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Probing the band structure of LaTe2 using angle resolved photoemission spectroscopy

Description: With the current interest in the rare-earth tellurides ashigh temperature charge density wave materials, a greater understandingof the physics of these systems is needed, particularly in the case ofthe ditellurides. We report a detailed study of the band structure ofLaTe_2 in the charge density wave state using high-resolution angleresolved photoemission spectroscopy (ARPES). From thiswork we hope toprovide insights into the successes and weaknesses of past theoreticalstudy as well as helping to clear up prior ambiguities by providing anexperimental basis for future work inthe tellurides.
Date: November 1, 2006
Creator: Garcia, Daniel R.; Zhou, Shuyun Y.; Gweon, Gey-Hong; Jung, M.H.; Kwon, Y.S. & Lanzara, Alessandra
Partner: UNT Libraries Government Documents Department

Topology of charge density and elastic properties of Ti3SiC2 polymorphs

Description: Using an all-electron, full potential first-principles method, we have investigated the topology of charge density and elastic properties of the two polymorphs, alpha and beta, of Ti3SiC2. The bonding effect was analyzed based on Bader's quantum theory of ''atoms in molecules'' (AIM). It was found that the Ti-Si bonding effect is significantly weaker in beta than in alpha, giving less stabilizing effect for beta. The Si-C bonds, which are absent in alpha, are formed in beta and provide additional stabilizing effect for beta. In contrast to conventional thinking, there is no direction interaction between Ti atoms in both alpha and beta. The calculated elastic properties are in good agreement with the experimental results, giving the bulk modulus of about 180 GPa and the Poisson's ratio of 0.2. The beta phase is generally softer than the alpha phase. As revealed by the direction dependent Young's modulus, there is only slight elastic anisotropy in Ti3SiC2. For alpha, Young's modulus is minimum in the c direction and maximum in the directions 42o from c. For beta, the maximum lies in the c direction, in part due to the formation of Si-C bonds in this direction.
Date: June 24, 2004
Creator: Yu, Rong; Zhang, Xiao Feng; He, Lian Long & Ye, Heng Qiang
Partner: UNT Libraries Government Documents Department

Revealing Charge Density Wave Formation in the LaTe2 System byAngle Resolved Photoemission Spectroscopy

Description: We present the first direct study of charge density wave(CDW) formation in quasi-2D single layer LaTe2 using high-resolutionangle resolved photoemission spectroscopy (ARPES) and low energy electrondiffraction (LEED). CDW formation is driven by Fermi surface (FS)nesting, however characterized by a surprisingly smaller gap (~;50 meV)than seen in the double layer RTe3 compounds, extending over the entireFS. This establishes LaTe2 as the first reported semiconducting 2D CDWsystem where the CDW phase is FS nesting driven. In addition, the layerdependence of this phase in the tellurides and the possible transitionfrom a stripe to a checkerboard phase is discussed.
Date: November 15, 2006
Creator: Garcia, D. R.; Gweon, G.-H.; Zhou, S. Y.; Graf, J.; Jozwiak, C. M.; Jung, M. H. et al.
Partner: UNT Libraries Government Documents Department

PetaScale calculations of the electronic structures ofnanostructures with hundreds of thousands of processors

Description: Density functional theory (DFT) is the most widely used ab initio method in material simulations. It accounts for 75% of the NERSC allocation time in the material science category. The DFT can be used to calculate the electronic structure, the charge density, the total energy and the atomic forces of a material system. With the advance of the HPC power and new algorithms, DFT can now be used to study thousand atom systems in some limited ways (e.g, a single selfconsistent calculation without atomic relaxation). But there are many problems which either requires much larger systems (e.g, >100,000 atoms), or many total energy calculation steps (e.g. for molecular dynamics or atomic relaxations). Examples include: grain boundary, dislocation energies and atomic structures, impurity transport and clustering in semiconductors, nanostructure growth, electronic structures of nanostructures and their internal electric fields. Due to the O(N{sup 3}) scaling of the conventional DFT algorithms (as implemented in codes like Qbox, Paratec, Petots), these problems are beyond the reach even for petascale computers. As the proposed petascale computers might have millions of processors, new computational paradigms and algorithms are needed to solve the above large scale problems. In particular, O(N) scaling algorithms with parallelization capability up to millions of processors are needed. For a large material science problem, a natural approach to achieve this goal is by divide-and-conquer method: to spatially divide the system into many small pieces, and solve each piece by a small local group of processors. This solves the O(N) scaling and the parallelization problem at the same time. However, the challenge of this approach is for how to divide the system into small pieces and how to patch them up without the trace of the spatial division. Here, we present a linear scaling 3 dimensional fragment (LS3DF) method which uses a novel ...
Date: April 1, 2006
Creator: Wang, Lin-Wang; Zhao, Zhengji & Meza, Juan
Partner: UNT Libraries Government Documents Department

