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Optical manipulation of ultrafast electron and nuclear motion on metal surfaces

Description: We study the unoccupied electronic structure and dynamics of chemisorbed atoms and molecules on metal surfaces by time resolved two-photon photoemission (TR-2PP). spectroscopy, low temperature scanning tunneling microscopy (LT-STM), and theory. Our research concerns simple atomic adsorbates such as alkali and alkaline earth atoms, which provide fundamentally important models for adsorbate-surface interactions, and more complex adsorbates such as fullerenes on noble metals, which illustrate emergent interfacial properties that derive from intrinsic molecular attributes, and moleculemolecule and molecule-surface interactions. Our goal is to understand how these interactions contribute to formation of the interfacial electronic structure, and how thus formed electronic properties affect interfacial phenomena of importance to energy transduction and storage. Moreover, we explore how the interfacial electronic excitation drives dynamical phenomena such as charge transfer and surface femtochemistry.
Date: December 2, 2009
Creator: Petek, Hrvoje
Partner: UNT Libraries Government Documents Department

Magnetism in Non-Traditional Materials

Description: We performed a systematic microscopic investigation of two completely dissimilar materials (namely, ZnO and rhombohedral-C{sub 60} polymers) exhibiting ferromagnetism in the presence of defects, and showed that this new phenomena has a common origin and the mechanism responsible can be used as a powerful tool for inducing and tailoring magnetic features in systems which are not magnetic otherwise. Based on our findings we proposed a general recipe for developing ferromagnetism in new materials of great technological interest. Our results support the role of complimentary pairs of defects in inducing magnetism in otherwise non-magnetic materials belonging to two widely differing classes with no apparent correlation between them. In both classes, ferromagnetism is found to be enhanced when the two kinds of defects form structures (pathways) of alternating effective donor and acceptor crystal sites leading to the development of electron charge and spin density like waves. Using ab initio density functional theory calculations we predicted the existence of a new class of carbon cages formed via hybrid connection between planar graphene sheets and carbon nanotubes. The resulting novel structure has the appearance of ?nano-drum? and offers the exciting prospect of integrating useful device properties of both graphene as well as the nanotube into a single unit with tunable electronic properties. Creation of a hexagonal hole in the graphene portion of this structure results in significant magnetic moments for the edge atoms. The structure appears to be capable of sustaining ferrimagnetic state with the assistance of topological defects. The charge and spin distributions obtained in our calculations for the nano-drums are in striking contrast to those in planar graphene nanoribbons with a central hole. In this case, the central hole appears as the complimentary defect to those of the ribbon edges. Similar situation is found in case of the nano-drum in which the ...
Date: September 17, 2013
Creator: Menon, Madhu
Partner: UNT Libraries Government Documents Department

Characterization of a high-temperature superconducting conductor on round core cables in magnetic fields up to 20 T

Description: The next generation of high-field magnets that will operate at magnetic fields substantially above 20 T, or at temperatures substantially above 4.2 K, requires high-temperature superconductors (HTS). Conductor on round core (CORC) cables, in which RE-Ba{sub 2}Cu{sub 3}O{sub 7-{delta}} (RE = rare earth) (REBCO) coated conductors are wound in a helical fashion on a fl‚exible core, are a practical and versatile HTS cable option for low-inductance, high-field magnets. We performed the first tests of CORC magnet cables in liquid helium in magnetic fields of up to 20 T. A record critical current I{sub c} of 5021 A was measured at 4.2 K and 19 T. In a cable with an outer diameter of 7.5 mm, this value corresponds to an engineering current density J{sub e} of 114 A mm{sup -2} , the highest J{sub e} ever reported for a superconducting cable at such high magnetic fields. Additionally, the first magnet wound from an HTS cable was constructed from a 6 m-long CORC cable. The 12-turn, double-layer magnet had an inner diameter of 9 cm and was tested in a magnetic field of 20 T, at which it had an I{sub c} of 1966 A. The cables were quenched repetitively without degradation during the measurements, demonstrating the feasibility of HTS CORC cables for use in high-field magnet applications.
Date: February 13, 2013
Creator: van der Laan, Danko; Noyes, Patrick; Miller, George; Weijers, Hubertus & Willering, Gerard
Partner: UNT Libraries Government Documents Department

