Search Results (575 total)

Based on 58×10⁶ J/ψ events collected with the BESII detector at the Beijing Electron-Positron Collider, the baryon pair processes J/ψ→Σ⁺Σ̅ ^- and J/ψ→Ξ⁰Ξ̅ ⁰ are observed for the first time. The branching fractions are measured to be B(J/ψ→Σ⁺Σ̅ ^-)=(1.50±0.10±0.22)×10^-3 and B(J/ψ→Ξ⁰Ξ̅ ⁰)=(1.20±0.12±0.21)×10^-3, where the first errors are statistical and the second ones are systematic.

We report the first observation of the strong effect of quantum rotational wave packets in atmospheric air on the long-range filamentary propagation of intense femtosecond laser pulses. In a pump-probe experiment, we find that the probe filament can be sucked into the pump filament’s molecular quantum wake and trapped or be destroyed by it.

We study the packing of colloidal microspheres mixed with polymers in oil-in-water emulsion droplets by evaporation. The addition of polymers produces non-unique configurations of final clusters when the number of particles N inside the droplet is larger than 4. The cluster configurations are classified into three categories based on symmetry. Stablized colloidal clusters of spherical packings are observed. Our observations on packing process suggest the mechanisms which cause different and nonunique structures. The osmotic pressure and the interparticle interaction due to polymers changes the force balance between microspheres and result in different structures.

We present the first measurements of the ρ(770)⁰,K^*(892),Δ(1232)⁺⁺,Σ(1385), and Λ(1520) resonances in d+Au collisions at sqrt[s_NN]=200 GeV, reconstructed via their hadronic decay channels using the STAR detector (the solenoidal tracker at the BNL Relativistic Heavy Ion Collider). The masses and widths of these resonances are studied as a function of transverse momentum p_T. We observe that the resonance spectra follow a generalized scaling law with the transverse mass m_T. The 〈p_T〉 of resonances in minimum bias collisions are compared with the 〈p_T〉 of π,K, and p̅ . The ρ⁰/π^-,K^*/K^-,Δ⁺⁺/p,Σ(1385)/Λ, and Λ(1520)/Λ ratios in d+Au collisions are compared with the measurements in minimum bias p+p interactions, where we observe that both measurements are comparable. The nuclear modification factors (R_dAu) of the ρ⁰,K^*, and Σ^* scale with the number of binary collisions (N_bin) for p_T> 1.2 GeV/c.

Raman scattering was performed on Ge_xSi_1−x (x=0.54 or 0.28) alloy nanocrystals embedded in amorphous Si oxide. An asymmetric, depolarized, and size-dependent low-frequency Raman peak was observed and identified as the superposition of two surface acoustic vibration modes of the alloy nanocrystals. The current theoretical models can be used to explain the mode frequencies but not the dampings observed experimentally. Based on energy-dispersive x-ray microanalysis and density-functional-theory total energy optimization of structures, a modified core-shell-matrix model in which the effects of neighboring nanocrystals in the matrix are taken into account is in good agreement with experiments. This work provides good insight into the frequencies and dampings of acoustic vibrations of the nanocrystals embedded in the matrix.

We reduce the dimensionless interaction strength α in graphene by adding a water overlayer in ultrahigh vacuum, thereby increasing dielectric screening. The mobility limited by long-range impurity scattering is increased over 30%, due to the background dielectric constant enhancement leading to a reduced interaction of electrons with charged impurities. However, the carrier-density-independent conductivity due to short-range impurities is decreased by almost 40%, due to reduced screening of the impurity potential by conduction electrons. The minimum conductivity is nearly unchanged, due to canceling contributions from the electron-hole puddle density and long-range impurity mobility. Experimental data are compared with theoretical predictions with excellent agreement.

We systematically study Raman spectroscopy of cleaved Na_xCoO₂ single crystals with 0.37≤x≤0.80. The Raman shift of A_1g mode is found to be linearly dependent on Na content, while the Raman shift of E_1g mode has an abnormal shift to high frequency around x=0.5. The abnormal shift is ascribed to the occurrence of Na rearrangement in O1 structure. Temperature-dependent Raman spectrum for x=0.56 sample shows that Na rearrangement transition from O1 structure to H1 structure occurs around 240 K. Electronic transport and susceptibility for the sample with x=0.56 show a response to the Na rearrangement transition from O1 to H1 structure and that different Na ordering pattern causes distinct physical properties. These results give a direct evidence to prove Na ordering effect on physical properties of Co-O plane.

