147 Matching Results

Search Results

Advanced search parameters have been applied.

The Low-Energy State ofCirculating Stored Ion Beams: Crystalline Beams

Description: Molecular dynamics is employed to study the low energy states of a beam of charged particles subject to circumferentially varying guiding and focusing forces and with Coulomb forces between the particles. In a constant gradient ring, the lowest energy state is never ordered, but in an alternating gradient structure, operating below the transition energy, the lowest state is ordered. The nature and characteristics of the ground state depends upon the beam density and the ring parameters. For zero temperature the crystal remains intact for a very long time, but at nonzero temperatures it gains energy from the lattice. A critical temperature exists above which the crystal melts rapidly.
Date: March 10, 1994
Creator: Wei, J.; Li, X.-P. & Sessler, Andrew M.
Partner: UNT Libraries Government Documents Department

Lattice QCD Thermodynamcis : P4 Action for new beta and MILC Nt=6

Description: These results are from the continuing Lattice Quantum Chromodynamics runs on BG/L. We show analyzed thermodynamics results for 6-10k trajectories ({beta} dependent), of the 32{sup 3}{chi}8 runs with the p4rhmc v2.0 QMP{_}MPI.X (semi-optimized p4 code using qmp over mpi). These jobs had a number of omitted trajectories, due to zero size and over-sized data files. For this interim report these errors were removed from the output to save space. The results also include the output of a new ''histogram.perl'' script, used to plot the gauge action < S{sub g} >= 10*(1- < plaq >)-(1- < rect >) for three values of {beta}=3.51, 3.54, 3.57. This output will be used to determine the new {beta} values that will be run to define the critical temperature. We also show a preliminary analysis on the first 5,000 trajectories of the 32{sup 3}{chi}6 runs for the milc code, using the new su3{_}rhmc{_}susc{_}eos.3g1f.qmp-bgl2 faster RHMC algorithm.
Date: May 7, 2007
Creator: Soltz, R; Vranas, P & Gupta, R
Partner: UNT Libraries Government Documents Department

Growth and characterization of superconducting spinel oxide LiTi2O4 thin films

Description: Epitaxial films of LiTi{sub 2}O{sub 4} on single crystalline substrates of MgAl{sub 2}O{sub 4}, MgO, and SrTiO{sub 3} provide model systems to systematically explore the effects of lattice strain and microstructural disorder on the superconducting state. Lattice strain that affects bandwidth gives rise to variations in the superconducting and normal state properties. Microstructural disorder, such as antiphase boundaries that give rise to Ti network disorder, reduces the critical temperature, and Ti network disorder combined with Mg interdiffusion lead to a much more dramatic effect on the superconducting state. Surface sensitive X-ray absorption spectroscopy has identified Ti to retain site symmetry and average valence of the bulk material regardless of film thickness.
Date: March 10, 2009
Creator: Chopdekar, R.V.; Wong, F.; Takamura, Y.; Arenholz, E. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

Structure and anisotropic properties of single crystals nickel doped barium iron arsenide

Description: The crystal structure, anisotropic electrical resistivity and magnetic susceptibility, as well as specific heat results of the pure single crystals of BaFe{sub 2}As{sub 2}, BaFeNiAs{sub 2}, and BaNi{sub 2}As{sub 2} are surveyed. BaFe{sub 2}As{sub 2} properties demonstrate the equivalence of C(T), Fisher's d({chi}T)/dT and d{rho}/dT results in determining the antiferromagnetic transition at T{sub N} = 132(1) K. BaNi{sub 2}As{sub 2} shows a structural phase transition from a high-temperature tetragonal phase to a low-temperature triclinic pol symmetry at T{sub 0} 131 K, with superconducting critical temperature well below at {Tc} 0.69 K. BaFeNiAs{sub 2} does not show any sign of superconductivity and gives behavioral similarity to BaCo{sub 2}As{sub 2}, a renomalized paramagnetic metal.
Date: January 1, 2009
Creator: Ronning, Filip; Bauer, Eric D; Sefat, A S; Jin, R; Mcguire, M A; Sales, B C et al.
Partner: UNT Libraries Government Documents Department

