Deciphering Azimuthal Correlations in Relativistic Heavy-Ion Collisions

We discuss various sources of azimuthal correlations in relativistic heavy-ion collisions. The integral measure Phi is applied to quantify the correlations. We first consider separately the correlations caused by the elliptic flow, resonance decays, jets and transverse momentum conservation. An effect of randomly lost particles is also discussed. Using the PYTHIA and HIJING event generators we produce a sample of events which mimic experimental data. By means of kinematic cuts and particle's selection criteria, the data are analyzed to identify a dominant source of correlations.Comment: 8 pages, minor corrections, to appear in Phys. Rev.

High gas pressure and high-temperature synthesis (HP-HTS) technique and its impact on iron-based superconductors

The high-pressure growth technique generally plays an important role in the improvement of the sample quality and the enhancement of various physical and magnetic properties of materials. The high gas pressure technique provides a large sample space (10-15 cm) to grow various kinds of materials. In this paper, we introduce the high gas pressure and high-temperature synthesis (HP-HTS) technique that is present at our institute and is applied to the growth process of different kinds of superconducting materials, particularly iron-based superconductors. More details and the working principle of this HP-HTS technique are discussed. We have also demonstrated the current results based on the iron-based superconductors by using this unique HP-HTS technique. These results demonstrate the enhancement of the superconducting properties with the improved sample quality compared to the conventional synthesis process at ambient pressure.Comment: 12 pages, 8 figure

Fluctuations of the azimuthal particle distribution in NA49 at the CERN SPS

Event-by-event fluctuations and correlations in azimuthal angle are currently widely investigated in various experiments. In this paper the $\Phi$ measure (earlier used in experiments to evaluate fluctuations in transverse momentum) is now applied to azimuthal angle $\phi$. Properties of this $\Phi_{\phi}$ function are investigated through fast generators and with complex models such as Pythia, Hijing, and UrQMD. Preliminary results of NA49 on $\Phi_{\phi}$ are also presented. The system size dependence (p+p, C+C, Si+Si and 6 centralities of Pb+Pb) at the highest SPS energy (158$A$ GeV) is shown, as well as the energy dependence (20$A$ - 158$A$ GeV) for the 7.2\% most central Pb+Pb interactions.Comment: Proceedings from Hot Quarks 2010,June 21-2

High-pressure synthesis and the enhancement of the superconducting properties of FeSe0.5Te0.5

A series of FeSe0.5Te0.5 bulk samples have been prepared through the high gas pressure and high-temperature synthesis (HP-HTS) method to optimize the growth conditions, for the first time and investigated for their superconducting properties using structural, microstructure, transport, and magnetic measurements to reach the final conclusions. Ex-situ and in-situ processes are used to prepare bulk samples under a range of growth pressures using Ta-tube and without Tatube. The parent compound synthesized by convenient synthesis method at ambient pressure (CSP) exhibits a superconducting transition temperature of 14.8 K. Our data demonstrate that the prepared FeSe0.5Te0.5 sealed in a Ta-tube is of better quality than the samples without a Ta-tube, and the optimum growth conditions (500 MPa, 600{\deg}C for 1 h) are favourable for the development of the tetragonal FeSe0.5Te0.5 phase. The optimum bulk FeSe0.5Te0.5 depicts a higher transition temperature of 17.3 K and a high critical current density of the order of >10^4 A/cm^2 at 0 T, which is improved over the entire magnetic field range and almost twice higher than the parent compound prepared through CSP. Our studies confirm that the high-pressure synthesis method is a highly efficient way to improve the superconducting transition, grain connectivity, sample density, and also pinning properties of a superconductor

Comparison of Gd addition effect on the superconducting properties of FeSe0.5Te0.5 bulks under ambient and high-pressure conditions

We have prepared a series of (FeSe0.5Te0.5 + xGd) bulk samples, with x = 0, 0.03, 0.05, 0.07, 0.1 and 0.2, through the convenient solid-state reaction method at ambient pressure (CSP). High gas pressure and high-temperature synthesis methods (HP-HTS) are also applied to grow the parent compound (x = 0) and 5-wt% of Gd-added bulks. Structural, microstructural, transport and magnetic characterizations have been performed on these samples in order to draw the final conclusion. Our analysis results that the HP-HTS applied for the parent compound enhances the transition temperature (Tc) and the critical current density (Jc) with the improved sample density and intergrain connections. The lattice parameter c is increased with Gd additions, suggesting a small amount of Gd enters the tetragonal lattice of FeSe0.5Te0.5 and the Gd interstitial sites are along the c-axis. A systematic decrease of the onset transition temperature Tc is observed with Gd additions, however, the calculated Jc of these Gd-added samples is almost the same as that of the parent compound prepared by CSP. It specifies that there is no improvement of the grain connections or pinning properties due to these rare earth additions. However, Gd-added FeSe0.5Te0.5 bulks prepared by HP-HTS have revealed a slightly improved critical current density due to improved grain connections and sample density but have a lower transition temperature than that of the parent compounds.Comment: 33 pages, 9 figures, 3 table