Charge Imbalance

Description: The purpose of this article is to review the theory of charge imbalance, and to discuss its relevance to a number of experimental situations. We introduce the concepts of quasiparticle charge and charge imbalance, and discuss the generation and detection of charge imbalance by tunneling. We describe the relaxation of the injected charge imbalance by inelastic scattering processes, and show how the Boltzmann equation can be solved to obtain the steady state quasiparticle distribution and the charge relaxation rate. Details are given of experiments to measure charge imbalance and the charge relaxation rate when inelastic scattering is the predominant relaxation mechanism. Experiments on and theories of other charge relaxation mechanisms are discussed, namely relaxation via elastic scattering in the presence of energy gap anisotropy, or in the presence of a pair breaking mechanism such as magnetic impurities or an applied supercurrent or magnetic field. We describe three other situations in which charge imbalance occurs, namely the resistance of the NS interface, phase slip centers, and the flow of a supercurrent in the presence of a temperature gradient.
Date: September 1, 1980
Creator: Clarke, John
Partner: UNT Libraries Government Documents Department

Cluster-Variation Method for the Triangular Lattice Gas [Part] 1. Three-Sublattice Point Approximation

Description: The triangular lattice gas is examined as a model for the ordering of Li{sup +} ions in intercalated transition metal dichalcogenides. A three-sublattice Bragg~ Williams (point approximation) calculation illustrates the meaning of the features in the incremental capacity versus concentration curves: (a) minima are associated with ordered structures, (b) sharp maxima (divergences) are caused by the coexistence of ordered and disordered phases over small concentration intervals and (c) smooth maxima can be found at concentrations of no direct relevance to ordering.
Date: August 1, 1980
Creator: Osorio, R. & Falicov, L. M.
Partner: UNT Libraries Government Documents Department

Cluster-Variation Method for the Triangular Lattice Gas [Part] 2. Triangle Approximation

Description: A triangular lattice gas with nearest~neighbor repulsions and three-particle attractions is examined in a three-sublattice triangle approximation as a model for lithium intercalation in the transition metals dichalcogenides. Order-disorder phase diagrams and thermodynamic functions are computed. Peaks in the incremental capacity as function of concentration in the experimental data of Thompson are reproduced in our model.
Date: August 1, 1980
Creator: Osorio, R. & Falicov, L. M.
Partner: UNT Libraries Government Documents Department

Fourier-Transform Analysis of Normal Photoelectron Diffraction Data for Surface-Structure Determination

Description: A direct method for surface-structure determination from normal emission photoelectron diffraction (NPD) data is presented. Fourier transforms of the calculated NPD intensities yield peaks at adsorbate-substrate normal interlayer distances. Applications are demonstrated using theoretical NPD curves for the Se/Ni system calculated by dynamical theory. These results show that interplanar spacings between the overlayer and as many as four substrate layers could be determined with an accuracy of better than 3%.
Date: May 1, 1981
Creator: Hussain, Z.; Shirley, D. A.; Li, C. H. & Tong, S. Y.
Partner: UNT Libraries Government Documents Department

Study of Liquid Crystal Conformation by Multiple Quantum NMR: n-Pentyl Cyanobiphenyl

Description: The multiple quantum NMR spectrum of 4-cyano-4'-n-pentyl-d{sub 11}-biphenyl in the nematic phase is presented. The sub-spectra of the higher quantum transitions are easier to interpret than the single quantum spectrum. A preliminary analysis of the biphenyl conformation indicates that the dihedral angle is 32° ± 1°.
Date: September 1, 1980
Creator: Sinton, S. & Pines, A.
Partner: UNT Libraries Government Documents Department

Substrate-Dependent C(1s) Shape Resonance in CO Overlayers on Ni(111) and (001)

Description: A large resonance was observed in the C(1s) angle-resolved photoemission cross-section of CO overlayers on Ni(111) and Ni(001), using synchrotron radiation at SSRL, while none was observed for O(1s). Energy-, angular-, and polarization-dependence measurements showed that the C(1s) resonance, which is peaked at h{nu} = 311 eV, is closely related to the shape resonance predicted for the C(1s) level in isolated CO. However, the surface potential introduces strong substrate-dependent deviations from gas-phase theory.
Date: August 1, 1980
Creator: Davis, R. F.; Kevan, S. D.; Rosenblatt, D. H.; Mason, M. G.; Tobin, J. G. & Shirley, D. A.
Partner: UNT Libraries Government Documents Department