Spontaneous Generation of Voltage in Single-Crystal Gd5Si2Ge2 During Magnetostructural Phase Transformations

Description: The spontaneous generation of voltage (SGV) in single-crystal and polycrystalline Gd{sub 5}Si{sub 2}Ge{sub 2} during the coupled magnetostructural transformation has been examined. Our experiments show reversible, measurable, and repeatable SGV responses of the materials to the temperature and magnetic field. The parameters of the response and the magnitude of the signal are anisotropic and rate dependent. The magnitude of the SGV signal and the critical temperatures and critical magnetic fields at which the SGV occurs vary with the rate of temperature and magnetic-field changes.
Date: April 19, 2006
Creator: Zou, M.; Tang, H.; Schlagel, D.L.; Lograsso, T.A.; K.A. Gschneidner,jr. & Pecharsky, V.K.
Partner: UNT Libraries Government Documents Department

High Temperature Structural Behavior of SrRuO{sub 3}

Description: The unusual metal SrRuO{sub 3} is perhaps the only known 4d transition metal based ferromagnet (Tc = 162K) with a sizable moment. To complement low T polarized neutron diffraction measurements of the magnetization density, high T neutron diffraction measurements are reported here. Two structural phase transitions are observed. Between 10K and 800K SrRuO{sub 3} is orthorhombic and at 800K it appears to be tetragonal until 975K, where it becomes cubic. The temperature variation of the lattice parameters are reported along with a structural description of the tetragonal phase.
Date: July 15, 1997
Creator: Chakoumakos, B.C., Nagler, S.E., Misture, S.T., Christen, H.M.
Partner: UNT Libraries Government Documents Department

On the ``inverse'' isotope effect in organic superconductors: New findings and implications

Description: The inverse deuterium isotope effect, previously found in K-(BEDT-TTF){sub 2}CU(NCS){sub 2}, is also found in two other BEDT-TTF-based superconductors with different packing motifs and different types of anions. Remarkably, the magnitude of the isotope shift is essentially identical in all three superconductors, ca. +0.26 {+-} 0.06 K. These results, when taken together with the recent results of Lang et al. on the uniaxial pressure derivatives of T{sub c}, suggest that the inverse isotope effect may not have a direct relationship to the pairing mechanism but instead is a reflection of the change in the internal lattice pressure.
Date: July 24, 2000
Creator: Kini, A. M.; Schlueter, J. A.; Ward, B. H.; Geiser, U. W. & Wang, H. H.
Partner: UNT Libraries Government Documents Department

Magnetic imaging of interlayer Josephson vortices.

Description: The authors have magnetically imaged interlayer Josephson vortices emerging parallel to the planes of single crystals of the organic superconductor {kappa}-(BEDT-TTF){sub 2} Cu(NCS){sub 2}, and the single layer cuprate high-T{sub c} superconductors Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}} (Tl-2201) and (Hg,Cu)Ba{sub 2}CuO{sub 6+{delta}} (Hg-1201), using a scanning Superconducting Quantum Interference Device (SQUID) microscope. These images provide a direct measurement of the interlayer penetration depth, which is approximately 63 {micro}m for {kappa}-(BEDT-TTF){sub 2} Cu(NCS){sub 2}, 18 {micro}m for Tl-2201 and 8 {micro}m for Hg-1201. The lengths for the cuprates are about a factor of 10 larger than originally predicted by the interlayer tunneling model for the mechanism of superconductivity in layered compounds, indicating that this mechanism alone cannot account for the high critical temperatures in these materials.
Date: January 15, 1999
Creator: Kirtley, J. A.
Partner: UNT Libraries Government Documents Department

Disorder-driven hysteresis-loop criticality in Co/CoO-films.

