Jun 2010, Volume 5 Issue 2

Cover illustration

  • The Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory was the first one that consisted of two independent rings. It was designed to operate at high luminosity over a wide range of beam energies and with particle species ranging from polarized proton to heavy ions. From central gold-gold collisions at the top center-of-mass energy of 200 GeV per nucleon-nucleon pair, thousands of particles were produced (see cover figure). With several years of data taki [Detail] ...

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    Jie LIU (刘杰), Bin LIU (刘斌)

    In this article, we review our recent theoretical works on producing ultracold molecules from ultracold bosonic atoms via magnetically tunable Feshbach resonances. Our analysis relies on a two-channel quantum microscopic model that accounts for many-body effects in the association process. We show that the picture of two-body molecular production depicted by the Landau–Zener model is signifi-cantly altered due to many-body effects. We derive an analytic expression for molecular conversion efficiency for the nonadiabatic linearly swept Feshbach resonance, that explains the discrepancy between the prediction of the Landau–Zener formula and the experimental data. With including the thermal dephasing effects in the oscillating magnetic field modulation Feshbach resoance, we reproduce the Lorentzian resonance lineshape and explain the maximum conversion efficiency observed in experiment.

    Bao-sen SHI (史保森), Chang ZHAI (翟畅), Fu-yuan WANG (王福源), Guang-can GUO (郭光灿)

    Transferring a quantum state between a photon and a quantum memory is the key point for realizing a long-distance quantum communication, and is also a basic ingredient of linear optical quantum computation. In an atomic-based network, the efficient coupling between a photon and an atomic system is a prerequisite for realizing the transfer of information between them, which requires that the photon should have a comparable bandwidth with the natural bandwidth of an atom. Therefore, generating a narrow-band photon has become a very important topic in the quantum information field. One simple and efficient way is cavity-enhanced spontaneously parametric down-conversion. In this paper, we will review and introduce a series of experiments done in our group for realizing this goal. We believe these works are very useful for the research in this direction.

    Xian-hui CHEN (陈仙辉)
  • Feng-jie MA(马锋杰), Zhong-yi LU(卢仲毅), Tao XIANG(向涛)
    We have studied the electronic and magnetic structures of the ternary iron arsenides AFe2As2 (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As 4p orbitals. The correction from the spin–orbit interaction to the electronic band structure is given. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based on the calculations, we propose that the low energy dynamics of these materials can be described effectively by a t−JH−J1−J2-type model [arXiv: 0806.3526v2, 2008].
  • Research articles
    Shi-liang LI (李世亮), Dao-xin YAO (姚道新), Yi-ming QIU (邱义铭), Hye Jung KANG, E. W. CARLSON, Jiang-ping HU (胡江平), Gen-fu CHEN (陈根富), Nan-lin WANG (王楠林), Peng-cheng DAI (戴鹏程),
    We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO0.84F0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of "Graphic". On warming to above "Graphic", these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature "Graphic". For superconducting CeFeAsO0.84F0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce–Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.
  • Research articles
    Dao-xin YAO (姚道新), E. W. CARLSON,
    We study the magnetic excitations of undoped iron oxypnictides using a three-dimensional Heisenberg model with single-ion anisotropy. Analytic forms of the spin-wave dispersion, velocities, and structure factor are given. Aside from quantitative comparisons that can be made to inelastic neutron scattering experiments, we also give qualitative criteria that can distinguish various regimens of coupling strength. The magnetization reduction due to quantum zero point fluctuations shows clear dependence on the c-axis coupling.
  • Research articles
    Mike GUIDRY, Yang SUN (孙扬), Cheng-li WU (吴成礼),
    Copper oxides become superconductors rapidly upon doping with electron holes, suggesting a fundamental pairing instability. The Cooper mechanism explains normal superconductivity as an instability of a fermi-liquid state, but high-temperature superconductors derive from a Mott-insulator normal state, not a fermi liquid. We show that precocity to pair condensation with doping is a natural property of competing antiferromagnetism and d-wave superconductivity on a singly-occupied lattice, thus generalizing the Cooper instability to doped Mott insulators, with significant implications for the high-temperature superconducting mechanism.
    Er-jia GUO, Hui-bin Lü, Kui-juan JIN, Guo-zhen YANG