Three-Sublattice Model for a Triangular Lattice Gas

Description: A three-sublattice Bragg-Williams model of a triangular lattice gas with repulsive nearest-neighbor interactions is presented. It is intended to represent intercalated Li{sup +} in Li{sub x}TiS{sub 2} . An order-disorder phase diagram and thermodynamic functions are computed. In particular the incremental capacity is calculated and compared with the experimental data of Thompson. It is found that (a) ordering results ln minima in the incremental capacity, (b) sharp maxirna (divergences) are caused by small regions of two-phase coexistence and (c) smooth maxima are found at concentrations of no relevance to ordering.
Date: March 1, 1980
Creator: Osorlo, R. & Falicov, L. M.
Partner: UNT Libraries Government Documents Department

UPS Study of the Adsorption of Oxygen on Reduced SrTiO{Sub 3} Surfaces

Description: The adsorption of oxygen on a reduced SrTiO{sub 3} (111) surface occurs in two distinct phases. At low exposures (less than 1 L), the adsorption leads to the incorporation of O{sup =} ions into the vacant lattice oxygen sites. At higher exposures a second different oxygen species adsorbs.
Date: October 1, 1980
Creator: Ferrer, S. & Somorjai, G. A.
Partner: UNT Libraries Government Documents Department

Fourier-Transform Analysis of Normal Photoelectron Diffraction Data for Surface-Structure Determination

Description: A direct method for surface-structure determination from normal emission photoelectron diffraction (NPD) data is presented. Fourier transforms of the calculated NPD intensities yield peaks at adsorbate-substrate normal interlayer distances. Applications are demonstrated using theoretical NPD curves for the Se/Ni system calculated by dynamical theory. These results show that interplanar spacings between the overlayer and as many as four substrate layers could be determined with an accuracy of better than 2%.
Date: October 1, 1980
Creator: Hussain, Z.; Shirley, D. A.; Li, C. H. & Tong, S. Y.
Partner: UNT Libraries Government Documents Department

Particle-Size-Induced Valence Changes in Samarium Clusters

Description: Samarium clusters exhibit mixed-valence behavior which is sensitive to particle size. XPS and UPS data show samarium to be primarily divalent (4f{sup 6} ) at small particle size. The trivalent state (4f{sup 5} ) becomes progressively more abundant with increasing s1ze, becoming the dominant state for the bulk metal. These results are interpreted using a model in which band narrowing, due to reduced surface coordination, is more dominant than surface tension effects in establishing the valence of small samarium clusters.
Date: May 1, 1981
Creator: Mason, M. G.; Lee, S. T.; Apai, G.; Davis, R. F.; Shirley, D. A.; Franciosi, A. et al.
Partner: UNT Libraries Government Documents Department

Quantum Noise Theory for the dc Squid

Description: The noise temperature of a de SQUID coupled to a tuned input circuit is computed using the complete quantum expression for the equilibrium noise in the shunt resistance of each junction. At T=0, where the noise reduces to zero-point fluctuations. the noise temperature for an optimized system is h{nu}/k{sub B}ln2, where {nu} is the signal frequency. The computation is extended to non-zero temperatures, and it is shown that a SQUID operated at 1K can approach the quantum limit.
Date: October 1, 1980
Creator: Koch, Roger H.; Van Harlingen, D. J. & Clarke, John
Partner: UNT Libraries Government Documents Department

NMR Study of Molecular Reforientation Under Fivefold Symmetry Solid Permethylferrocene

Description: The ring reorientation in permethylferrocene has been studied using high resolution solid state {sup 13}C NMR. The constraints which symmetry places upon the number and types of motional parameters which may be determined from the NMR spectrum are discussed. From comparison of the experimental lineshapes in the slow reorientation temperatures range with theoretical models for random rotations and symmetry related jumps, it is concluded that the reorientation occurs as jumps between symmetry related orientations with jumps of 2{pi}/5 highly favored over 4{pi}/5. The activation energy derived for the jump process is 13.5 kjoules/mole.
Date: August 1, 1980
Creator: Wemmer, D. E.; Ruben, D. J. & Pines, A.
Partner: UNT Libraries Government Documents Department

Nodal Quasiparticle Meltdown in Ultra-High Resolution Pump-Probe Angle-Resolved Photoemission