Description: The effect of magnetic disorder on the magnetization reversal process in thin Co/CoO-films has been investigated. The antiferromagnetic CoO layer allows a reversible tuning of the magnetic disorder by simple temperature variation. For temperatures above a critical temperature T{sub c}, we observe a discontinuous magnetization reversal, whereas smooth magnetization loops occur for T < T{sub c}. Our measurements establish the existence of a disorder-driven critical point in the non-equilibrium phase diagram. In addition, we observe scaling behavior in the vicinity of the critical point and determine the critical exponents to {beta} = 0.022 {+-} 0.006 and {beta}{delta} = 0.30 {+-} 0.03.
Date: November 1, 2000
Creator: Berger, A.; Inomata, A.; Jiang, J. S.; Pearson, J. E. & Bader, S. D.
Partner: UNT Libraries Government Documents Department

Ion-Beam-Induced Defects and Defect Interactions in Perovskite-Structure Titanates

Description: Ion-beam irradiation of perovskite structures results in the production and accumulation of defects. Below a critical temperature, irradiation also leads to a crystalline-to-amorphous transformation. The critical temperature for amorphization under 800 keV Kr{sup +} ion irradiation is 425,440 and 550 K for SrTiO{sub 3}, CaTiO{sub 3} and BaTiO{sub 3}, respectively. The results of ion-channeling studies on SrTiO{sub 3} irradiated with 1.0 MeV Au{sup 2+} ions suggest that the crystalline-to-amorphous transformation is dominated by the accumulation and interaction of irradiation-induced defects. In SiTiO{sub 3} irradiated with He{sup +} and 0{sup +} ions at 180 K, isochronal annealing studies indicate that there is significant recovery of defects on both the oxygen and cation sublattices between 200 and 400 K. These results suggest that defect recovery processes may control the kinetics of amorphization. A fit of the direct-impact/defect-stimulated model to the data for SrTiO{sub 3} suggests that the kinetics of amorphization are controlled by both a nearly athermal irradiation-assisted recovery process with an activation energy of 0.1 plus or minus 0.05 eV and a thermal defect recovery process with an activation energy of 0.6 plus or minus 0.1 eV. In SrTi0{sub 3} implanted with 40 keV H{sup +} to 5.0 x 10{sup 16} and 1.0 x 10{sup 17} ions/cm{sup 2}, annealing at 470 K increases the backscattering yield from Sr and Ti and is mostly likely due to the coalescence of H{sub 2} into bubble nuclei. Annealing at 570 K and higher results in the formation of blisters or large cleaved areas.
Date: August 23, 1999
Creator: Boatner, L.A.; Jiang, W.; Meldrum, A.; Thevuthasan, S.; Weber, W.J. & Williford, R.E.
Partner: UNT Libraries Government Documents Department

Hadron production at RHIC - chemical freeze-out, effective masses and expanding matter.

Description: The properties of the strongly interacting matter produced at RHIC are investigated within a chemical and thermal equilibrium chiral SU(3) {sigma}-{omega} approach. The focus is on the freeze-out conditions, the effective masses for the produced hadrons and the expansion of the system. For the freeze-out conditions the commonly adopted noninteracting gas calculations yield temperatures and chemical potentials close to or above the critical temperature for the chiral phase transition, but without taking into account any interactions. Contrary, the chiral SU(3) model predicts temperature and density dependent effective hadron masses and effective chemical potentials in the medium and a transition to a chirally restored phase at high temperatures or chemical potentials. Three different parametrizations of the model, which show different types of phase transition behavior, are investigated. They show that if a chiral phase transition occurred in those collisions, freezing of the relative hadron abundances in the symmetric phase is excluded by the data. Therefore, either very rapid chemical equilibration must occur in the broken phase, or the measured hadron ratios are the outcome of the dynamical symmetry breaking. Furthermore, the extracted chemical freeze-out parameters differ considerably from those obtained in simple noninteracting gas calculations. In particular, the three models yield up to 35 MeV lower temperatures than the free gas approximation. The in-medium masses turn out differ up to 150 MeV from their vacuum values.
Date: September 26, 2002
Creator: Zschiesche, D.; Schramm, S.; Schaffner-Bielich, J.; Stocker, H. & Greiner, W.
Partner: UNT Libraries Government Documents Department


