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A molecular dynamics simulation study of defect production in vanadium

Description: We performed molecular dynamics simulations to investigate the process of defect production in pure vanadium. The interaction of atoms was described by the EAM interatomic potential modified at short range to merge smoothly with the universal potential for description of the high energy recoils in cascades. The melting point of this EAM model of vanadium was found to be consistent with the experimental melting temperature. The threshold energies of displacement events in the model system are also consistent with experimental minimum threshold in vanadium, and its average was found to be 44 eV. We evaluated the efficiencies of defect production in the displacement events initiated by recoils with kinetic energy up to 5 keV, and found that the probability of cluster formation is smaller than that of simulated events in fcc metals reported in the literature.
Date: January 23, 1995
Creator: Morishita, K. & Diaz de la Rubia, T.
Partner: UNT Libraries Government Documents Department

Particle physics with kaons, muons and neutrinos. Summary of JHF K-Arena working groups 1a/1b

Description: The areas of rare K and {mu} decays and of neutrino oscillations are very active at the present time, and there is little reason to doubt that this will continue to be the case in the JHF era. The session featured talks describing the present status and projected future activity in these areas.
Date: August 1, 1998
Creator: Kuno, Yoshitaka & Littenberg, L.
Partner: UNT Libraries Government Documents Department

Carrier localization in gallium nitride

Description: In wide bandgap GaN, a large number of interesting and important scientific questions remain to be answered. For example, the large free electron concentration reaching 10{sup 19} to 10{sup 20} cm{sup - 3} in nominally undoped material are ascribed to intrinsic defects because no chemical impurity has been found at such high concentrations. According to theoretical models, a nitrogen vacancy acts as a donor but its formation energy is very large in n-type materials, making this suggestion controversial. We have investigated the nature of this yet unidentified donor at large hydrostatic pressure. Results from infrared reflection and Raman scattering indicate strong evidence for localization of free carriers by large pressures. The carrier density is drastically decreased by two orders of magnitude between 20 and 30 GPa. Several techniques provide independent evidence for results in earlier reports and present the first quantitative analysis. A possible interpretation of this effect in terms of the resonant donor level is presented.
Date: September 1, 1996
Creator: Wetzel, C.; Walukiewicz, W. & Haller, E.E.
Partner: UNT Libraries Government Documents Department

Neutron Scattering Studies of the Chains and Ladders in Sr{sub 14}Cu{sub 24}O{sub 41} and Related Compounds

Description: A review is given of the neutron scattering studies on the S=1/2 quasi-one-dimensional magnet Sr14Cu24O41 and related compounds which have both simple chains and two-leg ladders of copper ions. Both of the building blocks show interesting singlet ground states and magnetic excitations. In this system it is possible to change the number of hole carriers by substitution for Sr. A dramatic change has been observed in the ground state and the magnetic excitations with decreasing the number of holes.
Date: Autumn 1997
Creator: Matsuda, M.
Partner: UNT Libraries Government Documents Department

Magnetic properties of the S=1/2 quasi-one-dimensional antiferromagnet: Sr{sub 14-x}Y{sub x}Cu{sub 24}O{sub 41}

Description: Magnetic susceptibility and inelastic neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system Sr{sub 14-x}Y{sub x}Cu{sub 24}O{sub 41}, which has both simple chains and two-leg ladder chains of copper ions. The experimental results show that the simple chain in Sr{sub 14}Cu{sub 24}O{sub 41} has a spin gap in the excitation spectrum, which originates from a dimerized state. We have also studied the effect of Y{sup 3+} substitution for Sr{sup 2+} site on the dimerized state. It was found that the yttrium substitution suppresses the gap energy drastically.
Date: July 1, 1996
Creator: Matsuda, M.; Katsumata, K.; Shapiro, S.M. & Shirane, G.
Partner: UNT Libraries Government Documents Department