Description: High-T{sub c} cuprate superconductors are characterized by a strong momentum-dependent anisotropy between the low energy excitations along the Brillouin zone diagonal (nodal direction) and those along the Brillouin zone face (antinodal direction). Most obvious is the d-wave superconducting gap, with the largest magnitude found in the antinodal direction and no gap in the nodal direction. Additionally, while antin- odal quasiparticle excitations appear only below T{sub c}, superconductivity is thought to be indifferent to nodal excitations as they are regarded robust and insensitive to T{sub c}. Here we reveal an unexpected tie between nodal quasiparticles and superconductivity using high resolution time- and angle-resolved photoemission on optimally doped Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}}#14;. We observe a suppression of the nodal quasiparticle spectral weight following pump laser excitation and measure its recovery dynamics. This suppression is dramatically enhanced in the superconducting state. These results reduce the nodal-antinodal dichotomy and challenge the conventional view of nodal excitation neutrality in superconductivity. The electronic structures of high-Tc cuprates are strongly momentum-dependent. This is one reason why the momentum-resolved technique of angle-resolved photoemission spectroscopy (ARPES) has been a central tool in the #12;field of high-temperature superconductivity. For example, coherent low energy excitations with momenta near the Brillouin zone face, or antinodal quasiparticles (QPs), are only observed below T{sub c} and have been linked to superfluid density. They have therefore been the primary focus of ARPES studies. In contrast, nodal QPs, with momenta along the Brillouin zone diagonal, have received less attention and are usually regarded as largely immune to the superconducting transition because they seem insensitive to perturbations such as disorder, doping, isotope exchange, charge ordering, and temperature. Clearly, finding any strong dependencies of the nodal QPs will alter the conventional view and enrich our understanding of high temperature superconductivity. Time resolution through pump-and-probe techniques adds a ...
Date: June 3, 2011
Creator: Graf, Jeff; Jozwiak, Chris; Smallwood, Chris L.; Eisaki, H.; Kaindl, Robert A.; Lee, Dung-Hai et al.
Partner: UNT Libraries Government Documents Department

Normal Emission Photoelectron Diffraction Studies at SSRL

Description: A series of experiments designed to characterize the surface structural sensitivity of photoelectron diffraction is described. The technique's relation to LEED and to surface EXAFS is explored, and useful comparisons to the latter are pointed out. The application of normal emission photoelectron diffraction to disordered overlayers is shown to be reasonably straightforward. Data on both two dimensionally and three dimensionally (multiplesite) disordered overlayers are presented. The localized, atom-specific nature of PhD is shown to lend the technique real potential in the study of molecular adsorbate systems.
Date: June 1, 1980
Creator: Kevan, S. D.
Partner: UNT Libraries Government Documents Department

Surface-induced anisotropic orientations of interfacial ethanol molecules at air/sapphire (1-102) and ethanol/sapphire (1-102) interfaces

Description: Sum frequency vibrational spectroscopy was used to study the interfacial arrangement of ethanol molecules at the vapor/α-Al{sub 2}O{sub 3} (1{bar 1}02 ) and α-Al{sub 2}O{sub 3} (1{bar 1}02 )/ethanol liquid interfaces. The spectra in the C-H range show that ethanol molecules adsorbed from vapor onto α-Al{sub 2}O{sub 3} (1{bar 1}02 ) surface have a welldefined anisotropic arrangement following the structure of the α-Al{sub 2}O{sub 3} (1{bar 1}02 ) surface. The arrangement can be explained by the formation of two specific hydrogen bonds between the adsorbed ethanol molecule and hydroxyls on the sapphire surface. At the α-Al{sub 2}O{sub 3} (1{bar 1}02 )/ethanol liquid interface, the first ethanol monolayer assumes a similar anisotropic arrangement as in the case of an ethanol monolayer on the dry sapphire surface. The second monolayer has a rather broad orientation distribution that is azimuthally nearly isotropic, but with molecules flipped 180 degrees with respect to those in the first monolayer.
Date: June 1, 2011
Creator: Sung, J.; Waychunas, G. A. & Shen, Y. R.
Partner: UNT Libraries Government Documents Department

Bounds and self-consistent estimates for elastic constants of polycrystals of hcp solid He{sup 4}

Description: Recent advances in methods for computing both Hashin-Shtrikman bounds and related selfconsistent (or CPA) estimates of elastic constants for polycrystals composed of randomly oriented crystals can be applied successfully to hexagonal close packed solid He{sup 4}. In particular, since the shear modulus C{sub 44} of hexagonal close-packed solid He is known to undergo large temperature variations when 20 mK {<=}#20; T {<=}#20; 200 mK, bounds and estimates computed with this class of effective medium methods, while using C{sub 44} {r_arrow} 0 as a proxy for melting, are found to be both qualitatively and quantitatively very similar to prior results obtained using Monte Carlo methods. Hashin- Shtrikman bounds provide significantly tighter constraints on the polycrystal behavior than do the traditional Voigt and Reuss bounds.
Date: February 15, 2012
Creator: Berryman, J. G.
Partner: UNT Libraries Government Documents Department