Description: An investigation was made to increase the basic knowledge of nucleate and film boiling heat transfer to fluids up to their critical pressures. An extensive literature survey of the subject indicated a scarcity of consistent data and an absence of proven methods for analysis. This situation led to the design and construction of experimental apparatus suitable for measuring rates of free convection heat transfer and associated temperature differences for fluid conditions up to 4000 psi and 800 deg F, with provision for visual observation. The apparatus incorporated a stainless steel system for containing the fluid. Ten-mil-diameter cylindrical and 1/8-in.-high vertical-plate platinum test sections were placed in the fluid inside a pressure vessel, 5 in. in inside diameter and 21 in. high. The pressure and temperature of the fluid envirorment were controlled by constant-volume heating of the fluid in the pressure vessel with the aid of seven zones of radiant guard heaters located between the external wall of the pressure vessel and the insulation. Each of these zones was controlled by a differential-thermocouple-sensed circuit that automatically kept the pressure vessel at a prescribed uniform temperature. Fluid pressure was measured with a deadweight tester and a 0-4000-psi Heise gauge. Fluid temperature was measured with a calibrated resistance thermometer and Mueller bridge, purchased specifically for that purpose. The heat flux in the test section was obtained from electrical measurements of the directcurrent power supplied for Joule heating. The temperature of the test section was measured by means of a resistance thermometer. The system was outgassed and filled under a vacuum with water that was deionized, degassed, and deionized again. The quality of the water was maintained during an experiment with the aid of a thermalsiphon deionizing loop which was connected to the system. The results were tabulated and plotted in terms of heat ...
Date: August 1, 1961
Creator: Holt, V.E.
Partner: UNT Libraries Government Documents Department


Description: We have measured the critical magnetic field for superconductivity In tungsten from 5.5 to 15 mK using a {gamma}-ray anisotropy thermometer, and we have measured the heat capacity between 0.35 and 25 K. Analysis of the data gives H{sub 0} = 1.237 Oe for the 0 K critical field, T{sub c} = 16.0 mK for the critical temperature, {gamma} = 1.008 mJ/mole K{sup 2} for the coefficient of the electronic heat capacity, and {Theta}{sub 0} = 383 K for the 0 K Debye temperature. The measured values of the critical field H{sub c} are consistently higher than those reported by Black, Johnson and Wheatley (BJW) on the CMN temperature scale, but the temperature dependence is similar. This discrepancy and the temperature dependence of H{sub c} suggest that both sets of H{sub c} data are affected by magnetic impurities. Use of the calorimetric {gamma} value permits an improved test of the CMN temperature scale with the very low temperature H{sub c} data obtained by BJW.
Date: March 1, 1973
Creator: Triplett, B.B.; Phillips, N.E.; Thorp, T.L.; Shirley, D.A. & Brewer, W.D.
Partner: UNT Libraries Government Documents Department


Description: We review studies of QCD thermodynamics by lattice QCD simulations with dynamical Wilson quarks. After explaining the basic properties of QCD with Wilson quarks at finite temperature including the phase structure and the scaling properties around the chiral phase transition, we discuss the critical temperature, the equation of state and heavy-quark free energies.
Date: November 20, 2007
Creator: Ejiri, S.
Partner: UNT Libraries Government Documents Department

Asymmetric Ferromagnet-Superconductor-Ferromagnet Switch

Description: In layered ferromagnet-superconductor-ferromagnet F{sub 1} /S/F{sub 2} structures, the critical temperature T{sub c} of the superconductors depends on the magnetic orientation of the ferromagnetic layers F{sub 1} and F{sub 2} relative to each other. So far, the experimentally observed magnitude of change in T{sub c} for structures utilizing weak ferromagnets has been 2 orders of magnitude smaller than is expected from calculations. We theoretically show that such a discrepancy can result from the asymmetry of F/S boundaries, and we test this possibility by performing experiments on structures where F{sub 1} and F{sub 2} are independently varied. Our experimental results indicate that asymmetric boundaries are not the source of the discrepancy. If boundary asymmetry is causing the suppressed magnitude of T{sub c} changes, it may only be possible to detect in structures with thinner ferromagnetic layers.
Date: November 4, 2011
Creator: Cadden-Zimansky, P.; Bazaliy, Ya.B.; Litvak, L.M.; Jiang, J.S.; Pearson, J.; Gu, J.Y. et al.
Partner: UNT Libraries Government Documents Department

