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A New Spin on Photoemission Spectroscopy

Description: The electronic spin degree of freedom is of general fundamental importance to all matter. Understanding its complex roles and behavior in the solid state, particularly in highly correlated and magnetic materials, has grown increasingly desirable as technology demands advanced devices and materials based on ever stricter comprehension and control of the electron spin. However, direct and efficient spin dependent probes of electronic structure are currently lacking. Angle Resolved Photoemission Spectroscopy (ARPES) has become one of the most successful experimental tools for elucidating solid state electronic structures, bolstered by-continual breakthroughs in efficient instrumentation. In contrast, spin-resolved photoemission spectroscopy has lagged behind due to a lack of similar instrumental advances. The power of photoemission spectroscopy and the pertinence of electronic spin in the current research climate combine to make breakthroughs in Spin and Angle Resolved Photoemission Spectroscopy (SARPES) a high priority . This thesis details the development of a unique instrument for efficient SARPES and represents a radical departure from conventional methods. A custom designed spin polarimeter based on low energy exchange scattering is developed, with projected efficiency gains of two orders of magnitude over current state-of-the-art polarimeters. For energy analysis, the popular hemispherical analyzer is eschewed for a custom Time-of-Flight (TOF) analyzer offering an additional order of magnitude gain in efficiency. The combined instrument signifies the breakthrough needed to perform the high resolution SARPES experiments necessary for untangling the complex spin-dependent electronic structures central to today's condensed matter physics.
Date: December 1, 2008
Creator: Jozwiak, Chris
Partner: UNT Libraries Government Documents Department

Rhodium Catalysts in the Oxidation of CO by O<sub>2</sub> and NO: Shape, Composition, and Hot Electron Generation

Description: It is well known that the activity, selectivity, and deactivation behavior of heterogeneous catalysts are strongly affected by a wide variety of parameters, including but not limited to nanoparticle size, shape, composition, support, pretreatment conditions, oxidation state, and electronic state. Enormous effort has been expended in an attempt to understand the role of these factors on catalytic behavior, but much still remains to be discovered. In this work, we have focused on deepening the present understanding of the role of nanoparticle shape, nanoparticle composition, and hot electrons on heterogeneous catalysis in the oxidation of carbon monoxide by molecular oxygen and nitric oxide. These reactions were chosen because they are important for environmental applications, such as in the catalytic converter, and because there is a wide range of experimental and theoretical insight from previous single crystal work as well as experimental data on nanoparticles obtained using new state-of-the-art techniques that aid greatly in the interpretation of results on complex nanoparticle systems. In particular, the studies presented in this work involve three types of samples: ~ 6.5 nm Rh nanoparticles of different shapes, ~ 15 nm Rh<sub>1-x</sub>Pd<sub>x</sub> core-shell bimetallic polyhedra nanoparticles, and Rh ultra-thin film (~ 5 nm) catalytic nanodiodes. The colloidal nanoparticle samples were synthesized using a co-reduction of metal salts in alcohol and supported on silicon wafers using the Langmuir-Blodgett technique. This synthetic strategy enables tremendous control of nanoparticle size, shape, and composition. Nanoparticle shape was controlled through the use of different organic polymer capping layers. Bimetallic core-shell nanoparticles were synthesized by careful choice of metal salt precursors. Rh/TiO<sub>x</sub> and Rh/GaN catalytic nanodiodes were fabricated using a variety of thin film device fabrication techniques, including reactive DC magnetron sputtering, electron beam evaporation, and rapid thermal annealing. The combination of these techniques enabled control of catalytic nanodiode morphology, geometry, and electrical ...
Date: March 8, 2010
Creator: Renzas, James R.
Partner: UNT Libraries Government Documents Department

In Situ Studies of Surface Mobility on Noble Metal Model Catalysts Using STM and XPS at Ambient Pressure

