Search Results (732 total)

A wide variety of real-life networks share two remarkable generic topological properties: scale-free behavior and modular organization, and it is natural and important to study how these two features affect the dynamical processes taking place on such networks. In this paper, we investigate a simple stochastic process—trapping problem, a random walk with a perfect trap fixed at a given location, performed on a family of hierarchical networks that exhibit simultaneously striking scale-free and modular structure. We focus on a particular case with the immobile trap positioned at the hub node having the largest degree. Using a method based on generating functions, we determine explicitly the mean first-passage time (MFPT) for the trapping problem, which is the mean of the node-to-trap first-passage time over the entire network. The exact expression for the MFPT is calculated through the recurrence relations derived from the special construction of the hierarchical networks. The obtained rigorous formula corroborated by extensive direct numerical calculations exhibits that the MFPT grows algebraically with the network order. Concretely, the MFPT increases as a power-law function of the number of nodes with the exponent much less than 1. We demonstrate that the hierarchical networks under consideration have more efficient structure for transport by diffusion in contrast with other analytically soluble media including some previously studied scale-free networks. We argue that the scale-free and modular topologies are responsible for the high efficiency of the trapping process on the hierarchical networks.

We report measurements of the exclusive cross section for e⁺e^-→D⁰D^*-π⁺ as a function of center-of-mass energy from the D⁰D^*-π⁺ threshold to 5.2 GeV with initial-state radiation. No evidence is found for Y(4260)→D⁰D^*-π⁺ decays. The analysis is based on a data sample collected with the Belle detector at or near a center-of-mass energy of 10.58 GeV with an integrated luminosity of 695  fb^-1 at the KEKB asymmetric-energy e⁺e^- collider.

We predict a magnetoelectric (ME) effect in graphene nanoribbons on silicon substrates by first-principles calculations. It is shown that a bias voltage can produce strong linear ME effect by driving charge transfer between the nanoribbons and substrate, thus tuning the exchange splitting of magnetic edge states; moreover, the bias induced n-to-p-type transition in the ribbon layer can switch the ME coefficient from negative to positive due to the unique symmetry of band structures. This mechanism is proven to be robust against variations in material and physical configurations, thus opening a new avenue for ME coupling in metal-free magnet systems of practical importance.

Using spin-polarized density-functional theory calculations, we investigate the competition between charge and spin orderings in dangling-bond (DB) wires of increasing lengths fabricated on an H-terminated Si(001) surface. For wires containing less than ten DBs as studied in recent experiments, we find antiferromagnetic (AF) ordering to be energetically much more favorable than charge ordering. The energy preference of AF ordering shrinks in an oscillatory way as the wire length increases and preserves its sign even for DB wires of infinite length. The oscillatory behavior can be attributed to quantum size effects as the DB electrons fill discrete quantum levels. The predicted AF ordering is in startling contrast with the prevailing picture of charge ordering due to Jahn-Teller distortion or Peierls instability for wires of finite or infinite lengths, respectively.

The STAR Collaboration at the Relativistic Heavy Ion Collider presents measurements of J/ψ→e⁺e^- at midrapidity and high transverse momentum (p_T>5 GeV/c) in p+p and central Cu+Cu collisions at sqrt[s_NN]=200 GeV. The inclusive J/ψ production cross section for Cu+Cu collisions is found to be consistent at high p_T with the binary collision-scaled cross section for p+p collisions. At a confidence level of 97%, this is in contrast to a suppression of J/ψ production observed at lower p_T. Azimuthal correlations of J/ψ with charged hadrons in p+p collisions provide an estimate of the contribution of B-hadron decays to J/ψ production of 13%±5%.

The results of midrapidity (0<y<0.8) neutral pion spectra over an extended transverse momentum range (1<p_T<12 GeV/c) in sqrt[s_NN]=200 GeV Au+Au collisions, measured by the STAR experiment, are presented. The neutral pions are reconstructed from photons measured either by the STAR Barrel Electro-Magnetic Calorimeter or by the Time Projection Chamber via tracking of conversion electron-positron pairs. Our measurements are compared to previously published π^± and π⁰ results. The nuclear modification factors R_CP and R_AA of π⁰ are also presented as a function of p_T. In the most central Au+Au collisions, the binary collision scaled π⁰ yield at high p_T is suppressed by a factor of about 5 compared to the expectation from the yield of p+p collisions. Such a large suppression is in agreement with previous observations for light quark mesons and is consistent with the scenario that partons suffer considerable energy loss in the dense medium formed in central nucleus-nucleus collisions at the Relativistic Heavy Ion Collider.