Analysis of Bulk and Thin Film Model Samples Intended for Investigating the Strain Sensitivity of Niobium-Tin

Description: Bulk samples and thin films were fabricated and characterized to determine their suitability for studying the effect of composition and morphology on strain sensitivity. Heat capacity and resistivity data are used to determine the critical temperature distribution. It is found that all bulk samples contain stoichiometric Nb{sub 3}Sn regardless of their nominal Nb to Sn ratio. Furthermore, in bulk samples with Cu additions, a bi-modal distribution of stoichiometric and off-stoichiometric Nb-Sn is found. Thus the nominally off-stoichiometric bulk samples require additional homogenization steps to yield homogeneous off-stoichiometric samples. A binary magnetron-sputtered thin film has the intended off-stoichiometric Nb-Sn phase with a mid-point critical temperature of 16.3 K. This type of sample is a suitable candidate for investigating the strain sensitivity of A15 Nb{sub 1-{beta}}Sn{sub {beta}}, with 0.18 < {beta} < 0.25. The strain sensitivity of Nb-Sn as a function of composition and morphology is important for an in-depth understanding of the strain sensitivity of composite Nb{sub 3}Sn wires.
Date: August 1, 2010
Creator: Mentink, M. G. T.; Anders, A.; Dhalle, M. M. J.; Dietderich, D. R.; Godeke, A.; Goldacker, W. et al.
Partner: UNT Libraries Government Documents Department

Normal State Spectral Lineshapes of Nodal Quasiparticles in Single Layer Bi2201 Superconductor

Description: A detailed study of the normal state photoemission lineshapes and quasiparticle dispersion for the single layer Bi{sub 2}Sr{sub 2-x}La{sub x}CuO{sub 6+{delta}}(Bi2201) superconductor is presented. We report the first experimental evidence of a double peak structure and a dip of spectral intensity in the energy distribution curves (EDCs) along the nodal direction. The double peak structure is well identified in the normal state, up to ten times the critical temperature. As a result of the same self-energy effect, a strong mass renormalization of the quasiparticle dispersion, i.e. kink, and an increase of the quasiparticle lifetime in the normal state are also observed. Our results provide unambiguous evidence on the existence of bosonic excitation in the normal state, and support a picture where nodal quasiparticles are strongly coupled to the lattice.
Date: April 30, 2010
Creator: Lanzara, A.
Partner: UNT Libraries Government Documents Department

The finite temperature QCD using 2+1 flavors of domain wall fermions at Nt = 8

Description: We study the region of the QCD phase transition using 2+1 flavors of domain wall fermions (DWF) and a 16{sup 3} x 8 lattice volume with a fifth dimension of L{sub s} = 32. The disconnected light quark chiral susceptibility, quark number susceptibility and the Polyakov loop suggest a chiral and deconfining crossover transition lying between 155 and 185 MeV for our choice of quark mass and lattice spacing. In this region the lattice scale deduced from the Sommer parameter r{sub 0} is a{sup -1} {approx} 1.3 GeV, the pion mass is {approx} 300 MeV and the kaon mass is approximately physical. The peak in the chiral susceptibility implies a pseudo critical temperature T{sub c} = 171(10)(17) MeV where the first error is associated with determining the peak location and the second with our unphysical light quark mass and non-zero lattice spacing. The effects of residual chiral symmetry breaking on the chiral condensate and disconnected chiral susceptibility are studied using several values of the valence L{sub s}.
Date: November 30, 2009
Creator: Cheng, M; Christ, N H; Li, M; Mawhinney, R D; Renfrew, D; Hegde, P et al.
Partner: UNT Libraries Government Documents Department

BG/L Calculations of the QCD Critical Temperature and Equation of State

Description: We summarize the latest results from performing Lattice QCD calculations with two different discretization schemes, asqtad and p4fat3, on the LLNL BG/L supercomputer, and present a plan for continued running to complete the calculation of the QCD transition temperature and equation of state.
Date: December 2, 2007
Creator: Battacharya, T.; Bazavov, A.; Cheng, M.; Christ, N.; DeTar, C.; Gottlieb, S. et al.
Partner: UNT Libraries Government Documents Department

