Search Results (61 total)

We report experimental measurements of particle dynamics on slowly sheared granular matter in a three-dimensional Couette cell. A closely packed ensemble of transparent spherical beads is confined by an external pressure and filled with fluid to match both the density and refractive index of the beads. This allows us to track tracer particles embedded in the system and obtain three-dimensional trajectories [r(t),θ(t),z(t)] as a function of time. We study the probability distribution function of the vertical and radial displacements, finding Gaussian and exponential distributions, respectively. For slow shear rates, the mean-square fluctuations in all three directions are found to be dependent only on the angular displacement of the Couette cell, Δθ_e, ⟨Δz²⟩∼Δθ_e, ⟨Δr²⟩∼Δθ_e^α, ⟨Δθ²⟩∼Δθ_e^β, where α and β are constants. With Δθ_e proportional to the time between measurements, the values of the constants, α and β, are found to be subdiffusive and superdiffusive, respectively. ThFe linear relation between ⟨Δz²⟩ and angular displacement implies a diffusive process, from which we can calculate an “effective temperature,” T_eff, in the vertical direction, through a fluctuation-dissipation relation. It is of interest to determine whether these systems can be described by analogous equilibrium statistical mechanics concepts such as “effective temperature” and “compactivity.” By studying the dynamics of tracer particles, we find the effective temperature defined by the Stokes-Einstein relation to be independent of the tracer particle characteristic features, such as density and size, and dependent only on the packing density of the system. For slow shear rate, both the diffusivity and mobility of tracer particles are proportional to the shear rate, giving rise to a constant effective temperature, characteristic of the jammed system. We finally discuss the significance of the existence of T_eff for a statistical mechanics formulation of granular matter.

Connectivity correlations play an important role in the structure of scale-free networks. While several empirical studies exist, there is no general theoretical analysis that can explain the largely varying behavior of real networks. Here, we use scaling theory to quantify the degree of correlations in the particular case of networks with a power-law degree distribution. These networks are classified in terms of their correlation properties, revealing additional information on their structure. For instance, the studied social networks and the Internet at the router level are clustered around the line of random networks, implying a strongly connected core of hubs. On the contrary, some biological networks and the WWW exhibit strong anticorrelations. The present approach can be used to study robustness or diffusion, where we find that anticorrelations tend to accelerate the diffusion process.

The High Resolution Fly’s Eye (HiRes) experiment has observed the Greisen-Zatsepin-Kuzmin suppression (called the GZK cutoff) with a statistical significance of five standard deviations. HiRes’ measurement of the flux of ultrahigh energy cosmic rays shows a sharp suppression at an energy of 6×10¹⁹  eV, consistent with the expected cutoff energy. We observe the ankle of the cosmic-ray energy spectrum as well, at an energy of 4×10¹⁸  eV. We describe the experiment, data collection, and analysis and estimate the systematic uncertainties. The results are presented and the calculation of the statistical significance of our observation is described.

We for the first time applied x-ray diffraction microscopy to the imaging of mineral crystals inside biological composite materials—intramuscular fish bone—at the nanometer scale resolution. We identified mineral crystals in collagen fibrils at different stages of mineralization. Based on the experimental results and biomineralization analyses, we suggested a dynamic model to account for the nucleation and growth of mineral crystals in the collagen matrix. The results obtained from this study not only further our understanding of the complex structure of bone, but also demonstrate that x-ray diffraction microscopy will become an important tool to study biological materials.

We report the first demonstration of resonant x-ray diffraction microscopy for element specific imaging of buried structures with a pixel resolution of ∼15  nm by exploiting the abrupt change in the scattering cross section near electronic resonances. We performed nondestructive and quantitative imaging of buried Bi structures inside a Si crystal by directly phasing coherent x-ray diffraction patterns acquired below and above the Bi M₅ edge. We anticipate that resonant x-ray diffraction microscopy will be applied to element and chemical state specific imaging of a broad range of systems including magnetic materials, semiconductors, organic materials, biominerals, and biological specimens.