The glasma initial state and JIMWLK factorization

Description: We review recent work on understanding the next to leading order corrections to the classical fields that dominate the initial stages of a heavy ion collision. We have recently shown that the leading ln 1/x divergences of these corrections to gluon multiplicities can be factorized into the JIMWLK evolution of the color charge density distributions.
Date: August 26, 2008
Creator: Gelis,F.; Lappi, T. & Venugopalan, R.
Partner: UNT Libraries Government Documents Department

Numerical Study of Coulomb Scattering Effects on Electron Beamfrom a Nano-Tip

Description: Nano-tips with high acceleration gradient around the emission surface have been proposed to generate high brightness beams. However, due to the small size of the tip, the charge density near the tip is very high even for a small number of electrons. The stochastic Coulomb scattering near the tip can degrade the beam quality and cause extra emittance growth and energy spread. In the paper, we present a numerical study of these effects using a direct relativistic N-body model. We found that emittance growth and energy spread, due to Coulomb scattering, can be significantly enhanced with respect to mean-field space-charge calculations.
Date: June 25, 2007
Creator: Qiang, Ji; Corlett, John N.; Lidia, Steven M.; Padmore, HowardA.; Wan, Weishi; Zholent, Andrew A. et al.
Partner: UNT Libraries Government Documents Department

Half Metallic Digital Ferromagnetic Heterostructure Composed of a (Delta)-doped Layer of Mn in Si

Description: The authors propose and investigate the properties of a digital ferromagnetic heterostructure (DFH) consisting of a {delta}-doped layer of Mn in Si, using ab initio electronic-structure methods. They find that (1) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (2) the heterostructure is a two-dimensional half metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-d and nearest-neighbor Si-p states, and the corresponding carriers are responsible for the ferromagnetic order in the Mn layer. The minority-spin channel has a calculated semiconducting gap of 0.25 eV. Analysis of the total and partial densities of states, band structure, Fermi surfaces and associated charge density reveals the marked two-dimensional nature of the half metallicity. The band lineup is found to be favorable for retaining the half metal character to near the Curie temperature (T{sub C}). Being Si based and possibly having a high T{sub C} as suggested by an experiment on dilutely doped Mn in Si, the heterostructure may be of special interest for integration into mature Si technologies for spintronic applications.
Date: May 30, 2006
Creator: Qian, M C; Fong, C Y; Liu, K; Pickett, W E; Pask, J E & Yang, L H
Partner: UNT Libraries Government Documents Department

Solving the quasi-static field model of the pulse-line accelerator; relationship to a circuit model

Description: The Pulse-Line Ion Accelerator (PLIA) is a promising approach to high-gradient acceleration of an ion beam at high line charge density [1, 2, 3, 4, 5, 6]. A recent note by R. J. Briggs [7] suggests that a ''sheath helix'' model of such a system can be solved numerically in the quasi-static limit. Such a model captures the correct macroscopic behavior from ''first principles'' without the need to time-advance the full Maxwell equations on a grid. This note describes numerical methods that may be used to effect such a solution, and their connection to the circuit model that was described in an earlier note by the author [8]. Fine detail of the fields in the vicinity of the helix wires is not obtained by this approach, but for purposes of beam dynamics simulation such detail is not generally needed.
Date: February 1, 2006
Creator: Friedman, A
Partner: UNT Libraries Government Documents Department

ARPES study of the evolution of band structure and charge density wave properties in RTe3 ( R=Y , La, Ce, Sm, Gd, Tb, and Dy)