Cometal Addition Effect on Superconducting Properties and Granular Behaviours of Polycrystalline FeSe_0.5Te_0.5

The enhanced performance of superconducting FeSe0.5Te0.5 materials with added micro-sized Pb and Sn particles is presented. A series of Pb- and Sn-added FeSe0.5Te0.5 (FeSe0.5Te0.5 + xPb + ySn; x = y = 0–0.1) bulks are fabricated by the solid-state reaction method and characterized through various measurements. A very small amount of Sn and Pb additions (x = y ≤ 0.02) enhance the transition temperature (Tconset) of pure FeSe0.5Te0.5 by ~1 K, sharpening the superconducting transition and improving the metallic nature in the normal state, whereas larger metal additions (x = y ≥ 0.03) reduce Tconset by broadening the superconducting transition. Microstructural analysis and transport studies suggest that at x = y > 0.02, Pb and Sn additions enhance the impurity phases, reduce the coupling between grains, and suppress the superconducting percolation, leading to a broad transition. FeSe0.5Te0.5 samples with 2 wt% of cometal additions show the best performance with their critical current density, Jc, and the pinning force, Fp, which might be attributable to providing effective flux pinning centres. Our study shows that the inclusion of a relatively small amount of Pb and Sn (x = y ≤ 0.02) works effectively for the enhancement of superconducting properties with an improvement of intergrain connections as well as better phase uniformity

Role of double doping with C and RE2O3 oxides on the critical temperature and critical current of MgB2 phase

A series of MgB2 samples doped with C, RE2O3 oxides (RE = Ce, Nd), and C and RE2O3oxides was prepared by the hot isostatic pressing method at high pressure: 1 GPa under argon gas. Based on X-ray diffraction and magnetization measurements it was found that the RE2O3 is not incorporated into MgB2. A secondary REB4 phase is present after the synthesis. No decrease in the critical temperature (Tc) is observed after the RE2O3 doping. The Tc of C and RE2O3 doped MgB2 is decreased by 3 K. The doping decreases the diamagnetic response in comparison to pure MgB2 phase. The highest critical current density (Jc) was 3.4 x 105 A/cm2 at 4.2 K and 0.5 T for the bulk material, and the irreversibility field for the Nd2O3 doped sample was located at high field at T = 30 K. The results indicate that inside grains of pinning centers increase Jc from 4.2 K to 25 K and decrease Jc at 30 K. On the other hand, pinning centers between grains (precipitate) increase Jc at 30 K and decrease Jc from 4.2 K to 25 K in the middle and high magnetic fields. Our research shows that carbon-encapsulated boron creates a point pinning centers

The new resistance jump: the detection of damage in Nb barrier in MgB2 wires

We present a new method for detecting damage to Nb barriers in MgB2 wires by using a four-contact probe. The transport measurements and scanning electron microscope images indicate that a newly identified jump in resistance means that there is damage to the Nb barrier. Damage detection is important for application because it allows us to avoid reactions between the filament and the sheath material, and to develop implementation methods for MgB2 with high critical current density. Our methods for damage detection proposed in this paper are simple, fast, and easy to use

Superconducting joints of reacted monofilament MgB2 wires sintered by hot uniaxial pressing system

Successful superconducting joints of reacted magnesium diboride (MgB2) monofilament wires are reported in this paper. The absence of a reliable method to develop superconducting joints between reacted MgB2 wires presents a major obstacle to the wider adoption of MgB2 as a material for magnet winding. A hot uniaxial pressing (HUP) system was exploited for sintering purposes since it can facilitate the formation of condensed in situ bulk on the wire filament. The wires were manufactured with an extra thick barrier material to protect the filament from damage during HUP sintering. The sintering temperature and pressure of the HUP system were varied to comprehend the best-performing joint. The performance of joints could be improved by depreciating the pores within the intermediate bulk of the joint. To prove this point, joints were cut to study their morphology. However, due to sintering in pressurised conditions, the reaction of the in situ intermediate bulk was not completed. The x-ray diffraction result detected a significant unreacted magnesium phase in the intermediate bulk. This work obtained joints of reacted MgB2 wires which can be considered for industrial MgB2 magnetic resonance imaging magnets fabrication.</p

Superior critical current density obtained in MgB2 bulks through low-cost carbon-encapsulated boron powder

The unavailability of high quality precursor is encouraging researchers to seek effective ways to fabricate magnesium diboride (MgB2) wire. Herein, cost-effective amorphous boron powder produced through a diborane (B2H6) gas process is investigated for the possibility of further industrial application. A thin carbon layer to encapsulate the boron particles is simultaneously deposited by pyrolysis of hydrocarbon. We found that the carbon-encapsulated amorphous boron has a high upper critical field due to impurity scattering, and thereby, enhanced high-field critical current density