Copyright (c) 2009 All rights reserved. http://works.bepress.com/hkliu Recent documents in Professor Hua Kun Liu en-us Sun, 31 May 2009 06:33:50 PDT 3600 http://works.bepress.com/hkliu/63 http://works.bepress.com/hkliu/63 Wed, 28 May 2008 14:40:45 PDT The magnetic field Hdis(T) where an order-disorder transition of the vortex lattice in high-Tc superconductors occurs, is investigated by measurements of the magnetization M(H) in Bi2.1Sr1.9CaCu2O8 + delta (Bi2212) single crystals doped with iron and lead. Comparative studies are made of the temperature dependences of the field Hpeak(T), where the second peak occurs in |M(H)|, and the fields Hmin(T), and Hinfl(T) where a minimum and an inflection point occur at the low-field side of this peak. It is proposed that Hdis(T) lies close to Hinfl. In Bi2.1Sr1.9Ca1.0(Cu1-yFey)2O8 + delta single crystals with Fe concentration y = 0, 0.005, 0.016, and 0.022, a pronounced peak in the derivative |dM/dH| is observed, whose position Hinfl(T) is independent of temperature T. We relate this peak to the field Hdis(T), which separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. In heavily Pb-doped single Bi2212 crystals, Hinfl(T) decreases with increasing T. For the same crystals, a minimum in the normalized relaxation rate S(H) is observed at Hinfl, indicating two different flux-creep mechanisms above and below that field and two different solid vortex phases. It is argued that the negative slope of Hdis(T) in heavily-Pb-doped Bi2212 crystals is related to the enhanced c axis conductivity caused by the Pb sitting between the CuO2 layers and causing three-dimensional vortex lines, while in Fe-doped Bi2212 crystals the Fe ions sit on the CuO2 planes and thus do not enhance the coupling between pancake vortices. K. K. Uprety http://works.bepress.com/hkliu/62 http://works.bepress.com/hkliu/62 Wed, 28 May 2008 14:40:40 PDT Measurements of the production of forward pi(0) mesons from p+p and d+Au collisions at root s(NN) = 200 GeV are reported. The p+p yield generally agrees with next-to-leading order perturbative QCD calculations. The d+Au yield per binary collision is suppressed as eta increases, decreasing to similar to 30% of the p+p yield at <eta >=4.00, well below shadowing expectations. Exploratory measurements of azimuthal correlations of the forward pi(0) with charged hadrons at eta approximate to 0 show a recoil peak in p+p that is suppressed in d+Au at low pion energy. These observations are qualitatively consistent with a saturation picture of the low-x gluon structure of heavy nuclei. J. Adams http://works.bepress.com/hkliu/61 http://works.bepress.com/hkliu/61 Wed, 28 May 2008 14:40:36 PDT Nonstoichiometric amorphous Mg-Ni and Mg-Ni-V alloys were synthesized by mechanical milling the crystalline Mg2Ni alloy with Ni and/or V powders. All the nonstoichiometric amorphous Mg-based alloys result in Mg-based alloy electrodes with very high discharge capacities by comparison with crystalline Mg2Ni alloys. The results indicate that nonstoichiometric amorphous Mg-based alloys can be obtained either by increasing the Ni content, adding a range of other elements, or both, using a mechanical milling method. The nonstoichiometric Mg-based alloy electrodes studied have shown improved initial discharge capacities compared with the stoichiometric amorphous MgNi alloy. These results describe a method of achieving better Mg-based alloy electrodes with high discharge capacities and improved cycle life. The method enables a larger composition range to be achieved with a range of vanadium additions. L. Sun http://works.bepress.com/hkliu/60 http://works.bepress.com/hkliu/60 Wed, 28 May 2008 14:40:32 PDT We present the transverse momentum (PT) spectra for identified charged pions, protons and anti-protons from p + p and d + Au collisions at root sNN = 200 GeV. The spectra are measured around midrapidity (vertical bar y vertical bar < 0.5) over the range of 0.3 < PT < 10 GeV/c with particle identification from the ionization energy loss and its relativistic rise in the time projection chamber and time-of-flight in STAR. The charged pion and proton + anti-proton spectra at high PT in p + p and d + Au collisions are in good agreement with a phenomenological model (EPOS) and with next-to-leading order perturbative quantum chromodynamic (NLO pQCD) calculations with a specific fragmentation scheme and factorization scale. We found that all proton, anti-proton and charged pion spectra in p + p collisions follow x(T)-scating for the momentum range where particle production is dominated by hard processes (p(T) greater than or similar to 2 GeV/c). The nuclear modification factor around midrapidity is found to be greater than unity for charged pions and to be even larger for protons at 2 < PT < 5 GeV/c. (c) 2006 Elsevier B.V. All rights reserved. J. Adams http://works.bepress.com/hkliu/59 http://works.bepress.com/hkliu/59 Wed, 28 May 2008 14:40:27 PDT Sample size dependent magnetic critical current density has been observed in magnesium diboride superconductors. At high fields, larger samples provide higher critical current densities, while at low fields, larger samples give rise to lower critical current densities. The explanation for this surprising result is proposed in this study based on the electric field generated in the superconductors. The dependence of the current density on the