Yield stress anomaly in B2 FeAl

Description: The studies on yield stress anomaly of B2 FeAl single crystals are reviewed in this paper. A positive temperature dependence of yield stress, so-called yield stress anomaly, is observed in B2 FeAl in which excess vacancies are fully annealed out. Associated with the anomaly, characteristic asymmetry is found between tension and compression. While the strain-rate sensitivity is almost zero in the temperature range of the yield stress anomaly, the stress relaxation becomes significant with increasing temperature, indicating that a recovery process is thermally activated. It is ascertained by the two-surface trace analysis that slip transition from <111> direction at intermediate temperature to <100> at high temperature occurs around the peak temperature. Even at the peak temperature, in addition, operative slip vector for yielding is confirmed to be predominantly <111> by TEM. Also, it is observed that <111>-type superdislocations are frequently climb-dissociated in the temperature range of the anomaly. APB formation on {l_brace}111{r_brace} plane is energetically favorable, which is in agreement with the Flinn`s calculation for the B2 superlattice that APB energy on {l_brace}111{r_brace} plane is lower than that on {l_brace}110{r_brace} plane. Such an anisotropy of APB energy would offer specific driving force for the climb dissociation on <111> superdislocations. On the basis of the observed results, the anomalous strengthening behavior of B2 FeAl single crystals is discussed.
Date: December 31, 1996
Creator: Yoshimi, K.; Hanada, S. & Yoo, M.H.
Partner: UNT Libraries Government Documents Department

Behavior of excess electrons in supercritical fluids -- Electron attachment

Description: The behavior of excess electrons in supercritical ethane was investigated by measuring mobility and reaction rates. Mobilities wee measured by means of a time-of-flight method at 306--320 K as a function of pressure. Mobility values decreased at all temperatures with increasing pressure, but showed a small minimum or a shoulder at the pressure where the compressibility {chi}{sub T} has a peak. Electron attachment to CO{sub 2}, NO, pyrimidine and C{sub 2}F{sub 4} over the same temperature range was studied as a function of pressure. Both attachment rate constants k{sub a} for NO and C{sub 2}F{sub 4}, and equilibrium constants K({double_bond}k{sub a}/k{sub d}) for CO{sub 2} and pyrimidine increased sharply at pressures of {chi}{sub T} peaks. Activation volumes V{sub a}* and reaction volumes {Delta}V{sub r} are very large and negative in the critical region. The volume change is mainly due to electrostriction around ions formed. The results are compared to volume changes predicted by a compressible continuum model.
Date: July 1, 1999
Creator: Nishikawa, Masaru; Holroyd, R.A. & Itoh, Kengo
Partner: UNT Libraries Government Documents Department

Materials processing with intense pulsed ion beams

Description: We review research investigating the application of intense pulsed ion beams (IPIBs) for the surface treatment and coating of materials. The short range (0.1-10 {mu}m) and high-energy density (1-50 J/cm{sup 2}) of these short-pulsed ({le} 1 {mu}s) beams (with ion currents I = 5 - 50 kA, and energies E = 100 - 1000 keV) make them ideal to flash-heat a target surface, similar to the more familiar pulsed laser processes. IPIB surface treatment induces rapid melt and solidification at up to 10{sup 10} K/s to cause amorphous layer formation and the production of non-equilibrium microstructures. At higher energy density the target surface is vaporized, and the ablated vapor is condensed as coatings onto adjacent substrates or as nanophase powders. Progress towards the development of robust, high-repetition rate IPIB accelerators is presented along with economic estimates for the cost of ownership of this technology.
Date: December 1996
Creator: Rej, D. J.; Davis, H. A. & Olson, J. C.
Partner: UNT Libraries Government Documents Department

Coexistence of Haldane Gap Excitations and Long-Range Order in R{sub 2}BaNiO{sub 5} (R=Rare Earth)

Description: R2BaNiO5 (R = rare earth) quasi-1-D antiferromagnets are structurally equivalent to the well-studied 1-D S = 1 Haldane-gap compound Y2BaNiO5. Unlike the Y-nickelate though, these materials undergo 3-D magnetic ordering at finite temperatures. Recent inelastic neutron scattering studies of Pr2BaNiO5 and (Nd(x)Y(1-x))2BaNiO5 revealed purely one-dimensional gap excitations that propagate exclusively on the Ni-chains and are strikingly similar to Haldane gap modes in Y2BaNiO5. In the ordered phase these excitations survive and actually coexist with conventional spin waves. The results suggest that the Haldane singlet ground state of the Ni-chains is not fully destroyed by Neel ordering.
Date: June 19, 1997
Creator: Zheludev, A.
Partner: UNT Libraries Government Documents Department