The controlled motion of objects through narrow channels is important in many fields. We have fabricated asymmetric weak-pinning channels in a superconducting thin-film strip for controlling the dynamics of vortices. The lack of pinning allows the vortices to move through the channels with the dominant interaction determined by the shape of the channel walls. We present measurements of vortex dynamics in the channels and compare these with similar measurements on a set of uniform-width channels. While the uniform-width channels exhibit a symmetric response for both directions through the channel, the vortex motion through the asymmetric channels is quite different, with substantial asymmetries in both the static depinning and dynamic flux flow. This vortex ratchet effect has a rich dependence on magnetic field and driving force amplitude.

Comprehensive x-ray scattering studies, including resonant scattering at Mn L, Tb L, and M edges, were performed on single crystals of TbMn₂O₅ for crystallographic data to elucidate the nature of its commensurate and incommensurate phases. The scattering results provide direct evidence of symmetry lowering to the ferroelectric phase driven by magnetically induced lattice modulations and show the presence of multiple magnetic orders. The competing orders under spin-frustrated geometry are believed to cause discommensuration and result in the commensurate-to-incommensurate phase transition around 24 K. It is proposed that the low temperature incommensurate phase consists of commensurate domains separated by antiphase domain walls which change both signs of spontaneous polarizations and x-ray scattering amplitudes for forbidden reflections.

A search for TeV-PeV muon neutrinos from unresolved sources was performed on AMANDA-II data collected between 2000 and 2003 with an equivalent live time of 807 days. This diffuse analysis sought to find an extraterrestrial neutrino flux from sources with nonthermal components. The signal is expected to have a harder spectrum than the atmospheric muon and neutrino backgrounds. Since no excess of events was seen in the data over the expected background, an upper limit of E²Φ_90%C.L.<7.4×10^-8  GeV cm^-2 s^-1 sr^-1 is placed on the diffuse flux of muon neutrinos with a Φ∝E^-2 spectrum in the energy range 16 TeV to 2.5 PeV. This is currently the most sensitive Φ∝E^-2 diffuse astrophysical neutrino limit. We also set upper limits for astrophysical and prompt neutrino models, all of which have spectra different from Φ∝E^-2.

We present the first experimental demonstration of lensless diffractive imaging using coherent soft x rays generated by a tabletop soft-x-ray source. A 29 nm high harmonic beam illuminates an object, and the subsequent diffraction is collected on an x-ray CCD camera. High dynamic range diffraction patterns are obtained by taking multiple exposures while blocking small-angle diffraction using beam blocks of varying size. These patterns reconstruct to images with 214 nm resolution. This work demonstrates a practical tabletop lensless microscope that promises to find applications in materials science, nanoscience, and biology.

X-ray and neutron scattering studies were performed on DyB₄ which exhibits both a quadrupolar ordering and a macroscopic lattice distortion. A forbidden reflection at 7.792 keV near the Dy L₃ absorption edge is identified as a quadrupolar ordering peak, and the quadrupolar order and a monoclinic structural distortion develop concomitantly below 12.3 K as second-order–type phase transitions. Coupling between the quadrupolar order and the strain in DyB₄ is directly demonstrated by observing that both order parameters are proportional to each other.

The IceCube neutrino detector is a cubic kilometer TeV to PeV neutrino detector under construction at the geographic South Pole. The dominant population of neutrinos detected in IceCube is due to meson decay in cosmic-ray air showers. These atmospheric neutrinos are relatively well understood and serve as a calibration and verification tool for the new detector. In 2006, the detector was approximately 10% completed, and we report on data acquired from the detector in this configuration. We observe an atmospheric neutrino signal consistent with expectations, demonstrating that the IceCube detector is capable of identifying neutrino events. In the first 137.4 days of live time, 234 neutrino candidates were selected with an expectation of 211±76.1(syst)±14.5(stat) events from atmospheric neutrinos.