Description: High Pressure Scanning Tunneling Microscopy (HP-STM) and Ambient Pressure X-ray Photoelectron Spectroscopy were used to study the structural properties and catalytic behavior of noble metal surfaces at high pressure. HP-STM was used to study the structural rearrangement of the top most atomic surface layer of the metal surfaces in response to changes in gas pressure and reactive conditions. AP-XPS was applied to single crystal and nanoparticle systems to monitor changes in the chemical composition of the surface layer in response to changing gas conditions. STM studies on the Pt(100) crystal face showed the lifting of the Pt(100)-hex surface reconstruction in the presence of CO, H<sub>2</sub>, and Benzene. The gas adsorption and subsequent charge transfer relieves the surface strain caused by the low coordination number of the (100) surface atoms allowing the formation of a (1 x 1) surface structure commensurate with the bulk terminated crystal structure. The surface phase change causes a transformation of the surface layer from hexagonal packing geometry to a four-fold symmetric surface which is rich in atomic defects. Lifting the hex reconstruction at room temperature resulted in a surface structure decorated with 2-3 nm Pt adatom islands with a high density of step edge sites. Annealing the surface at a modest temperature (150 C) in the presence of a high pressure of CO or H<sub>2</sub> increased the surface diffusion of the Pt atoms causing the adatom islands to aggregate reducing the surface concentration of low coordination defect sites. Ethylene hydrogenation was studied on the Pt(100) surface using HP-STM. At low pressure, the lifting of the hex reconstruction was observed in the STM images. Increasing the ethylene pressure to 1 Torr, was found to regenerate the hexagonally symmetric reconstructed phase. At room temperature ethylene undergoes a structural rearrangement to form ethylidyne. Ethylidyne preferentially binds at the three-fold ...
Date: June 1, 2010
Creator: Butcher, Derek Robert
Partner: UNT Libraries Government Documents Department

SQUID-Detected MRI in the Limit of Zero Static Field

Description: This thesis describes an implementation of the so-called&quot;zero-field MRI&quot; (ZFMRI) pulse sequence, which allows for imaging in an arbitrarily low B<sub>0</sub> field. The ZFMRI sequence created an effective unidirectional gradient field by using a train of pi pulses to average out the concomitant gradient components during encoding. The signals were acquired using a low-transition temperature dc Superconducting QUantum Interference Device (low-Tc dc SQUID) coupled to a first-order axial gradiometer. The experiments were carried out in a liquid helium dewar which was magnetically shielded with a single-layer mu-metal can around the outside and a superconducting Pb can contained within the helium space. We increased the filling factor of the custom-made, double-walled Pyrex insert by placing the liquid alcohol sample, at a temperature of approximately -50 degrees C, at the center of one loop of the superconducting gradiometer, which was immersed in the helium bath.
Date: December 14, 2009
Creator: Kelso, Nathan Dean
Partner: UNT Libraries Government Documents Department

Studies on two classes of positive electrode materials for lithium-ion batteries

Description: The development of advanced lithium-ion batteries is key to the success of many technologies, and in particular, hybrid electric vehicles. In addition to finding materials with higher energy and power densities, improvements in other factors such as cost, toxicity, lifetime, and safety are also required. Lithium transition metal oxide and LiFePO<sub>4</sub>/C composite materials offer several distinct advantages in achieving many of these goals and are the focus of this report. Two series of layered lithium transition metal oxides, namely LiNi<sub>1/3</sub>Co<sub>1/3-y</sub>M<sub>y</sub>Mn<sub>1/3</sub>O<sub>2</sub> (M=Al, Co, Fe, Ti) and LiNi<sub>0.4</sub>Co<sub>0.2-y</sub>M<sub>y</sub>Mn<sub>0.4</sub>O<sub>2</sub> (M = Al, Co, Fe), have been synthesized. The effect of substitution on the crystal structure is related to shifts in transport properties and ultimately to the electrochemical performance. Partial aluminum substitution creates a high-rate positive electrode material capable of delivering twice the discharge capacity of unsubstituted materials. Iron substituted materials suffer from limited electrochemical performance and poor cycling stability due to the degradation of the layered structure. Titanium substitution creates a very high rate positive electrode material due to a decrease in the anti-site defect concentration. LiFePO<sub>4</sub> is a very promising electrode material but suffers from poor electronic and ionic conductivity. To overcome this, two new techniques have been developed to synthesize high performance LiFePO<sub>4</sub>/C composite materials. The use of graphitization catalysts in conjunction with pyromellitic acid leads to a highly graphitic carbon coating on the surface of LiFePO<sub>4</sub> particles. Under the proper conditions, the room temperature electronic conductivity can be improved by nearly five orders of magnitude over untreated materials. Using Raman spectroscopy, the improvement in conductivity and rate performance of such materials has been related to the underlying structure of the carbon films. The combustion synthesis of LiFePO<sub>4</sub> materials allows for the formation of nanoscale active material particles with high-quality carbon coatings in a quick and inexpensive fashion. The carbon coating ...
Date: December 1, 2008
Creator: Wilcox, James Douglas
Partner: UNT Libraries Government Documents Department