Neutron study of fracton excitations in percolating antiferromagnets

Description: The authors report the results of an inelastic neutron scattering experiment on nearly-percolating Heisenberg antiferromagnets (RbMn{sub c}Mg{sub 1{minus}o}F{sub 3}), in which the Mn concentrations (C = 0.31, 0.34 and 0.39) are very close to the percolation threshold (c{sub p} = 0.312). A broad peak superimposed on Ising-cluster excitations was observed throughout the Brillouin zone. The intensity of a broad peak increased on approaching the percolation threshold. The origin of this broad peak is attributed to the excitation of fractons in a percolating network.
Date: June 27, 1997
Creator: Ikeda, H.; Takahashi, M.; Fernandez-Baca, J.A. & Nicklow, R.M.
Partner: UNT Libraries Government Documents Department

Composite particle production in relativistic Au+Au collisions at AGS: First results from the E866 forward spectrometer @ 2, 4, and 10.8 A{center_dot}GeV

Description: Particle spectra were measured for Au + Au collisions at 2, 4, and 10. 8 A{center_dot}GeV using the E866 spectrometers. Recent results on proton emission and composite particle production form the E866 forward spectrometer data taken in 1994 together with the first results from the 1995/6 AGS running period are presented. Preliminary results indicate a decrease in the coalescence scaling coefficient with increasing projectile energy and centrality.
Date: December 31, 1996
Creator: Ashktorab, K.
Partner: UNT Libraries Government Documents Department

Particle production in Au + Au collisions from BNL E866

Description: In this article, we present recent results on particle production from Au+Au collisions at 11 A GeV/c obtained by the E866 experiment. The experiment studies the particle production in high baryon density matter created in central Au + Au collisions. Preliminary results of proton and pion production have been reported in previous Quark Matter conferences. Two particle correlation data for Au + Au collisions in this experiment are presented in another paper in this conference.
Date: November 1, 1996
Creator: Akiba, Y.; Hamagaki, H.; Homma, S. & Sako, H.
Partner: UNT Libraries Government Documents Department

Two particle correlations at mid-rapidity in Si+A and Au+Au from E859/E866

Description: Two particle correlation measurements for {ital Si-A} and {ital Au- Au} collisions from Brookhaven E859 and E866 are discussed. These measurements allow us, with some interpretation, to deduce the size of the participant region in a heavy ion collision. We show that various source parameterizations yield consistent results and we explore the dependence of the apparent source size on the pion yield.
Date: February 1, 1997
Creator: Baker, M.D. & Collaboration, E802
Partner: UNT Libraries Government Documents Department

Recent results in p{anti p} collisions at {radical}s = 1.8 TeV from the CDF Collaboration at the Fermilab Tevatron collider

Description: During the 1992--95 Tevatron collider Run 1, the Collider Detector at Fermilab (CDF) collected a data sample of {anti p}p collisions at a center of mass energy of 1.8 TeV with an integrated luminosity of > 100 pb{sup {minus}1}. This was split into two separate data-taking runs: Run 1A ({approximately} 20 pb{sup {minus}1}) and the present Run 1B (> 80 pb{sup {minus}1}). Data-taking is planned to continue into 1996 and a total sample of about 120 pb{sup {minus}1} is expected. This data sample, in combination with improvements to the data acquisition system, the muon coverage, and the installation of the CDF SVX silicon vertex detector, has allowed many new effects to be studied. This paper reports results on heavy quark (charm, bottom and top) physics, electroweak phenomena, quantum chromodynamics, and searches for new particles beyond the Standard Model.
Date: February 1, 1996
Creator: Barnett, B. A.
Partner: UNT Libraries Government Documents Department

Measurement of the polarization in the decays B{sup 0} {yields} J/{psi}K*{sup 0} and B{sub s}{sup 0} {yields} J/{psi}{phi}