Magnetization and local Co structure of 4  at.  % Co-doped ZnO insulating films, prepared by direct current reactive magnetron cosputtering, have been studied as a function of substrates. Although all the Co:ZnO films possess a common feature that Co²⁺ replaces Zn²⁺, room-temperature ferromagnetism of Co:ZnO films is strongly dependent on the substrates. The reorganization of defects due to magnetoelectric coupling and a slight dissimilarity among Co K-edge x-ray-absorption near-edge structure spectra on different substrates reveal that the combination of Co:ZnO film grown on ferroelectric substrates and a suitable Co-O bond length likely serves as a key in enhancing the magnetic ordering. Furthermore, electric property measurements demonstrate trapped electrons at defect sites, which is of help for a better understanding of the bound magnetic polaron mechanism. These observations and the corresponding mechanisms responsible for the ferromagnetism enhancement would advance the insight into the controversial magnetic behaviors in Co:ZnO system.

Jammed matter is by definition impenetrable to light, such that little is known about the geometry of jammed systems. Using confocal microscopy to image an emulsion in 3D, we first explain the origin of the enhanced fluorescence at the droplet contacts and then determine the contact network inside the model frictionless system. This enables the experimental determination of the average coordination number ⟨Z⟩ which agrees with the isostatic predicted value of ⟨Z⟩≈6. Furthermore, we calculate the entropy of the packing from the network of contacts.

We report the results of a five-year survey of the northern sky to search for point sources of high energy neutrinos. The search was performed on the data collected with the AMANDA-II neutrino telescope in the years 2000 to 2004, with a live time of 1001 days. The sample of selected events consists of 4282 upward going muon tracks with high reconstruction quality and an energy larger than about 100 GeV. We found no indication of point sources of neutrinos and set 90% confidence level flux upper limits for an all-sky search and also for a catalog of 32 selected sources. For the all-sky search, our average (over declination and right ascension) experimentally observed upper limit Φ⁰=(E / 1  TeV)^γ·dΦ / dE to a point source flux of muon and tau neutrino (detected as muons arising from taus) is Φ_{νμ+ν̅ μ}⁰+Φ_{ντ+ν̅ τ}⁰=11.1×  10^-11  TeV^-1 cm^-2 s^-1, in the energy range between 1.6 TeV and 2.5 PeV for a flavor ratio Φ_{νμ+ν̅ μ}⁰/Φ_{ντ+ν̅ τ}⁰=1 and assuming a spectral index γ=2. It should be noticed that this is the first time we set upper limits to the flux of muon and tau neutrinos. In previous papers we provided muon neutrino upper limits only neglecting the sensitivity to a signal from tau neutrinos, which improves the limits by 10% to 16%. The value of the average upper limit presented in this work corresponds to twice the limit on the muon neutrino flux Φ_{νμ+ν̅ μ}⁰=5.5×10^-11  TeV^-1 cm^-2 s^-1. A stacking analysis for preselected active galactic nuclei and a search based on the angular separation of the events were also performed. We report the most stringent flux upper limits to date, including the results of a detailed assessment of systematic uncertainties.

Here we will derive the general theory of the beam-breakup (BBU) instability in recirculating linear accelerators with coupled beam optics and with polarized higher-order dipole modes. The bunches do not have to be at the same radio-frequency phase during each recirculation turn. This is important for the description of energy recovery linacs (ERLs) where beam currents become very large and coupled optics are used on purpose to increase the threshold current. This theory can be used for the analysis of phase errors of recirculated bunches, and of errors in the optical coupling arrangement. It is shown how the threshold current for a given linac can be computed and a remarkable agreement with tracking data is demonstrated. General formulas are then analyzed for several analytically solvable problems: (a) Why can different higher order modes (HOM) in one cavity couple and why can they then not be considered individually, even when their frequencies are separated by much more than the resonance widths of the HOMs? For the Cornell ERL as an example, it is noted that optimum advantage is taken of coupled optics when the cavities are designed with an x-y HOM frequency splitting of above 50 MHz. The simulated threshold current is then far above the design current of this accelerator. To justify that the simulation can represent an actual accelerator, we simulate cavities with 1 to 8 modes and show that using a limited number of modes is reasonable. (b) How does the x-y coupling in the particle optics determine when modes can be considered separately? (c) How much of an increase in threshold current can be obtained by coupled optics and why does the threshold current for polarized modes diminish roughly with the square root of the HOMs’ quality factors. Because of this square root scaling, polarized modes with coupled optics increase the threshold current more effectively for cavities that have rather large HOM quality factors, e.g. those without very elaborate HOM absorbers. (d) How does multiple-turn recirculation interfere with the threshold improvements obtained with a coupled optics? Furthermore, the orbit deviations produced by cavity misalignments are also generalized to coupled optics. It is shown that the BBU instability always occurs before the orbit excursion becomes very large.