Description: We present a detailed angle-resolved photoemission spectroscopy (ARPES) investigation of the RTe3 family, which sets this system as an ideal"textbook" example for the formation of a nesting driven charge density wave (CDW). This family indeed exhibits the full range of phenomena that can be associated to CDWinstabilities, from the opening of large gaps on the best nested parts of Fermi surface (up to 0.4 eV), to the existence of residual metallic pockets. ARPES is the best suited technique to characterize these features, thanks to its unique ability to resolve the electronic structure in k space. An additional advantage of RTe3 is that theband structure can be very accurately described by a simple two dimensional tight-binding (TB) model, which allows one to understand and easily reproduce many characteristics of the CDW. In this paper, we first establish the main features of the electronic structure by comparing our ARPES measurements with the linear muffin-tinorbital band calculations. We use this to define the validity and limits of the TB model. We then present a complete description of the CDW properties and of their strong evolution as a function of R. Using simple models, we are able to reproduce perfectly the evolution of gaps in k space, the evolution of the CDW wave vector with R, and the shape of the residual metallic pockets. Finally, we give an estimation of the CDWinteraction parameters and find that the change in the electronic density of states n (EF), due to lattice expansion when different R ions are inserted, has the correct order of magnitude to explain the evolution of the CDW properties.
Date: January 16, 2008
Creator: Hussain, Zahid; Brouet, Veronique; Yang, Wanli; Zhou, Xingjiang; Hussain, Zahid; Moore, R.G. et al.
Partner: UNT Libraries Government Documents Department

Design of an electro-optical sampling experiment at the AWA facility

Description: The free space electro-optical (EO) sampling technique is a powerful tool for analyzing the longitudinal charge density of an ultrashort e-beam. In this paper, we present (1) experimental results for a laser-based mock-up of the EO experiment [1] and (2) a design for a beam-based, single-shot, EO sampling experiment using the e-beam from the Argonne Wakefield Accelerator (AWA) RF photoinjector. For the mock-up, a tabletop terahertz experiment is conducted in the AWA laser room. The mock-up uses an IR beam incident on <110> ZnTe crystal to produce a THz pulse via optical rectification. Detection is based on the cross correlation between the THz field and the probe IR laser field in a second <110> ZnTe crystal.
Date: June 1, 2007
Creator: Ruan, J.; Edwards, H.; Tan, Cheng Yang; Thurman-Keup, R.; Scarpine, V.; /Fermilab et al.
Partner: UNT Libraries Government Documents Department

Single-particle and collective mode couplings associated with 1- and 2-directional electronic ordering in metallic RTe3 (R = Ho, Dy, Tb)

Description: The coupling of phonons with collective modes and single-particle gap excitations associated with one (1d) and two-directional (2d) electronically-driven charge-density wave (CDW) ordering in metallic RTe{sub 3} is investigated as a function of rare-earth ion chemical pressure (R = Tb, Dy, Ho) using femtosecond pump-probe spectroscopy. From the T-dependence of the CDW gap {Delta}{sub CDW} and the amplitude mode (AM) we find that while the transition to a 1d-CDW ordered state at Tc1 initially proceeds in an exemplary mean-field (MF)-like fashion, below T{sub c1}, {Delta}{sub CDW} is depressed and departs from the MF behavior. The effect is apparently triggered by resonant mode-mixing of the amplitude mode (AM) with a totally symmetric phonon at 1.75 THz. At low temperatures, when the state evolves into a 2d-CDW ordered state at T{sub c2} in the DyTe{sub 3} and HoTe{sub 3}, additional much weaker mode mixing is evident but no soft mode is observed.
Date: February 15, 2010
Creator: Yusupov, R.V.; Mertelj, T.; /Stefan Inst., Ljubljana; Chu, J.-H.; Fisher, I.R.; /Stanford U., Geballe Lab. et al.
Partner: UNT Libraries Government Documents Department

Helical spin-density wave in Fe/Cr trilayers with perfect interfaces

Description: Despite the presence of only collinear, commensurate (C) and incommensurate (I) spin-density waves (SDW`s) in bulk Cr, the interfacial steps in Fe/Cr multilayers are now believed to stabilize a helical (H) SDW within the Cr spacer. Yet H SDW`s were first predicted in an Fe/Cr trilayer with perfect interfaces when the orientation of the Fe moments does not favor C ordering: if the number of Cr monolayers is even (odd) and the Fe moments are pointing in the same (opposite) direction, then a C SDW does not gain any coupling energy. Under these circumstances, a simple model verifies that H ordering is indeed favored over 1 ordering provided that the Fermi surface mismatch is sufficiently small or the temperature sufficiently high.
Date: July 1, 1998
Creator: Fishman, R.S.
Partner: UNT Libraries Government Documents Department

