245,567 research outputs found

    Strangeness Production in Chemically Non-Equilibrated Parton Plasma

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    Strangeness production was investigated during the equilibration of a gluon dominated parton plasma produced at RHIC and LHC energies. The time evolution of parton densities are followed by a set of rate equations in a 1-dimensional expanding system. The strangeness production will depend on the initial chemical equilibration level and in our case the parton densities will remain far from the full equilibrium. We investigate the influence of gluon fragmentation on final strangeness content.Comment: 12 pages (LaTeX) + 2 postscript figures (tarred, compressed, uuencoded) included. Review to appear in Proceedings of Strangeness'95, Tucson, Arizona, Jan. 4--6 1995. (American Institute of Physics

    Optical spectroscopy study of Nd(O,F)BiS2 single crystals

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    We present an optical spectroscopy study on F-substituted NdOBiS2_2 superconducting single crystals grown using KCl/LiCl flux method. The measurement reveals a simple metallic response with a relatively low screened plasma edge near 5000 \cm. The plasma frequency is estimated to be 2.1 eV, which is much smaller than the value expected from the first-principles calculations for an electron doping level of x=0.5, but very close to the value based on a doping level of 7%\% of itinerant electrons per Bi site as determined by ARPES experiment. The energy scales of the interband transitions are also well reproduced by the first-principles calculations. The results suggest an absence of correlation effect in the compound, which essentially rules out the exotic pairing mechanism for superconductivity or scenario based on the strong electronic correlation effect. The study also reveals that the system is far from a CDW instability as being widely discussed for a doping level of x=0.5.Comment: 5 pages, 5 figure

    Optical properties of TlNi2Se2: Observation of pseudogap formation

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    The quasi-two-dimensional nickel chalcogenides TlNi2Se2TlNi_2Se_2 is a newly discovered superconductor. We have performed optical spectroscopy study on TlNi2Se2TlNi_2Se_2 single crystals over a broad frequency range at various temperatures. The overall optical reflectance spectra are similar to those observed in its isostructure BaNi2As2BaNi_2As_2. Both the suppression in R(ω)R(\omega) and the peaklike feature in σ1(ω)\sigma_1(\omega) suggest the progressive formation of a pseudogap feature in the midinfrared range with decreasing temperatures, which might be originated from the dynamic local fluctuation of charge-density-wave (CDW) instability. We propose that the CDW instability in TlNi2Se2TlNi_2Se_2 is driven by the saddle points mechanism, due to the existence of van Hove singularity very close to the Fermi energy.Comment: 5 pages, 4 figure

    Phase fluctuations versus Gaussian fluctuations in optimally-doped YBa2_2Cu3_3O7_7

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    We analyze recent torque measurements of the magnetization MdM_d vs. magnetic field HH in optimally doped YBa2_2Cu3_3O7−y_{7-y} (OPT YBCO) to argue against a recent proposal by Rey et al that the magnetization results above TcT_c are consistent with Gaussian fluctuations. We find that, despite its strong interlayer coupling, OPT YBCO displays an anomalous non-monotonic dependence of MdM_d on HH which represents direct evidence for the locking of the pair wavefunction phase θn\theta_n at TcT_c and the subsequent unlocking by a relatively weak HH. These unusual features characterize the unusual nature of the transition to the Meissner state in cuprate superconductors. They are absent in low-TcT_c superconductors to our knowledge. We also stress the importance of the vortex liquid state, as well as the profiles of the melting field Hm(T)H_m(T) and the upper critical field curve Hc2(T)H_{c2}(T) in the TT-HH plane. Contrary to the claims of Rey et al, we show that the curves of the magnetization and the Nernst signal illustrate the inaccessibility of the Hc2H_{c2} line near TcT_c. The prediction of the Hc2H_{c2} line by Rey et al is shown to be invalid in OPT YBCO.Comment: 6 pages, 6 figure

    Optical spectroscopy study of the collapsed tetragonal phase of CaFe2_2(As0.935_{0.935}P0.065_{0.065})2_2 single crystals

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    We present an optical spectroscopy study on P-doped CaFe2_2As2_2 which experiences a structural phase transition from tetragonal to collapsed tetragonal (cT) phase near 75 K. The measurement reveals a sudden reduction of low frequency spectral weight and emergence of a new feature near 3200 \cm (0.4 eV) in optical conductivity across the transition, indicating an abrupt reconstruction of band structure. The appearance of new feature is related to the interband transition arising from the sinking of hole bands near Γ\Gamma point below Fermi level in the cT phase, as expected from the density function theory calculations in combination with the dynamical mean field theory. However, the reduction of Drude spectral weight is at variance with those calculations. The measurement also indicates an absence of the abnormal spectral weight transfer at high energy (near 0.5-0.7 eV) in the cT phase, suggesting a suppression of electron correlation effect.Comment: 6 pages, 4 figure
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