Synthesis and structural, magnetic, thermal, and transport properties of several transition metal oxides and aresnides

Description: Oxide compounds containing the transition metal vanadium (V) have attracted a lot of attention in the field of condensed matter physics owing to their exhibition of interesting properties including metal-insulator transitons, structural transitions, ferromagnetic and an- tiferromagnetic orderings, and heavy fermion behavior. Binary vanadium oxides VnO<sub>2n-1</sub> where 2 ≤ n ≤ 9 have triclinic structures and exhibit metal-insulator and antiferromagnetic transitions.[1–6] The only exception is V<sub>7</sub>O<sub>13</sub> which remains metallic down to 4 K.[7] The ternary vanadium oxide LiV<sub>2</sub>O<sub>4</sub> has the normal spinel structure, is metallic, does not un- dergo magnetic ordering and exhibits heavy fermion behavior below 10 K.[8] CaV<sub>2</sub>O<sub>4</sub> has an orthorhombic structure[9, 10] with the vanadium spins forming zigzag chains and has been suggested to be a model system to study the gapless chiral phase.[11, 12] These provide great motivation for further investigation of some known vanadium compounds as well as to ex- plore new vanadium compounds in search of new physics. This thesis consists, in part, of experimental studies involving sample preparation and magnetic, transport, thermal, and x- ray measurements on some strongly correlated eletron systems containing the transition metal vanadium. The compounds studied are LiV<sub>2</sub>O<sub>4</sub>, YV<sub>4</sub>O<sub>8</sub>, and YbV<sub>4</sub>O<sub>8</sub>. The recent discovery of superconductivity in RFeAsO<sub>1-x</sub>F<sub>x</sub> (R = La, Ce, Pr, Gd, Tb, Dy, Sm, and Nd), and AFe<sub>2</sub>As<sub>2</sub> (A = Ba, Sr, Ca, and Eu) doped with K, Na, or Cs at the A site with relatively high T<sub>c</sub> has sparked tremendous activities in the condensed matter physics community and a renewed interest in the area of superconductivity as occurred following the discovery of the layered cuprate high T<sub>c</sub> superconductors in 1986. To discover more supercon- ductors with hopefully higher T<sub>c</sub>’s, it is extremely important to investigate compounds having crystal structures related to the compounds showing high T<sub>c</sub> superconductivity. Along with the vanadium oxide compounds described ...
Date: January 1, 2010
Creator: Das, Supriyo
Partner: UNT Libraries Government Documents Department