Description: The authors report on a measurement of the longitudinal polarization fraction in the decay B{sup 0} {yields} J/{psi}K*{sup 0} using data collected with the Collider Detector at Fermilab. B{sup 0} mesons are reconstructed through the decay chain B{sup 0} {yields} J/{psi} K*{sup 0}, J/{psi} {yields} {mu}{sup +}{mu}{sup {minus}}, K*{sup 0} {yields} K{sup +}{pi}{sup {minus}}. A sample of 65 {+-} 10 events is used to obtain the result {Lambda}{sub L}/{Lambda} = 0.65 {+-} 0.10 (stat) {+-} 0.04 (sys). The first measurement of {Lambda}{sub L}/{Lambda} in the decay B{sub s}{sup 0} {yields} J/{psi}{phi} is also presented.
Date: July 1995
Creator: Abe, F.
Partner: UNT Libraries Government Documents Department

Transverse momentum dependent two-pion Bose-Einstein correlations in Au + Au collisions at 11.6 A {center_dot} GeV/c

Description: Bose-Einstein correlations of {pi}{sup +}{pi}{sup +} and {pi}{sup {minus}}{pi}{sup {minus}} pairs collected by the BNL-E866 Forward Spectrometer in 11.6 A{center_dot}GeV/c Au + Au collisions have been measured. The data were analyzed using three-dimensional correlation functions parameterized by the Yano-Koonin-Potgoretskii and Bertsch-Pratt formalism to study transverse momentum dependent source parameters. Rapid decreases of longitudinal source radii and slower decreases in the transverse parameters with increasing transverse momentum were observed, which suggests a strong longitudinal and some transverse expansion. A freeze-out time {tau}{sub 0} was derived as 4.5--5 fm/c, under the assumption of the freeze-out temperature T = 130 MeV, and the duration of emission was found to be {delta}{tau} {approx} 2--4 fm/c.
Date: December 1998
Creator: Lee, J. H. & Collaboration, E866
Partner: UNT Libraries Government Documents Department

Centrality and collision system dependence of antiproton production from p+A to Au+Au collisions at AGS energies

Description: Antiproton production in heavy ion collisions reflects subtle interplay between initial production and absorption by nucleons. Because the AGS energies (10--20 A{center_dot}GeV/c) are close to the antiproton production threshold, antiproton may be sensitive to cooperative processes such as QGP and hadronic multi-step processes. On the other hand, antiproton has been proposed as a probe of baryon density due to large N{anti N} annihilation cross sections. Cascade models predict the maximum baryon density reaches about 10 times the normal nucleus density in central Au+Au collisions, where the strong antiproton absorption is expected. In this paper, the authors show systematic studies of antiproton production from p+A to Au+Au collisions.
Date: December 1997
Creator: Sako, H.; Ahle, L.; Akiba, Y. & Collaboration, E802
Partner: UNT Libraries Government Documents Department

Measurement of the lifetime of the B{sup o}{sub s} meson from D{sup minus}{sub s}{ell}{sup plus} correlations

Description: The lifetime of the B{sub s}{sup 0} meson is measured using the semileptonic decay B{sub s}{sup 0} {r_arrow} D{sub s}{sup {minus}}l{sup {plus}}{nu}X. The data sample consists of approximately 110 pb{sup {minus}1} of p{ovr p} collisions at {radical}s = 1.8 TeV collected by the CDF detector at the Fermilab Tevatron collider. There are 254{plus_minus}21 D{sub s}{sup {minus}}l{sup {plus}} signal events where the D{sub s}{sup {plus}} is reconstructed through its decay mode D{sub s}{sup {minus}} {r_arrow} {phi}{pi}{sup {minus}}, {phi} {r_arrow} K{sup {plus}}K{sup {minus}}. Using these events, the B{sub s}{sup 0} meson lifetime is determined to be {tau}(B{sub s}{sup 0}) {equals} 1.37{sup {plus}0.14}{sub {minus}0.12}(stat.){plus_minus}0.04(syst.) ps. The B{sub s}{sup 0} meson proper decay length distribution has been examined for a lifetime difference between the two CP eigenstates of the B{sub s}{sup 0} meson, B{sub s}{sup H} and B{sub s}{sup L}. 12 refs., 3 figs., 1 tab.
Date: July 1996
Creator: Burkett, K. & Paulini, M.
Partner: UNT Libraries Government Documents Department

Search for squarks and gluinos using dileptons

Description: Gluinos and squarks can be produced in {ital p{anti p}} collisions at {radical}s = 1.8 TeV and decay via charginos and neutralinos to final states containing two (or more) leptons. There is no charge correlation between the leptons of charginos from gluino decays as gluinos are Majorana particles. The strategy of this analysis is to search for an excess of events containing two like-sign isolated leptons, missing energy, and jets. The analysis is based on 81 pb{sup -1} of 1994-5 data recorded at CDF.
Date: October 1, 1996
Creator: Done, J. P.
Partner: UNT Libraries Government Documents Department

