The Single Electron Transistor (SET), especially its variation the radio frequency single electron transistor (RF-SET), is a fast and ultra-sensitive electrometer working in the vicinity of the quantum limit. In this thesis, the theory and techniques related to the SET in the normal state and the superconducting state are introduced The thesis focuses especially on our efforts of improving the sensitivity of superconducting RF-SET, including: the design and fabrication of an all superconducting near loss-less matching network for improving the RF modulation, our early investigation of the sensitivity and linearity of a superconducting SET (S-SET) subject to quantum fluctuations of quasiparticles and Cooper pairs under different tunneling mechanisms, and our recent work approaching the quantum sensitivity limit using a double Josephson quasiparticle (DJQP) cycle of a S-SET. Using an effective bath description, we have found that the S-SET provides damping of the resonator modes proportional to its differential conductance, and has an effective temperature that can be well below both the ambient temperature and the energy scale of the bias voltage. In the region of negative differential conductance (NDC), the S-SET shows negative damping and a negative temperature. In the final part of the thesis, I have demonstrated our first application of an RF-SET for real-time detection of electron tunneling through a quantum dot.

As the size of a physical system decreases toward the nanoscale, quantum mechanical effects such as the discretization of energy levels and the interactions of the electronic spins become readily observable. To understand what happens within submicrometer scale samples is one of the goals of modern condensed matter physics. Electron transport phenomena drew a lot of attention over the past two decades or so, not only because quantum corrections to the classical transport theory, but also they allow us to probe deeply into the microscopic nature of the system put to test. Although a significant amount of research was done in the past and thus extended our understanding in this field, most of these works were concentrated on simpler examples. Electron transport in strongly correlated systems is still a field that needs to be explored more thoroughly. In fact, experimental works that have been done so far to characterize coherence physics in correlated systems such as ferromagnetic metals are far from conclusive. One reason ferromagnetic samples draw such attention is that there exist correlations that lead to excitations (e.g. spin waves, domain wall motions) not present in normal metals, and these new environmental degrees of freedom can have profound effects on decoherence processes. In this thesis, three different types of magnetic samples were examined: a band ferromagnetism based metallic ferromagnet, permalloy, a III-V diluted ferromagnetic semiconductor with ferromagnetism from a hole-mediated exchange interaction, and magnetite nanocrystals and films. The first observation of time-dependent universal conductance fluctuations (TD-UCF) in permalloy is presented and our observations lead to three major conclusions. First, the cooperon contribution to the conductance is suppressed in this material. This is consistent with some theoretical expectations, and implies that weak localization will be suppressed as well. Second, we see evidence that domain wall motion leads to enhanced conductance fluctuations, demonstrating experimentally that domain walls can act as coherent scatterers of electrons. Third, the temperature dependence of the fluctuations is surprisingly strong, suggesting that the dominant decoherence mechanism in these wires is different than that in similar normal metal nanostructures. The first observation of TD-UCF in diluted magnetic semiconductors (DMS) is also presented. In contrast to analogous measurements on permalloy samples, we find a surprising suppression of TD-UCF noise in this material at low temperatures, independent of field orientation. We believe this implies that the suppression is not due to an orbital effect, and therefore some of the fluctuations originate with time-varying magnetic disorder. The temperature dependence of the TD-UCF implies either an unusual fluctuator spectrum or a nonstandard dephasing mechanism. Measurements of UCF as a function of magnetic field allow an order of magnitude estimate of the coherence length at 2 K of approximately 50 nm in this material. The last samples examined were magnetite nanocrystals and films. Magnetite has been used in technologies for millennia, from compasses to magnetoelectronic devices, although its electronic structure has remained controversial for seven decades, with a low temperature insulator and a high temperature "bad metal" separated by the Verwey transition at 120 K. A new electrically driven insulator-metal transition below the Verwey temperature in both magnetite films and nanocrystals was observed. The possibility that this was a thermal effect was tested through various methods, and we have shown that the transition is in fact truly electrically driven. This electrically driven transition also showed a great deal of rigidity against external magnetic field and high gate voltages.