    Perovskite oxides and heterojunctions have attracted much attention due to their multifunctional properties of electricity and optics and magnetic as well as the very good chemical and thermal stability. In this brief review, we describe the novel progress of researches in the optical characteristic, including ultrafast photoelectric effects of picosecond order in perovskite oxide single crystals, thin-films and heterojunctions, high-sensitive photovoltages, the enhanced transient lateral photovoltages in perovskite oxide thin-films and heterojunctions, and the high-sensitive ultraviolet (UV) photodetectors based on perovskite oxides. The recent advances present in this paper not only could stimulate theoretical studies on the mechanism but also would open up the possibilities in the developments of optoelectronic devices based on perovskite oxides and heterojunctions.

  • Research articles
    Guang-cun SHAN (单光存), Wei HUANG (黄维),
    The fantastic physical properties of single-walled silicon nanotubes (SWSiNTs) under mechanical strain make them promising materials for fabricating nanoscale electronic devices or transducers. Here we investigate the energy band and band-gap properties of the SWSiNTs calculated from the tight-binding model approximation. The results show that the band-gap properties are very sensitive to the deformation degree and the helicity of the SWSiNTs. The results can be employed to guide the design of nanoelectronic devices based on silicon nanotubes.
  • Research articles
    Hoi Yin SHIK (石海燕), Ru CHEN (陈茹),
    A complete dimerized state exists for one kind of two-leg spin half ladders, which has local antiferromagnetic ordering and frustration effect at the same time. The system’s low-lying excitations can be obtained exactly which enables us to calculate thermodynamic quantities such as specific heat and magnetic susceptibility at low temperatures. Our results also show that the subset energy spectrum is a good approximation to the whole spectrum even for the usual two-leg spin half ladder without frustration.
    Bin-bin Lü(吕彬彬), Yan-ping DENG(邓艳萍), Qiang TIAN (田强)

    We study a one-dimensional Sine–Gordon lattice of anharmonic oscillators with cubic and quartic nearest-neighbor interactions, in which discrete breathers can be explicitly constructed by an exact separation of their time and space dependence. DBs can stably exist in the one-dimensional Sine–Gordon lattice no matter whether the nonlinear interaction is cubic or quartic. When a parametric driving term is introduced in the factor multiplying the harmonic part of the on-site potential of the system, we can obtain the stable quasiperiodic discrete breathers and chaotic discrete breathers by changing the amplitude of the driver.

    Li-juan RUAN(阮丽娟)

    The results from data taken during the last several years at the Relativistic Heavy-Ion Collider (RHIC) will be reviewed in the paper. Several selected topics that further our understanding of constituent quark scaling, jet quenching and color screening effect of heavy quarkonia in the hot dense medium will be presented. Detector upgrades will further probe the properties of Quark Gluon Plasma. Future measurements with upgraded detectors will be presented. The discovery perspectives from future measurements will also be discussed.

    Ji-huan PAN(潘继环), Li-juan ZHOU(周丽娟), Wei-xing MA(马维兴)

    Based on our previous study of the QCD inspired eikonalized model for describing vector meson photoproduction, pp, and p ˉp elastic scattering at high energies, we apply the mode to high energy K±p elastic scattering. The total cross section σtot(s), differential cross section dσ/dt, the ratio of the real part to imaginary part of the forward scattering amplitude ρ(s), and nuclear slope parameter function β(s) are calculated in the model. Our results show that the theoretical prediction for σtot(s) is in a good agreement with the experimental data within error bars of the data. For the other theoretical predictions there are no data to test the predictive power of the model. We need the corresponding experimental data to examinate the validity of our QCD inspired eikonalized model. However, our calculations clearly show that the Odderon exchange in the process makes a significant contribution to the observable of ρ(s) and β(s). Therefore, we may conclude that there is a good opportunity to find the QCD Odderon in the K±p elastic scattering at high energies.