Mechanics of Metals with Phase Changes

Description: New experimental data is presented on some exotic metals that exhibit phase changes at cryogenic temperatures. The types of phase changes that were detected in the specific heat data range from martensitic (diffusion less) transitions to superconducting transitions. In addition, the charge density wave (CDW) state in uranium metal was detected in the specific heat. Specific-heat measurements were made in zero-magnetic field using an apparatus capable of obtaining temperatures as low as 0.4 K. Calibration performed on this apparatus, using a single-crystal copper sample, show its accuracy to be 0.50%, while the resolution was better than 0.1%. Our measurements demonstrate that similar high precision and accurate specific-heat measurements can be obtained on milligram-scale samples. In Chapters 2 and 3, specific-heat measurements are presented for the B2 (CsCl structure) alloy AuZn and for {alpha}-uranium (orthorhombic symmetry). The AuZn alloy exhibits a continuous transition at 64.75 K and an entropy of transition of ({Delta}S{sub tr}) 2.02 J K{sup {minus}1} mol{sup {minus}1}. Calculation of the Debye temperature, by extrapolating of the high temperature phase elastic constants to T = 0 K yields a value of 207 K ({+-}2 K), in favorable agreement with the calorimetric value of 219 K ({+-}0.50 K), despite the intervening martensitic transition. Reported results for single-crystal {alpha}-U show a low-temperature limiting {Theta}{sub D} of 256 K ({+-}0.50 K) and four low-temperature anomalies: a superconducting transition below 1 K, an electronic transition at 22 K, and two anomalies at 38 K and at 42 K indicative of the CDW state. In order to continue the study of the actinide series of elements, a program was initiated to first purify and then grow single crystals of plutonium. Accordingly, the focus of Chapters 4 through 6 will be a description of plutonium sample preparation. In this program plutonium metal was purified via ...
Date: January 1, 2001
Creator: Lashley, J.C.
Partner: UNT Libraries Government Documents Department

Effects of Fresnel fringes on TEM images of interfaces in X-ray multilayers

Description: Fresnel fringe effects make assessment of interfacial structures from high-resolution TEM images of cross-sectional specimens difficult, producing different apparent structures in the images. Fresnel fringes have been observed in many TEM images of W/C, WC/C, Ru/C, and Mo/Si, multilayers. Visibility of these fringes depends on the thickness of the specimen and the defocus value. Contrast of the fringes becomes higher with increasing defocus. The effects of these fringes have been commonly over-looked in efforts of making quantitative interpretation of interfacial profiles. In this report, we present the observations of the Fresnel fringes in nanometer period Mo/Si, W/C, and WC/C multilayers in through-focus-series TEM images. Calculation of the Fresnel fringes of a Mo/Si multilayer using charge density approximation is used to illustrate the characteristics of the fringes from different interfacial structures. We find that the potential difference and the abruptness of the interfacial composition change are a strong function of the fringe contrast, while the fringes spacing depends more strongly on the thickness of the transition or interfacial layer.
Date: March 2, 1992
Creator: Nguyen, Tai D.; O'Keefe, Michael A.; Kilaas, Roar; Gronsky, Ronald & Kortright, Jeffrey B.
Partner: UNT Libraries Government Documents Department