Single-Walled Carbon Nanotubes (SWNT) have been found to have excellent solubility in super acids such as 100+% H2SO4, and chlorsulfonic acid. The solutions display liquid crystalline behavior at high concentrations in super acids. Traditional wet-spinning method has been applied to SWNTs to make fibers from SWNTs only with the assistance of 100+% H2SO 4 (neat SWNT fibers). Extensive conditions, including concentrations, coagulation, and operation temperature, have been explored with Daca mixer and other custom-designed apparatuses (SBM and Refined Mixer). Fibers' properties have been tremendously improved through the research. Different characterizations have been done and all of them confirmed the neat SWNT fibers have the best alignment to-date among any macroscopic neat SWNT articles, as well as electrical conductivities. Meanwhile, neat SWNT fibers were used for X-ray diffraction study. For the first time, direct evidence has been provided to support the strong intercalation between SWNTs and super acids. Interestingly, for the first time, it has been reported that liquid sulfuric acid forms shell structure while exposed to SWNTs.

Coupled plasmonic systems have been in the limelight recently for both their interesting fundamental physical properties, and their possible applications in sensing and optoelectrical system integration. Planar plasmonic systems, which can couple propagating and localized plasmons, show promise for integration of future on-chip optoelectronic devices. This thesis investigates such a system consisting of a planar metallodielectric grating coupled to spherical gold nanoparticles. It is shown that the addition of gold nanoparticles to a silver metallodielectric grating system can produce a profound change in the resonance response of the system. This coupling is shown to depend strongly on the size and surface coverage density of the gold nanoparticles.

Charge detection at microsecond time-scales has far reaching consequences in both technology and in our understanding of electron dynamics in nanoscale devices such as quantum dots. Radio-frequency superconducting single electron transistors (RF-SET) and quantum point contacts (QPC) are ultra sensitive charge sensors operating near the quantum limit. The operation of RF-SETs outside the superconducting gap has been a topic of study; the sub-gap operation, especially in the presence of large quantum fluctuations of quasiparticles remains largely unexplored, both theoretically and experimentally. We have investigated the effects of quantum fluctuations of quasiparticles on the operation of RF-SETs for large values of the quasiparticle cotunneling parameter alpha = 8EJ/Ec, where EJ and Ec are the Josephson and charging energies. We find that, for alpha > 1, sub-gap RF-SET operation is still feasible despite quantum fluctuations that wash out quasiparticle tunneling thresholds. Such RF-SETs show linearity and signal-to-noise ratio superior to those obtained when quantum fluctuations are weak, while still demonstrating excellent charge sensitivity. We have operated a QPC charge detector in a radio frequency mode that allows fast charge detection in a bandwidth of several megahertz. The noise limiting the sensitivity of the charge detector is not the noise of a secondary amplifier, but the non-equilibrium device noise of the QPC itself. The noise power averaged over a measurement bandwidth of about 10MHz around the carrier frequency is in agreement with the theory of photon-assisted shot noise. Frequency-resolved measurements, however show several significant discrepancies with the theoretical predictions. The measurement techniques developed can also be used to investigate the noise of other semiconductor nanostructures such as quantum dots in the Kondo regime. A study of the noise characteristics alone can not determine whether the device is operating at the quantum limit; a characterization of back action is also necessary. The inelastic current through a double quantum dot system (DQD) is sensitive to the spectral density of voltage fluctuations in its electromagnetic environment. Electrical transport studies on a DQD system electrostatically coupled to an SET shows qualitative evidence of back-action of SET. The design and fabrication of a few electron DQD device with integrated RF-SET and QPC charge sensors for the study of back action of the sensors and real-time electron dynamics in the DQD are also discussed.

Resonant X-ray scattering has been observed in lanthanides and actinides. The observed scattering is sensitive to magnetization, but it arises primarily from electric, rather than magnetic, multipole transitions. The magnetic sensitivity is due to "exchange" effects and to spin-orbit correlations in one or both of the levels involved in the resonance. "Exchange" effects include the exclusion principle, which permits transitions only to vacant orbitals in partially filled shells, and exchange splitting in band states. The elastic scattering amplitude contains terms dependent on the direction of the magnetic moment up to quadratic order for E1 transitions and up to quartic order for E2 transitions. In coherent scattering of X-rays from spiral antiferromagnets, the magnetic sensitivity results in the formation of resonant magnetic satellites associated with the Bragg peaks. This effect provides a new type of probe for the investigation of magnetic structures.