A cone-shaped cloak whose cross section gradually increases along the axial direction (z direction) is proposed in this paper. We present full wave analysis of this cloak in response to electromagnetic waves, showing that a perfect conical cloak can support the propagation of any kind of fields. In addition, the reduced set of cloaking parameters is derived for azimuthally invariant (∂/∂φ=0) incident fields. The advantage of this simplified cloaking structure is that all the components of material parameters are spatially invariant with relative magnitude larger than one. Hence, conical cloak with small scattering is physically realizable within a wide band of frequency for this specific type of incident fields. Finally, we apply similar transformation to achieve a polarization rotator which can arbitrarily control the polarization of the electromagnetic wave getting through. The proposed design provides a practical way to realize invisible cloak and some other electromagnetic devices, especially in the conditions that the source distribution is rotationally symmetric.

We synthesized single phase La_0.85Sr_0.15FeAsO_1−δ samples and systematically studied the effect of oxygen deficiency in this system. It is found that partial substitution of Sr for La induces the hole carrier evidenced by positive thermoelectric power (TEP) but no bulk superconductivity is observed. The superconductivity can be realized by annealing the as-grown sample in vacuum to produce the oxygen deficiency. With increasing oxygen deficiency, the superconducting transition temperature (T_c) increases and the maximum T_c reaches 26 K—the same as that observed in the LaFeAsO_1−xF_x system. TEP changes the sign from positive for the nonsuperconducting as-grown sample to negative for the superconducting samples with oxygen deficiency, while R_H keeps negative for all samples. It suggests that the dominated carrier in La_0.85Sr_0.15FeAsO_1−δ is electronlike.

We observe an obvious anomalous line shape of the e⁺e^-→ hadrons total cross sections in the energy region between 3.700 and 3.872 GeV. It is inconsistent with the explanation for only one simple ψ(3770) resonance with a statistical significance of 7σ. The anomalous line shape may be explained by two possible enhancements of the inclusive hadron production near the center-of-mass energies of 3.764 and 3.779 GeV, indicating that either there is likely a new structure in addition to the ψ(3770) resonance around 3.773 GeV, or there are some physics effects reflecting the DD̅ production dynamics.

We report the observation of a strong coupling between an artificial “atom” and localized interface mode in the microwave regime. Transmittance is experimentally measured for the effective near-zero-index paired structures containing ε-negative and μ-negative materials made of composite right- and left-handed transmission lines. When the atom is embedded in the interface, because of the strong coupling between the atom and the interface mode, a Rabi splitting of 0.12  GHz is observed, which is in good agreement with numerical simulations. Different from the usual Rabi splitting observed in conventional cavities, the splitting modes in the effective zero-index media are invariant with the scaling change of the length.

We show that the optical properties of an oblique layered system with two kinds of isotropic materials can be described using the concept of transformation media, as long as the thickness of the layers is much smaller than the wavelength. Once the connection with transformation media is established, we then show that oblique layered system can serve as a universal element to build a variety of interesting functional optical components such as wave splitters, wave combiners, one-dimensional cloaking devices, and reflectionless field rotators.

We study the details of the U(3)–O(4) quantum phase transition in the U(4) vibron model. Both asymptotic analysis in the classical limit and rigorous calculations for finite boson number systems indicate that a second-order phase transition is still there even for the systems with boson number N ranging from tens to hundreds. Two kinds of effective order parameters, including E1 transition ratios B(E1:2₁→1₁)/B(E1:1₁→0₁) and B(E1:0₂→1₁)/B(E1:1₁→0₁), and the energy ratios E₂₁/E₀₂ and E₃₁/E₀₂ are proposed to identify the second-order phase transition in experiments. We also found that the critical point of phase transition can be approximately described by the E(3) symmetry, which persists even for moderate N~10 protected by the scaling behaviors of quantities at the critical point. In addition, a possible empirical example exhibiting roughly the E(3) symmetry is discussed.

A comparative definition for community in networks is proposed, and the corresponding detecting algorithm is given. A community is defined as a set of nodes, which satisfies the requirement that each node’s degree inside the community should not be smaller than the node’s degree toward any other community. In the algorithm, the attractive force of a community to a node is defined as the connections between them. Then employing an attractive-force-based self-organizing process, without any extra parameter, the best communities can be detected. Several artificial and real-world networks, including the Zachary karate club, college football, and large scientific collaboration networks, are analyzed. The algorithm works well in detecting communities, and it also gives a nice description of network division and group formation.