X-ray and neutron reflectivity

Description: The intensity of X-rays and neutrons reflected from an interface at near-grazing incidence depends sensitively on the refractive index profile normal to the interface. As the refractive index depends on the density, the measured reflectivity vs. incidence angle can be used to determine the Interfacial density profile with sub {angstrom} resolution. The density sampled is, however, different for the two probes. As neutrons interact with the atomic nuclei, they probe the scattering amplitude density profile of the nuclei. This amounts effectively to probing the average mass density profile, where the contribution of each atomic species is weighted by its nucleus-neutron interaction strength, which varies irregularly throughout the periodic table. X-rays, however, interact with the electronic charge and thus probe the total electronic density profile of the interface. The two probes provide, therefore, slightly different, though related, information on the interface structure. Since modern synchrotron X-ray sources are 5-6 orders of magnitude brighter than the best neutron sources, X-ray reflectivity measurements routinely achieve much higher resolutions and accuracies than their neutron counterparts. However, the magnetic interaction and the large variation of the neutron scattering length upon isotopic substitution renders neutron reflectivity a method-of-choice in special cases such as studies of surface magnetism and the adsorption of polymers at liquid and solid surfaces.
Date: October 4, 1994
Creator: Deutsch, M. & Ocko, B. M.
Partner: UNT Libraries Government Documents Department

Calculations of nonspherically averaged charge densities for subtitutionally disordered alloys

Description: Based on screening transformations of muffin-tin orbitals introduced by Andersen et al. [Phys. Rev. Lett. 53, 2571 (1984)], we have developed a formalism for calculating the non-spherically averaged charge densities of substitutionally disordered alloys using the Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) method in the atomic-sphere approximation (ASA). We have validated our method by calculating charge densities for ordered structures, where we find that our approach yields charge densities that are essentially indistinguishable from the results of full-potential methods. For substitutionally disordered alloys, where full-potential methods have not been implemented so far, our approach can be used to calculate reliable non-spherically averaged charge densities from spherically symmetric one-electron potentials obtained from the KKR-ASA CPA. We report on our study of differences in charge denisty between ordered AlLi in L1{sub o} phase and substitutionally disordered Al{sub 0.5}Li{sub 0.5} on face-centered cubic lattice.
Date: February 1, 1994
Creator: Singh, P. P. & Gonis, A.
Partner: UNT Libraries Government Documents Department

Confinement and stability of a Crystalline Beam

Description: This technical report defines and describes a Crystalline Beam. This is an ordered state of matter made of electrically charged ions which are moving together in a storage ring with very high density and small velocity spread. In particular, the paper analyses the requirements for the confinement and the stability of the Beam. It is demonstrated that a storage ring made of one circular weak-focusing magnet, similar to a Betatron, is the most suitable for the confinement and stability of the Crystalline Beam. The disruptive effects of drift insertions have also been investigated. Requirements on final densities and velocity spreads are also calculated and reported. A matrix formalism is developed for the design of the storage ring. The important issue of the disruption caused by the curvature of the closed trajectory is not here discussed; it is the subject of a subsequent paper.
Date: May 10, 1993
Creator: Ruggiero, A. G.
Partner: UNT Libraries Government Documents Department

Complexation of actinides with derivatives of oxydiaceticacid

Description: Complexation of Np(V), U(VI) and Nd(III) with dimethyl-3-oxa-glutaramic acid (DMOGA) and tetramethyl-3-oxa-glutaramide (TMOGA) was studied in comparison with the complexation with oxydiacetic acid (ODA). Stability constants and enthalpy of complexation were determined by potentiometry, spectrophotometry and calorimetry. Thermodynamic parameters, in conjunction with structural information of solid compounds, indicate that DMOGA and TMOGA form tridentate complexes with the ether-oxygen participating in bonding with actinide/lanthanide ions. The trends in the stability constants, enthalpy and entropy of complexation are discussed in terms of the difference in the hydration of the amide groups and carboxylate groups and the difference in the charge density of the metal ions.
Date: January 4, 2006
Creator: Rao, Linfeng & Tian, Guoxin
Partner: UNT Libraries Government Documents Department

Influence of electron beam parameters on coherent electron cooling

Description: Coherent electron cooling (CeC) promises to revolutionize the cooling of high energy hadron beams. The intricate dynamics of the CeC depends both on the local density and energy distribution of the beam. The variations of the local density (beam current) are inevitable in any realistic beam. Hence, in this paper we propose a novel method of beam conditioning. The conditioning provides compensation of effect from such variation by a correlated energy modulation. We use our analytical FEL model for an electron bunch with Gaussian line charge density and cosine-type energy variation along bunch. We analyze the phase variation between the electron density modulation at the exit of the FEL-amplifier and the ions inducing it in the modulator as a function of the peak current and the electron beam energy. Based on this analysis, electron bunch parameters for optimal CeC cooling are found numerically.
Date: May 20, 2012
Creator: G., Wang; Hao, Y.; Litvinenko, V.N. & Webb, S.
Partner: UNT Libraries Government Documents Department