Measurement of {sigma} (p{bar p} --> t{bar t}) at {radical}s = 1.8 TeV by the D0 experiment

Description: We present the results from counting {ital t{anti t}} event candidates in {ital p{anti p}} collisions at a center-of-mass energy of 1.8 TeV by the DO collaboration. The results are based on an analysis of {approximately}100 pb{sup -1} of data in seven different channels which can be broadly categorized as dilepton or single- lepton depending on whether there are two or one high-{ital p{sub T}} isolated leptons ({ital e} or {mu}) in the final state. We have three dilepton channels, {ital ee},{mu}{mu}, and {ital e}{mu}. The single-lepton analysis is further divided into two subcategories, event shape and tag, orthogonal by construction, each of which has two channels, {ital e} + jets and {mu} + jets. All channels combined, we observe 37 events in our data 2048 with an estimated background of 13.4 {+-} 3.0 events. Under the assumptions of the minimal standard model, this translates into a {ital t{anti t}} production cross section as a function of the mass of the top quark ({ital m{sub t}}). For {ital m{sub t}} = 180 GeV, our measurement amounts to {sigma}({ital p{anti p}} {r_arrow} {ital t{anti t}}) = 4.7 {+-} 1.6 pb.
Date: September 1, 1996
Creator: Butler, J.M.; Chakraborty, D. & Collaboration, D0
Partner: UNT Libraries Government Documents Department

Top physics prospects in Run II and TeV33

Description: The top production rate in the future Tevatron run at {radical}s = 2 TeV with the increased luminosity is summarized. The expected yield seen by the upgraded detectors which have larger acceptance and better efficiency will give statistical and systematic improvements for various top decay properties and standard model values.
Date: July 1, 1996
Creator: Byon-Wagner, A.
Partner: UNT Libraries Government Documents Department

Search for new exotic particles at CDF

Description: We present recent results of searches for new particles beyond the Standard Model at the Collider Detector at Fermilab (CDF). These include searches for heavy gauge bosons (Z{prime}, W{prime}), leptoquarks and stable massive charged particles. For most of these particles we set the currently most stringent limits. 15 refs., 7 figs.
Date: July 1, 1996
Creator: Wensel, H.
Partner: UNT Libraries Government Documents Department

Search for charginos and neutralinos using trileptons

Description: We search for supersymmetry (SUSY) using trilepton events in {ital p{anti p}} collisions at {radical}s = 1.8 TeV. In the Minimal Supersymmetric Standard Model (MSSM), trilepton events are expected from chargino-neutralino ({anti {chi}}{sub 1}{sup {+-}}{anti {chi}}{sub 2}{sup 0}) pair production, with subsequent decay into leptons. In all possible combinations of electron and muon channels in 100 pb{sup -1} of data, we observe no events which pass our trilepton selection criteria. Assuming the GUT hypothesis within the framework of the MSSM, our preliminary analysis excludes M({anti {chi}}{sub 1}{sup {+-}}) < 68 GeV/c{sup 2}.
Date: October 1, 1996
Creator: Done, J. P.
Partner: UNT Libraries Government Documents Department

Search for charged Higgs bosons in CDF

Description: We present results of a direct search for charged Higgs production from {ital p}{ital {anti p}} collisions at {radical}s = 1.8 TeV at Fermilab`s Tevatron collider using the CDF detector. An expanded Higgs sector containing charged Higgs bosons is a persistent feature of candidate theories to replace the Standard Model. The minimal supersymmetric extension of the Standard Model, for example predicts that the dominant decay mode of the top quark is {ital t} {r_arrow} H{sup +}{ital b} {r_arrow} {tau}{sup +} {ital vb} for large values of tan {Beta}. We use the hadronic decays of the tau lepton in this channel to exclude charged Higgs bosons with M{sub H{sup {+-}}} {lt} 140 GeV/c{sup 2} for large tan{Beta}.
Date: August 1, 1996
Creator: Loomis, C.
Partner: UNT Libraries Government Documents Department