Advanced Pressure Boundary Materials

Description: Increasing the operating temperatures of fossil power plants is fundamental to improving thermal efficiencies and reducing undesirable emissions such as CO{sub 2}. One group of alloys with the potential to satisfy the conditions required of higher operating temperatures is the advanced ferritic steels such as ASTM Grade 91, 9Cr-2W, and 12Cr-2W. These are Cr-Mo steels containing 9-12 wt% Cr that have martensitic microstructures. Research aimed at increasing the operating temperature limits of the 9-12 wt% Cr steels and optimizing them for specific power plant applications has been actively pursued since the 1970's. As with all of the high strength martensitic steels, specifying upper temperature limits for tempering the alloys and heat treating weldments is a critical issue. To support this aspect of development, thermodynamic analysis was used to estimate how this critical temperature, the A{sub 1} in steel terminology, varies with alloy composition. The results from the thermodynamic analysis were presented to the Strength of Weldments subgroup of the ASME Boiler & Pressure Vessel Code and are being considered in establishing maximum postweld heat treatment temperatures. Experiments are also being planned to verify predictions. This is part of a CRADA project being done with Alstom Power, Inc.
Date: January 1, 2007
Creator: Santella, Michael L. & Shingledecker, John P.
Partner: UNT Libraries Government Documents Department

Vapor pressure and critical point of rubidium

Description: The vapor pressure of rubidium was experimentally determined from 1400 deg F up to its critical temperature. An empirical equation of the form ln P = A + B/T + C ln T + DT/sup m/ fit the data well when m = --2. A critical pressure of 1942.9 plus or minus 3.5 psia (132.2 plus or minus 0.24 ata) was measured. The corresponding critical temperature, extrapolated from the pressure-- temperature curve, agrees with 3790.7 plus or minus 10 deg R (2106 plus or minus 5 deg K) of Chung and Bonilla, which was adopted. The technique employed was the pressure tube method developed earlier in this laboratory and used for determining the vapor pressure of cesium. This method messures the critical pressure directly, as well as the vapor pressure at lower temperatures. (3 tables) (auth)
Date: January 1, 1973
Creator: Bhise, V.S. & Bonilla, C.F.
Partner: UNT Libraries Government Documents Department

High temperature vapor pressure and the critical point of potassium

Description: The vapor pressure of potassium was experimentally determined from 2100 deg F up to-its critical temperature. An empirical equation of the form ln P = A + B/T + C ln T + DT/sup 1.5/ was found to best fit the data. A critical pressure of 2378.2 plus or minus 4.0 psia (161.79 plus or minus 0.27 ata) was measured. The corresponding critical temperature, extrapolated from the pressure-- temperature curve, is 4105.4 plus or minus 5 deg R (2280.8 plus or minus 3 deg K). The technique employed was tae pressure tube method developed earlier in this laboratory and used for determining the vapor pressure of rubidium and cesium. This method measures tae critical pressure directly, as well as the vapor pressure st lower temperatures. (4 tables, 6 figures, 26 references) (auth)
Date: January 1, 1973
Creator: Jerez, W.R.; Bhise, V.S.; Das Gupta, S. & Bonilla, C.F.
Partner: UNT Libraries Government Documents Department

Pseudogap and Incommensurate Magnetic Fluctuations in YBa{sub 2}Cu{sub 3}O{sub 6.6}

Description: Unpolarized inelastic neutron scattering is used to study the temperature and wave vector dependence of the dynamical magnetic susceptibility, {xi}`` (q,{omega}), of a well characterized single crystal YBa{sub 2}Cu{sub 3}O{sub 6.6} (T{sub c} = 62.7 K). We find that a pseudogap opens in the spin fluctuation spectrum at temperatures well above T{sub c}. We speculate that the appearance of the low frequency incommensurate fluctuations is associated with the opening of the pseudogap. To within the error of the measurements, a gap in the spin fluctuation spectrum is found in the superconducting state.
Date: July 24, 1997
Creator: Dai, P., Mook, H.A.,
Partner: UNT Libraries Government Documents Department