Forward-backward multiplicity correlation strengths have been measured with the STAR detector for Au+Au and p+p collisions at sqrt[s_NN]=200  GeV. Strong short- and long-range correlations (LRC) are seen in central Au+Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short-range correlations are observed in peripheral Au+Au collisions. Both the dual parton model (DPM) and the color glass condensate (CGC) predict the existence of the long-range correlations. In the DPM, the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC, longitudinal color flux tubes generate the LRC. The data are in qualitative agreement with the predictions of the DPM and indicate the presence of multiple parton interactions.

We study B→K^(*)ℓ⁺ℓ^- decays (ℓ=e, μ) based on a data sample of 657×10⁶ BB̅ pairs collected with the Belle detector at the KEKB e⁺e^- collider. We report the differential branching fraction, isospin asymmetry, K^* polarization, and the forward-backward asymmetry (A_FB) as functions of q²=M_ℓℓ²c². The fitted A_FB spectrum exceeds the standard model expectation by 2.7 standard deviations. The measured branching fractions are B(B→K^*ℓ⁺ℓ^-)=(10.7_-1.0^+1.1±0.9)×10^-7 and B(B→Kℓ⁺ℓ^-)=(4.8_-0.4^+0.5±0.3)×10^-7, where the first errors are statistical and the second are systematic, with the muon to electron ratios R_K^*=0.83±0.17±0.08 and R_K=1.03±0.19±0.06.

We have studied the electronic structure of the two-dimensional Heisenberg antiferromagnet VOCl using photoemission spectroscopy and density-functional theory including local Coulomb repulsion. From calculated exchange integrals and the observed energy dispersions we argue that the degree of one dimensionality regarding both the magnetic and electronic properties is noticeably reduced compared to the isostructural compounds TiOCl and TiOBr. Also, our analysis provides conclusive justification to classify VOCl as a multiorbital Mott insulator. In contrast to the titanium-based compounds density-functional theory here gives a better description of the electronic structure. However, a quantitative account of the low-energy features and detailed line shapes calls for further investigations including dynamical and spatial correlations.

We report ab initio calculations of magnetic and spin-polarized quantum transport properties of pure and nitrogen-doped carbon atomic wires. For finite-sized wires with even number of carbon atoms, total magnetic moment of 2μ_B is found. On the other hand, wires with odd number atoms have no net magnetic moment. Doped with one or two nitrogen atom(s), the carbon atomic wires exhibit a spin-density-wave-like state. The magnetic properties can be rationalized through bonding patterns and unpaired states. When the wire is sandwiched between Au electrodes to form a transport junction, perfect spin filtering effect can be induced by slightly straining the wire.

We present a measurement of the D⁰-D̅ ⁰ mixing parameter y_CP using a flavor-untagged sample of D⁰→K_S⁰K⁺K^- decays. The measurement is based on a 673  fb^-1 data sample recorded with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. Using a method based on measuring the mean decay time for different K⁺K^- invariant mass intervals, we find y_CP=(+0.11±0.61(stat.)±0.52(syst.))%.

Within the framework of the particle-number conserving (PNC) formalism, one-quasiparticle and low-lying high-K pair-broken (multiquasiparticle) bands systematically observed in Hf and Lu isotopes (170≤A≤178) are analyzed consistently. The PNC method deals with the cranked shell model with pairing interaction, in which the Pauli blocking effects are exactly accounted for, and the pairing interaction strength is determined by the experimental odd-even difference in binding energies. With an appropriate Nilsson level scheme that best fits the experimental bandhead energies of the one-quasiparticle bands, the experimental moments of inertia (MOIs) of these one-quasiparticle and multiquasiparticle bands (including configuration and frequency dependences, signature splitting, etc.) can be well reproduced without any additional free parameter. In most cases, the PNC formalism supports the configuration assignments in earlier works. the PNC calculation also reveals that the experimental systematics of low-lying high-K pair-broken bands in Hf and Lu isotopes are intimately related to the subshell effects near the Fermi surfaces of both protons and neutrons.

Using a sample of 58 million J/ψ events collected with the BESII detector at the BEPC, more than 100 000 J/ψ→pp̅ π⁰ events are selected, and a detailed partial wave analysis is performed. The branching fraction is determined to be Br(J/ψ→pp̅ π⁰)=(1.33±0.02±0.11)×10^-3. A long-sought missing N^*, first observed in J/ψ→pn̅ π^-, is observed in this decay too, with mass and width of 2040_-4⁺³±25   MeV/c² and 230_-8⁺⁸±52   MeV/c², respectively. Its spin-parity favors 3 / 2⁺. The masses, widths, and spin parities of other N^* states are obtained as well.