Yb-based heavy fermion compounds and field tuned quantum chemistry

Description: The motivation of this dissertation was to advance the study of Yb-based heavy fermion (HF) compounds especially ones related to quantum phase transitions. One of the topics of this work was the investigation of the interaction between the Kondo and crystalline electric field (CEF) energy scales in Yb-based HF systems by means of thermoelectric power (TEP) measurements. In these systems, the Kondo interaction and CEF excitations generally give rise to large anomalies such as maxima in ρ(T) and as minima in S(T). The TEP data were use to determine the evolution of Kondo and CEF energy scales upon varying transition metals for YbT<sub>2</sub>Zn<sub>20</sub> (T = Fe, Ru, Os, Ir, Rh, and Co) compounds and applying magnetic fields for YbAgGe and YbPtBi. For YbT<sub>2</sub>Zn<sub>20</sub> and YbPtBi, the Kondo and CEF energy scales could not be well separated in S(T), presumably because of small CEF level splittings. A similar effect was observed for the magnetic contribution to the resistivity. For YbAgGe, S(T) has been successfully applied to determine the Kondo and CEF energy scales due to the clear separation between the ground state and thermally excited CEF states. The Kondo temperature, T<sub>K</sub>, inferred from the local maximum in S(T), remains finite as magnetic field increases up to 140 kOe. In this dissertation we have examined the heavy quasi-particle behavior, found near the field tuned AFM quantum critical point (QCP), with YbAgGe and YbPtBi. Although the observed nFL behaviors in the vicinity of the QCP are different between YbAgGe and YbPtBi, the constructed H-T phase diagram including the two crossovers are similar. For both YbAgGe and YbPtBi, the details of the quantum criticality turn out to be complicated. We expect that YbPtBi will provide an additional example of field tuned quantum criticality, but clearly there are further experimental investigations left and more ideas ...
Date: January 1, 2010
Creator: Mun, Eundeok
Partner: UNT Libraries Government Documents Department

Analysis on linac quadrupole misalignment in FACET commissioning 2012

Description: In this note, the analysis on linac quadrupole misalignment is presented for the FACET linac section LI05-09 plus LI11-19. The effectiveness of the beam-based alignment technique is preliminarily confirmed by the measurement. Beam-based alignment technique was adopted at SLAC linac since SLC time. Here the beam-based alignment algorithms are further developed and applied in the FACET commissioning during 2012 run.
Date: July 5, 2012
Creator: Sun, Yipeng
Partner: UNT Libraries Government Documents Department

Application of electron-transfer theory to several systems of biological interest

Description: Electron-transfer reaction rates are compared with theoretically calculated values for several reactions in the bacterial photosynthetic reaction center. A second aspect of the theory, the cross-relation, is illustrated using protein-protein electron transfers. 22 refs., 3 tabs.
Date: January 1, 1985
Creator: Marcus, R.A. & Sutin, N.
Partner: UNT Libraries Government Documents Department

Salt Dependence of the Radius of Gyration and Flexibility of Single-stranded DNA in Solution probed by Small-angle X-ray Scattering

Description: Short single-stranded nucleic acids are ubiquitous in biological processes and understanding their physical properties provides insights to nucleic acid folding and dynamics. We used small angle x-ray scattering to study 8-100 residue homopolymeric single-stranded DNAs in solution, without external forces or labeling probes. Poly-T's structural ensemble changes with increasing ionic strength in a manner consistent with a polyelectrolyte persistence length theory that accounts for molecular flexibility. For any number of residues, poly-A is consistently more elongated than poly-T, likely due to the tendency of A residues to form stronger base-stacking interactions than T residues.
Date: July 6, 2012
Creator: Sim, Adelene Y.L.; Lipfert, Jan; Herschlag, Daniel & Doniach, Sebastian
Partner: UNT Libraries Government Documents Department

The LCLS Timing Event System

Description: The Linac Coherent Light Source requires precision timing trigger signals for various accelerator diagnostics and controls at SLAC-NAL. A new timing system has been developed that meets these requirements. This system is based on COTS hardware with a mixture of custom-designed units. An added challenge has been the requirement that the LCLS Timing System must co-exist and 'know' about the existing SLC Timing System. This paper describes the architecture, construction and performance of the LCLS timing event system.
Date: July 23, 2012
Creator: Dusatko, John; Allison, S.; Browne, M.; Krejcik, P. & /SLAC
Partner: UNT Libraries Government Documents Department