Final Report - Composite Fermion Approach to Strongly Interacting Quasi Two Dimensional Electron Gas Systems

Description: Work related to this project introduced the idea of an “effective monopole strength” Q* that acted as the effective angular momentum of the lowest shell of composite Fermions (CF). This allowed us to predict the angular momentum of the lowest band of energy states for any value of the applied magnetic field simply by determining N{sub QP} the number of quasielectrons (QE) or quasiholes (QH) in a partially filled CF shell and adding angular momenta of the N{sub QP} Fermions excitations. The approach reported treated the filled CF level as a “vacuum state” which could support QE and QH excitations. Numerical diagonalization of small systems allowed us to determine the angular momenta, the energy, and the pair interaction energies of these elementary excitations. The spectra of low energy states could then be evaluated in a Fermi liquid‐like picture, treating the much smaller number of quasiparticles and their interactions instead of the larger system of N electrons with Coulomb interactions.
Date: November 30, 2009
Creator: Quinn, John
Partner: UNT Libraries Government Documents Department

Optically Detected Magnetic Resonance Studies on π-conjugated semiconductor systems

Description: Optically Detected Magnetic Resonance (ODMR) techniques were used to investigate the dynamics of excitons and charge carriers in π-conjugated organic semiconductors. Degradation behavior of the negative spin-1/2 electroluminescence-detected magnetic resonance (ELDMR) was observed in Alq3 devices. The increase in the resonance amplitude implies an increasing bipolaron formation during degradation, which might be the result of growth of charge traps in the device. The same behavior of the negative spin-1/2 ELDMR was observed in 2wt% Rubrene doped Tris(8-hydroxyquinolinato)aluminium (Alq3) devices. However, with increasing injection current, a positive spin-1/2 ELDMR, together with positive spin 1 triplet powder patterns at {delta}m{sub S}={+-}1 and {delta}m{sub S}={+-}2, emerges. Due to the similarities in the frequency dependences of single and double modulated ELDMR and the photoluminescence-detected magnetic resonance (PLDMR) results in poly[2-methoxy-5-(2 -ethyl-hexyloxy)-1,4-phenyl ene vinylene] (MEH-PPV) films, the mechanism for this positive spin-1/2 ELDMR was assigned to enhanced triplet-polaron quenching under resonance conditions. The ELDMR in rubrene doped Alq3 devices provides a path to investigate charge distribution in the device under operational conditions. Combining the results of several devices with different carrier blocking properties and the results from transient EL, it was concluded trions not only exist near buffer layer but also exist in the electron transport layer. This TPQ model can also be used to explain the positive spin-1/2 PLDMR in poly(3-hexylthiophene) (P3HT) films at low temperature and in MEH-PPV films at various temperatures up to room temperature. Through quantitative analysis, TE-polaron quenching (TPQ) model is shown having the ability to explain most behaviors of the positive spin-1/2 resonance. Photocurrent detected magnetic resonance (PCDMR) studies on MEH-PPV devices revealed a novel transient resonance signal. The signal may originate from the higher concentration of deep traps near cathode. A quantitative analysis based on this assumption was carried out and found to be consistent with the experimental ...
Date: December 6, 2011
Creator: Chen, Ying
Partner: UNT Libraries Government Documents Department

First principles analysis of lattice dynamics for Fe-based superconductors and entropically-stabilized phases

Description: Modern calculations are becoming an essential, complementary tool to inelastic x-ray scattering studies, where x-rays are scattered inelastically to resolve meV phonons. Calculations of the inelastic structure factor for any value of Q assist in both planning the experiment and analyzing the results. Moreover, differences between the measured data and theoretical calculations help identify important new physics driving the properties of novel correlated systems. We have used such calculations to better and more e#14;ciently measure the phonon dispersion and elastic constants of several iron pnictide superconductors. This dissertation describes calculations and measurements at room temperature in the tetragonal phase of CaFe{sub 2}As{sub 2} and LaFeAsO. In both cases, spin-polarized calculations imposing the antiferromagnetic order present in the low-temperature orthorhombic phase dramatically improves the agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase. In addition, we discuss a relatively new approach called self-consistent ab initio lattice dynamics (SCAILD), which goes beyond the harmonic approximation to include phonon-phonon interactions and produce a temperature-dependent phonon dispersion. We used this technique to study the HCP to BCC transition in beryllium.
Date: July 20, 2012
Creator: Hahn, Steven
Partner: UNT Libraries Government Documents Department