Bidisperse colloidal suspensions of temperature-sensitive microgel spheres were quenched from liquid to glass states by a rapid temperature drop, and then the glass was permitted to age. Irreversible rearrangements, events that dramatically change a particle’s local environment, were observed to be closely related to dynamic heterogeneity. The rate of these irreversible events decreased during aging and the the number of particles required to move as part of these irreversible rearrangements increased. Thus, the slowing dynamics of aging were governed by growing, correlated domains of particles. Additionally, short-range order developed, and a spatial decay length scale associated with orientational order was found to grow during aging.

We report measurements of charmless hadronic B⁰ decays into the π⁺π^-K⁺π^- final state. The analysis uses a sample of 657×10⁶ BB̅ pairs collected with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider at the resonance. The decay B⁰→ρ⁰K⁺π^- is observed for the first time; the significance is 5.0σ and the corresponding partial branching fraction for M_Kπ∈(0.75,1.20)  GeV/c² is [2.8±0.5(stat)±0.5(syst)]×10^-6. We also obtain the first evidence for B⁰→f₀(980)K⁺π^- with 3.5σ significance and for B⁰→π⁺π^-K^*0 with 4.5σ significance. For the two-body decays B⁰→ρ⁰K^*0 and B⁰→f₀(980)K^*0, the significances are 2.7σ and 2.5σ, respectively, and the upper limits on the branching fractions are 3.4×10^-6 and 2.2×10^-6 at 90% confidence level.

We report K/π fluctuations from Au+Au collisions at sqrt[s_NN]=19.6, 62.4, 130, and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. K/π fluctuations in central collisions show little dependence on incident energy and are on the same order as those from NA49 at the Super Proton Synchrotron in central Pb+Pb collisions at sqrt[s_NN]=12.3 and 17.3 GeV. We report results for the collision centrality dependence of K/π fluctuations and results for charge-separated fluctuations. We observe that the K/π fluctuations scale with the charged particle multiplicity density.

We present a systematic analysis of two-pion interferometry in Au+Au collisions at sqrt[s_NN]=62.4 GeV and Cu+Cu collisions at sqrt[s_NN]=62.4 and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The multiplicity and transverse momentum dependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.

The differential cross section for the process γγ→ηπ⁰ has been measured in the kinematic range 0.84  GeV<W<4.0  GeV, |cos⁡θ^*|<0.8, where W and θ^* are the energy and π⁰ (or η) scattering angle, respectively, in the γγ center-of-mass system. The results are based on a 223  fb^-1 data sample collected with the Belle detector at the KEKB e⁺e^- collider. Clear peaks due to the a₀(980) and a₂(1320) are visible. The differential cross sections are fitted in the energy region 0.9  GeV<W<1.46  GeV to obtain the parameters of the a₀(980). Its mass, width and Γ_γγB(ηπ⁰) are measured to be 982.3_-0.7-4.7^+0.6+3.1  MeV/c², 75.6±1.6_-10.0^+17.4  MeV and 128_-2-43⁺³⁺⁵⁰²  eV, respectively. The energy and angular dependences above 3.1 GeV are compared with those measured in the π⁰π⁰ channel. The integrated cross section over |cos⁡θ^*|<0.8 has a W^-n dependence with n=10.5±1.2±0.5, which is slightly larger than that for π⁰π⁰. The differential cross sections show a sin⁡^-4θ^* dependence similar to γγ→π⁰π⁰. The measured cross section ratio, σ(ηπ⁰)/σ(π⁰π⁰)=0.48±0.05±0.04, is consistent with a QCD-based prediction.

From a Dalitz plot analysis of B→Kπ⁺ψ^′ decays, we find a signal for Z(4430)⁺→π⁺ψ^′ with a mass M=(4443_-12-13⁺¹⁵⁺¹⁹)  MeV/c², width Γ=(107_-43-56⁺⁸⁶⁺⁷⁴)  MeV, product branching fraction B(B̅ ⁰→K^-Z(4430)⁺)×B(Z(4430)⁺→π⁺ψ^′)=(3.2_-0.9-1.6^+1.8+5.3)×10^-5, and significance of 6.4σ that agrees with previous Belle measurements based on the same data sample. In addition, we determine the branching fraction B(B⁰→K^*(892)⁰ψ^′)=(5.52_-0.32-0.58^+0.35+0.53)×10^-4 and the fraction of K^*(892)⁰ mesons that are longitudinally polarized f_L=(44.8_-2.7-5.3^+4.0+4.0)%. These results are obtained from a 605  fb^-1 data sample that contains 657×10⁶ BB̅ pairs collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider.