We are studying the charge transfer in atom-surface scattering using a recently developed many-body theory. The final population of the atom is studied as a function of the surface workfunction, which has a strong effect on the final population. The effects caused by degeneracy, surface bandwidth and velocity are investigated. The formation of the Kondo peak, strongly controls both the initial population and the rate of charge transfer, and consequently the final population. The results show a strong degeneracy and velocity dependence for both the positive and negative ion. For the negative ion we also find a significant bandwidth dependence.

Spin-sensitive electron spectroscopies were used to study paramagnetic metal surfaces and thin films. In these experiments, a beam of low-energy spin-polarized electrons from a GaAs source is directed at the target surface, and the spin polarizations and the energy distributions of scattered electrons are measured by a compact Mott polarimeter equipped with either a retarding field analyzer or a concentric hemispherical analyzer. Secondary electrons ejected from a Cu(100) surface by a polarized incident beam of energies 14, 50 and 100 eV were investigated by spin-polarized secondary electron spectroscopy. The data are interpreted in terms of the fractional contributions of rediffused primary and true secondary electrons to the total secondary electron yield. Spin-polarized electron-energy-loss spectra were obtained from Cu(100) and Mo(110). The data reveal strong evidence for inelastic spin-flip exchange scattering due to electron-hole pair excitation. In particular, a prominent polarization-loss feature evident in the Mo(110) data correlates with the joint density of occupied and unoccupied states. The dramatically different behavior in the polarization of scattered electrons from molybdenum and copper was employed as a signature to determine the attenuation length for low energy electrons in molybdenum. When molybdenum thin films were grown on a Cu(100) substrate, the Mo loss feature becomes prominent upon deposition of only one monolayer.

Spin-polarized electron energy loss spectroscopy (SPEELS) has been used to investigate several paramagnetic surfaces. In this technique, a low energy beam of spin-polarized electrons from a GaAs source is directed at the surface to be studied and the spin-polarization of the emitted electrons, as well as the kinetic energy distribution, is measured via a micro-Mott polarimeter equipped with a retarding potential energy analyzer. The near-elastic-energy electrons contain information on the inelastic scattering channels available in the solid. The spin-polarization of these same electrons is sensitive to the shape of the final state manifold, i.e., the density of unoccupied states. The low energy and behavior of the spin-polarization spectrum mainly reflects the high number of unpolarized electrons present near zero kinetic energy. Results from Cu(100), polycrystalline Au, GaAs(110), GaAs(100) and Mo(110) surfaces suggest that exchange scattering with spin-flip is ubiquitous for the lowest energy electron beam energies studied ($\sim$14 eV). A simple convolution of the empty and occupied densities of states correctly predicts the shape of the energy dependent spin-flip rate.

Metastable (Atom) De-excitation Spectroscopy (MDS) provides a powerful technique with which to investigate surface electronic structure with extreme surface specificity. In this technique a thermal energy beam of noble-gas metastable atoms is directed at the surface under study and the kinetic energy distribution of ejected electrons that result from metastable atom de-excitation is measured. Although the measured distribution contains information about the electronic structure of the outermost surface layer, its detailed analysis requires knowledge of the dynamics of the metastable atom-surface interaction. In the present work, these dynamics have been investigated directly by use of spin-labeling techniques. The electron spins of the incident metastable atoms are polarized and the spin-polarization of the ejected electrons is measured with a Mott polarimeter. Energy resolve electron spin-polarization measurements are reported for a variety of sub-monolayer coverages of cesium on a Cu(100) surface, and for oxygen and cesium co-adsorption on a Cu(100) surface. The Cs/Cu(100) system exhibits large ($\sim$2.8 eV) change in the surface work function. The results of the current work suggest that MDS interactions in both high and low work function regimes are more complex than has previously been supposed. Several additional interactions are suggested to explain the data acquired. The question of the occupancy of the adsorbed cesium valence level at various coverages is also addressed.