sample size has been derived as a power law j}R1/n @n is the n factor characterizing E2j curve E 5Ec( j/ jc)n]. This dependence provides one with a method to derive the n factor and can also be used to determine the dependence of the activation energy on the current density. M. J. Qin http://works.bepress.com/hkliu/58 http://works.bepress.com/hkliu/58 Wed, 28 May 2008 14:40:23 PDT The STAR Collaboration at the Relativistic Heavy Ion Collider reports measurements of azimuthal correlations of high transverse momentum (p(T)) charged hadrons in Au+Au collisions at higher p(T) than reported previously. As p(T) is increased, a narrow, back-to-back peak emerges above the decreasing background, providing a clear dijet signal for all collision centralities studied. Using these correlations, we perform a systematic study of dijet production and suppression in nuclear collisions, providing new constraints on the mechanisms underlying partonic energy loss in dense matter. J. Adams http://works.bepress.com/hkliu/57 http://works.bepress.com/hkliu/57 Wed, 28 May 2008 14:40:19 PDT The Rutherford backscattering (RBS) method has been employed to study the incorporation of oxygen into MgB2 films during their fabrication by pulsed-laser deposition (PLD). A series of MgB2 thin film samples were analyzed, including two films produced in situ on Al2O3-c substrates (with higher Tc and lower Tc) with an on-axis geometry, one film produced in situ with an off-axis geometry, and one film produced ex situ, with a bulk-like Tc. The amount of oxygen detected by RBS, which is stable in the form of MgO, appears to be correlated with the Tc of the films, the higher the Tc the lower the oxygen content. The superconducting properties of the thin films are discussed in the context of the RBS results. Y. Zhao http://works.bepress.com/hkliu/56 http://works.bepress.com/hkliu/56 Wed, 28 May 2008 14:40:14 PDT YBa/sub 2/Cu/sub 3/O/sub 7/ (Y123) thin films were grown by pulsed laser deposition (PLD) on YSZ (100), SrTiO3 (100), and LaAlO3 (100) single crystal substrates. Prior to the film deposition, a discontinuous layer of Ag nano-dots was deposited on the substrates. The Y123 films grown on such surfaces modified with Ag nano-dots were characterized by Atomic Force Microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), AC susceptibility and DC magnetization. The effects of the density of Ag nano-dots, which was controlled by the numbers of PLD shots, on the microstructures and resultant critical current density J/sub c/ have been studied systematically. Results showed that at fixed physical deposition conditions J/sub c/ increased monotonically with number of Ag shots, n, for films grown on both STO and LAO substrates. At 77 K, the J/sub c/ increased from 10/sup 6/ to 3.2/spl times/10/sup 6/ A/cm/sup 2/ for LAO and from 8/spl times/10/sup 5/ to 3.5/spl times/106 A/cm/sup 2/ for STO as n increased from 0 to 150. At 5 K, the enhancement of J/sub c/ was approximately four times at both low and high fields. However, for films grown on YSZ substrate, J/sub c/ increased from 2/spl times/10/sup 5/ to 2/spl times/10/sup 6/ A/cm/sup 2/ as Ag shots increased from 0 to 30, and decreased to 9/spl times/10/sup 5/ for n/spl ges/60. Detailed microstructure investigations indicated that the crystallinity and ab alignment gradually improved as the number of Ag-nano-dots increased. A. H. Li http://works.bepress.com/hkliu/55 http://works.bepress.com/hkliu/55 Wed, 28 May 2008 14:40:11 PDT We present the directed flow (v(1)) measured in Au+Au collisions at root s(NN)=62.4 GeV in the midpseudorapidity region parallel to eta parallel to < 1.3 and in the forward pseudorapidity region 2.5 <parallel to eta parallel to < 4.0. The results are obtained using the three-particle cumulant method, the event plane method with mixed harmonics, and for the first time at the Relativistic Heavy Ion Collider, the standard method with the event plane reconstructed from spectator neutrons. Results from all three methods are in good agreement. Over the pseudorapidity range studied, charged particle directed flow is in the direction opposite to that of fragmentation neutrons. J. Adams http://works.bepress.com/hkliu/54 http://works.bepress.com/hkliu/54 Wed, 28 May 2008 14:40:07 PDT The influence of the sinter-forging rate on the critical current density (Jc) behaviour in an external field and on the contact resistance Rc for Bi-2223 current leads has been investigated. The current leads were fabricated by a combination of cold isostatic pressing (CIP) and sinter-forging methods with the thickness reduction rate ranging from 0% to 90%. The two silver contact terminals of each sample were also prepared during the sinter-forging. The results revealed that Jc was strongly affected by the deformation rate of sinter-forging and reached a maximum of 725 A/cm2 at a deformation rate of 80%. From the measurements of the external magnetic field dependence on Jc. It was determined that sinter-forging could improve the Jc behaviour in external fields, particularly in the regime below 10×10-13 Tesla (i.e. 10 mT). The measurements of the contact resistance Rc were conducted for different transport currents at 77 K. The results showed that the contact resistance for the samples with higher deformation rates became less dependent on the transport current over a range of 0.5 A to 50 A. X. K. Fu