The cross sections for e⁺e^-→ϕπ⁺π^- and e⁺e^-→ϕf₀(980) are measured from threshold to sqrt[s]=3.0  GeV using initial-state radiation. The analysis is based on a data sample of 673  fb^-1 collected on and below the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. First measurements are reported for the resonance parameters of the ϕ(1680) in the ϕπ⁺π^- mode: m=(1689±7±10)  MeV/c² and Γ=(211±14±19)  MeV/c². A structure at sqrt[s]=2.1  GeV/c², corresponding to the so called Y(2175), is observed; its mass and width are determined to be (2079±13_-28⁺⁷⁹)  MeV/c² and (192±23_-61⁺²⁵)  MeV/c², respectively.

We report the realization of superconductivity by 5d element Ir doping in LaFeAsO, a prototype parent compound of high-temperature iron-based superconductors. X-ray diffraction patterns indicate that the material has formed the ZrCuSiAs-type structure with a space group P4/nmm. The systematic evolution of the lattice constants demonstrated that the Fe ions were successfully replaced by Ir. Both electrical resistance and magnetization measurements show superconductivity up to 11.8 K in LaFe_1−xIr_xAsO. The superconducting transitions at different magnetic fields were also measured yielding a slope of −dH_c2/dT=6.7 T/K near T_c. Using the Werthamer-Helfand-Hohenberg formula H_c2=−0.69 (dH_c2/dT)∣_TcT_c, the upper critical field at zero K is found to be about 54 T. Hall-effect measurements indicate that the conduction in this material is dominated by electronlike charge carriers; the charge-carrier density determined at 100 K is about 3.71×10²⁰/cm³, which is close to the LaFeAsO_1−xF_x system. This is the example of bulk superconductivity induced by replacing the Fe sites with higher d-orbital electrons in FeAs-1111 family, which should add more ingredients to the underlying physics of the iron-based superconductors.

We investigate the combined effect of the repulsive vector interaction and the positive electric chemical potential on the chiral phase transition by considering neutral color superconductivity. The chiral condensate, diquark condensate, and quark number densities are solved in both two-flavor and two-plus-one-flavor Nambu-Jona-Lasinio models with the so called Kobayashi-Maskawa-’t Hooft term under the charge-neutrality constraint. We demonstrate that multiple chiral critical-point structures always exist in the Nambu-Jona-Lasinio model within the self-consistent mean-field approximation, and that the number of chiral critical points can vary from zero to four, which is dependent on the magnitudes of vector interaction and the diquark coupling. The difference between the dynamical chemical potentials induced by vector interaction for u and d quarks can effectively reduce the Fermi sphere disparity between the two flavors of diquark pairing. Thus the vector interaction works to significantly suppress the unstable region associated with chromomagnetic instability in the phase of neutral asymmetric homogeneous color superconductivity.

We argue from both technical and physical points of view that the main result shown in the Comment by Cherrolet [Phys. Rev. B 80, 037101 (2009)] as well as the authors’ interpretations of the result are not sufficient to draw the conclusion that the scaling law at the mobility edge takes the form T∝1/L². On the other hand, we believe that the result shows some evidence of T∝ln L/L² behavior found in S. K. Cheung and Z. Q. Zhang, Phys. Rev. B 72, 235102 (2005). More calculations with even larger L’s are necessary to give a more definitive answer to this question.

The biosonar system of bats utilizes physical baffle shapes around the sites of ultrasound emission for diffraction-based beam forming. Among these shapes, some extreme cases have evolved that include a long noseleaf protrusion (sella) in a species of horseshoe bat. We have evaluated the acoustic cost function associated with sella length with a computational physics approach and found that the extreme length can be predicted accurately from a fiducial point on this function. This suggests that some extreme cases of biological morphology can be explained from their physical function alone.

The complete knowledge of Laplacian eigenvalues and eigenvectors of complex networks plays an outstanding role in understanding various dynamical processes running on them; however, determining analytically Laplacian eigenvalues and eigenvectors is a theoretical challenge. In this paper, we study the Laplacian spectra and their corresponding eigenvectors of a class of deterministically growing treelike networks. The two interesting quantities are determined through the recurrence relations derived from the structure of the networks. Beginning from the rigorous relations one can obtain the complete eigenvalues and eigenvectors for the networks of arbitrary size. The analytical method opens the way to analytically compute the eigenvalues and eigenvectors of some other deterministic networks, making it possible to accurately calculate their spectral characteristics.