Description: Dynalene Inc has developed and patented a fuel cell coolant with the help of DOE SBIR Phase I and Phase II funding (Project DE-FG02-04ER83884). However, this coolant could only be produced in lab scale (500 ml to 2 L) due to problems in the optimization and scale-up of a nanoparticle ingredient. This project optimized the nanoparticle production process in 10 L and 100 L reactors (which translates to about 5000 gallons of coolant), optimized the filtration process for the nanoparticles, and develop a high throughput production as well as quality control method for the final coolant formulation. Scale-up of nanoparticle synthesis (using emulsion polymerization) is an extremely challenging task. Dynalene researchers, in collaboration with a university partner, identified all the parameters affecting the size, charge density and coagulation characteristics of the nanoparticles and then optimized these parameters to achieve the goals and the objectives of this project. Nanoparticle synthesis was demonstrated to be reproducible in the 10 L and 100 L scales.
Date: December 21, 2011
Creator: Mohapatra, Satish
Partner: UNT Libraries Government Documents Department

Implications of Pulser Voltage Ripple

Description: In a recent set of measurements obtained by G. Kamin, W. Manning, A. Molvik, and J. Sullivan, the voltage waveform of the diode pulser had a ripple of approximately {+-}1.3% of the 65 kV flattop voltage, and the beam current had a larger corresponding ripple of approximately {+-}8.4% of the 1.5 mA average current at the location of the second Faraday cup, approximately 1.9 m downstream from the ion source. The period of the ripple was about 1 {mu}s. It was initially unclear whether this large current ripple was in fact a true measurement of the current or a spurious measurement of noise produced by the pulser electronics. The purpose of this note is to provide simulations which closely match the experimental results and thereby corroborate the physical nature of those measurements, and to provide predictions of the amplitude of the current ripples as they propagate to the end of linear transport section. Additionally analytic estimates are obtained which lend some insight into the nature of the current fluctuations and to provide an estimate of what the maximum amplitude of the current fluctuations are expected to be, and conversely what initial ripple in the voltage source is allowed, given a smaller acceptable tolerance on the line charge density.
Date: December 21, 2011
Creator: Barnard, J J
Partner: UNT Libraries Government Documents Department

First-principles calculation of mechanical properties of Si <001> nanowires and comparison to nanomechanical theory

Description: We report the results of first-principles density functional theory calculations of the Young's modulus and other mechanical properties of hydrogen-passivated Si {l_angle}001{r_angle} nanowires. The nanowires are taken to have predominantly {l_brace}100{r_brace}surfaces, with small {l_brace}110{r_brace} facets according to the Wulff shape. The Young's modulus, the equilibrium length and the constrained residual stress of a series of prismatic beams of differing sizes are found to have size dependences that scale like the surface area to volume ratio for all but the smallest beam. The results are compared with a continuum model and the results of classical atomistic calculations based on an empirical potential. We attribute the size dependence to specific physical structures and interactions. In particular, the hydrogen interactions on the surface and the charge density variations within the beam are quantified and used both to parameterize the continuum model and to account for the discrepancies between the two models and the first-principles results.
Date: October 19, 2006
Creator: Lee, B & Rudd, R E
Partner: UNT Libraries Government Documents Department

Kirchhoff's Integral Representation and a Cavity Wake Potential

Description: A method is proposed for the calculation of the short-range wake field potentials of an ultra-relativistic bunch passing near some irregularities in a beam pipe. The method is based on the space-time domain integration of Maxwell's equations using Kirchhoff's formulation. We demonstrate this method on two cases where we obtain the wake potentials for the energy loss of a bunch traversing an iris-collimator in a beam pipe and for a cavity. Likewise, formulas are derived for Green's functions that describe the transverse force action of wake fields. Simple formulas for the total energy loss of a bunch with a Gaussian charge density distribution are derived as well. The derived estimates are compared with computer results and predictions of other models.
Date: February 17, 2012
Creator: Novokhatski, Alexander
Partner: UNT Libraries Government Documents Department