We report the detailed phase diagram and anomalous transport properties of Fe-based high-T_c superconductors SmFeAsO_1-xF_x. It is found that superconductivity emerges at x∼0.07, and optimal doping takes place in the x∼0.20 sample with the highest T_c∼54  K. T_c increases monotonically with doping; the anomaly in resistivity from structural phase or spin-density-wave order is rapidly suppressed, suggesting a quantum critical point around x∼0.14. As manifestations, a linear temperature dependence of the resistivity shows up at high temperatures in the x<0.14 regime but at low temperatures just above T_c in the x>0.14 regime; a drop in carrier density evidenced by a pronounced rise in the Hall coefficient is observed below the temperature of the anomaly peak in resistivity. A scaling behavior is observed between the Hall angle and temperature: cot⁡θ_H∝T^1.5 for all samples with different x in SmFeAsO_1-xF_x system.

Optical Tamm states, a kind of interface modes, are also called Tamm plasmon-polaritons. They are experimentally observed in photonic heterostructures based on microstrip transmission lines. The position of optical Tamm states can be designed exactly under effective impedance match and effective phase shift match conditions. Our results show that the photonic band gaps can have the effect of negative-permittivity or negative-permeability media in constructing the interface modes. The simulations and experimental results agree with each other quite well.

We demonstrate some interesting phenomena associated with a nonmonochromatic plane wave passing through a spherical invisibility cloak whose radial permittivity and permeability are of Drude and Lorentz types. We observe that the frequency center of a quasimonochromatic incident wave will suffer a blueshift in the forward scattering direction. Different frequency components have different depths of penetration, causing a rainbowlike effect within the cloak. The concept of group velocity at the inner boundary of the cloak needs to be revisited. Extremely low scattering can still be achieved within a narrow band.

We have performed extensive and systematic ab initio calculations to substantiate a recently proposed generalized electron counting (GEC) rule that governs the rich patterns of compound semiconductor reconstruction induced by metal adsorption. In this rule, the metal adsorbates serve as an electron bath, either donating or accepting the right number of electrons, with which the binary host system chooses a specific reconstruction under the classic electron counting rule and, meanwhile, the adsorbates stay in their optimal valency. The GEC rule is applied to different GaAs surfaces deposited by various classes of metal adsorbates, leading to a number of possible reconstructions, which can be further confirmed by first-principles calculations and/or experiments. The alkali-metal adsorption on the GaAs(110) surface up to the saturate coverage is a perfect example of the GEC rule. The application of the GEC rule to the prototype system of Mn/GaAs(001) not only predicts possible reconstruction patterns over a wide range of coverage but also provides an underlying link between the reconstruction structures and the local magnetic moments of the metal adsorbates. For Au/GaAs(100), we demonstrate the application of the GEC rule to those systems where metal adsorbates form covalent bonds with the substrate. The GEC rule, as a generic principle, is expected to be applicable to more metal-adsorbed compound semiconductor surfaces.

Using an ultrafast pulse of mega-electron-volt energy protons accelerated from a laser-irradiated foil, we have heated solid density aluminum plasmas to temperatures in excess of 15 eV. By measuring the temperature and the expansion rate of the heated Al plasma simultaneously and with picosecond time resolution we have found the predictions of the SESAME Livermore equation-of-state (LEOS) tables to be accurate to within 18%, in this dense plasma regime, where there have been few previous experimental measurements.

Using a 492  fb^-1 data sample collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider, we observe the decay B⁰→pp̅ K^*0 with a branching fraction of (1.18_-0.25^+0.29(stat)±0.11(syst))×10^-6. We study the decay dynamics of B⁰→pp̅ K^*0 and compare with B⁺→pp̅ K^*+. The K^*0 meson is found to be almost 100% polarized (with a fraction of (101±13±3)% in the helicity zero state), while the K^*+ meson has a (32±17±9)% fraction in the helicity zero state. The direct CP asymmetries for B⁰→pp̅ K^*0 and B⁺→pp̅ K^*+ are measured to be -0.08±0.20±0.02 and -0.01±0.19±0.02, respectively. In addition, we report improved measurements of the branching fractions B(B⁺→pp̅ K^*+)=(3.38_-0.60^+0.73±0.39)×10^-6 and B(B⁰→pp̅ K⁰)=(2.51_-0.29^+0.35±0.21)×10^-6, which supersede our previous measurements.

