA and PBE on the ferromagnetic elemental solids bcc Fe, fcc Ni, and hcp Co, and SCAN it is found to give negligible improvements, if not a worsening, in the description of these materials. Finally, the coupling between vibrational and magnetic degrees of freedom is discussed by reviewing the literature and proposing an investigation of the influence of vibrations on longitudinal spin fluctuations. These excitations are here studied by means of thermodynamic models based on Landau expansion of the energy in even powers of the magnitude of the local magnetic moments. We find that vibrational and magnetic disorder alter the energy landscapes as a function of moment size also in bcc Fe, which is often considered a Heisenberg system, inducing a more itinerant electron behavior.

In this project a microscopic description of the magnon-phonon coupling is rederived. This theory is then used to simulate the thermalization process between the spin and lattice system in NiO. It is argued that the Kusuya-LeCraw process (1 mg + 1 ph into 1 mg) cannot be the reason that the phononic and magnonic systems reach a thermal equilibrium. As a consequence a quasi-stable equilibrium, different from the thermal solution, is reached in less than 1 ps for NiO. Finally, the effect of applying an external magnetic field to the system, breaking the degeneracy between the magnons of opposite polarization, is studied.