Measurements of the L-shell emission of highly charged gold ions were made under controlled laboratory conditions using the SuperEBIT electron beam ion trap, allowing detailed spectral observations of lines from Fe-like Au⁵³⁺ through Ne-like Au⁶⁹⁺. Using atomic data from the Flexible Atomic Code, we have identified strong 3d_5∕2→2p_3∕2 emission features that can be used to diagnose the charge state distribution in high energy density plasmas, such as those found in the laser entrance hole of hot hohlraum radiation sources. We provide collisional-radiative calculations of the average ion charge ⟨Z⟩ as a function of temperature and density, which can be used to relate charge state distributions inferred from 3d_5∕2→2p_3∕2 emission features to plasma conditions, and investigate the effects of plasma density on calculated L-shell Au emission spectra.

We designed chiral phononic crystals and studied their properties by using the layer multiple scattering method. The transmittance curves and the corresponding band structures show that this kind of structure possesses significant polarization gaps. The chiral structures break the symmetry so that the degenerate transverse modes split into a pair of right-hand polarized mode and left-hand polarized mode. The polarization splitting in the low-frequency range is enhanced in systems of large filling ratios. We also demonstrate that chiral structures containing strongly resonant units can induce negative group velocity in elastic waves, and stronger resonance brings a wider band of negative group velocity.

We report a new STAR measurement of the longitudinal double-spin asymmetry A_LL for inclusive jet production at midrapidity in polarized p+p collisions at a center-of-mass energy of sqrt[s]=200  GeV. The data, which cover jet transverse momenta 5<p_T<30  GeV/c, are substantially more precise than previous measurements. They provide significant new constraints on the gluon spin contribution to the nucleon spin through the comparison to predictions derived from one global fit to polarized deep-inelastic scattering measurements.

We present the first spin alignment measurements for the K^*0(892) and ϕ(1020) vector mesons produced at midrapidity with transverse momenta up to 5 GeV/c at sqrt[s_NN]=200 GeV at RHIC. The diagonal spin-density matrix elements with respect to the reaction plane in Au+Au collisions are ρ₀₀=0.32±0.04 (stat) ± 0.09 (syst) for the K^*0 (0.8<p_T<5.0 GeV/c) and ρ₀₀=0.34±0.02 (stat) ± 0.03 (syst) for the ϕ (0.4<p_T<5.0 GeV/c) and are constant with transverse momentum and collision centrality. The data are consistent with the unpolarized expectation of 1/3 and thus no evidence is found for the transfer of the orbital angular momentum of the colliding system to the vector-meson spins. Spin alignments for K^*0 and ϕ in Au+Au collisions were also measured with respect to the particle's production plane. The ϕ result, ρ₀₀=0.41±0.02 (stat) ± 0.04 (syst), is consistent with that in p+p collisions, ρ₀₀=0.39±0.03 (stat) ± 0.06 (syst), also measured in this work. The measurements thus constrain the possible size of polarization phenomena in the production dynamics of vector mesons.

The geometrical evolution, electronic structures, and magnetic properties of small binary clusters Bi_nMn_m (n≤13, m≤6) are investigated using density-functional theory (DFT) with the view of explaining the experimentally observed magnetic moments in these systems. The results demonstrate that the magnetic moments of the dopant Mn atoms exhibit a weak dependence on the structure, composition, and environment, holding a general constant of 4μ_B/atom approximately, whereas the magnetic couplings among these Mn atoms are strongly dependent. We propose that the preferred ferromagnetic couplings in the dopant Mn_m components result in pronounced magnetic moments represented by certain combinations in which the Bi to Mn ratio is close to 2. Moreover, a faint antiferromagnetic perturbation is provided by the Bi_n units. The hybridization among Mn 3d, Mn 4s, and Bi 6p induces −0.1μ_B on Bi atoms and reduces the Mn atomic moments by 1μ_B. On the whole, the calculated magnetic trends of the different composition series qualitatively fit well with experimental measurements. The lowest-energy structures of the clusters are segregated cases in which the dopant Mn_m components assemble together, forming a pentagonal bipyramid shape and surrounded by irregular Bi_n components. Generally, an amorphous configuration is observed for low-Mn-concentration clusters, but the evolution of a defective polyicosahedron pattern with a Mn core shell is favored for high-Mn-concentration clusters and this tendency will keep the most stable geometrical structure for larger sizes. By analyzing the binding energies and the second-order energy differences, we find that the monatomic doping Bi_nMn series containing n=4, 6, 10, and 12 is more stable than its neighboring sizes. Furthermore, the highest occupied and lowest unoccupied molecular orbital gaps decrease as a function of Mn concentration in the clusters, indicating an enhancement of metallicity.