We have shown that, when the linear oversampling ratio ⩾2, exactly oversampled diffraction patterns can be directly obtained from measured data through deconvolution. By using computer simulations and experimental data, we have demonstrated that exact oversampling of diffraction patterns distinctively improves the quality of phase retrieval. Furthermore, phase retrieval based on the exact sampling scheme is independent of the oversampling ratio, which can significantly reduce the radiation dosage to the samples. We believe that the present work will contribute to high-quality image reconstruction of materials science samples and biological structures using x-ray diffraction microscopy.

On 27 December 2004, a giant γ flare from the Soft Gamma-Ray Repeater 1806-20 saturated many satellite gamma-ray detectors, being the brightest transient event ever observed in the Galaxy. AMANDA-II was used to search for down-going muons indicative of high-energy gammas and/or neutrinos from this object. The data revealed no significant signal, so upper limits (at 90% C.L.) on the normalization constant were set: 0.05(0.5)  TeV^-1 m^-2 s^-1 for γ=-1.47 (-2) in the gamma flux and 0.4(6.1)  TeV^-1 m^-2 s^-1 for γ=-1.47 (-2) in the high-energy neutrino flux.

In combination of direct phase retrieval of coherent x-ray diffraction patterns with a novel tomographic reconstruction algorithm, we, for the first time, carried out quantitative 3D imaging of a heat-treated GaN particle with each voxel corresponding to 17×17×17  nm³. We observed the platelet structure of GaN and the formation of small islands on the surface of the platelets, and successfully captured the internal GaN-Ga₂O₃ core shell structure in three dimensions. This work opens the door for nondestructive and quantitative imaging of 3D morphology and 3D internal structure of a wide range of materials at the nanometer scale resolution that are amorphous or possess only short-range atomic organization.

We have studied the atomic and electronic structures of silver-filled (n,0) single-walled carbon nanotubes (Ag@SWCNTs) for n=6, 7, 8, and 10 by using first-principles calculations. We find that silver atoms encapsulated in SWCNTs self-aggregate to form ultrathin nanowires, of which the atomic arrangement depends on the diameter of the SWCNTs as well as the silver content. The electronic structures of the Ag@SWCNTs can be tuned from semiconducting to metallic by controlling the silver content. The hybridization of electronic states and the charge transfer between the encapsulated silver nanowires (AgNWs) and the SWCNTs are also addressed by performing Milliken population analysis combined with the projected density of states.

Resonant x-ray scattering is performed near the Mn K-absorption edge for an epitaxial thin film of BiMnO₃. The azimuthal angle dependence of the resonant (003) peak (in monoclinic indices) is measured with different photon polarizations; for the σ→π^′ channel a threefold symmetric oscillation is observed in the intensity variation, while the σ→σ^′ scattering intensity remains constant. These features are accounted for in terms of the peculiar ordering of the manganese 3d orbitals in BiMnO₃. It is demonstrated that the resonant peak persists up to 770 K with an anomaly around 440 K; these high and low temperatures coincide with the structural transition temperatures, seen in bulk, with and without a symmetry change, respectively. A possible relationship of the orbital order with the ferroelectricity of the system is presented.

A ferromagnetic oxide Co_0.05(LiNb)_0.95O_3−δ was prepared by Co ion implantation into (104) LiNbO₃ wafers. Co is uniformly distributed to a depth of ∼220 nm by implanting at three different energies and doses. Electron energy loss spectroscopy and x-ray absorption near-edge structure (XANES) spectra reveal a solid solution of cobalt in an implanted layer, where Co is not metallic but in the 2+ state. Furthermore, full multiple-scattering ab initio calculations of Co XANES at the K edge clearly provide a structure fingerprint to determine Co substitution at Li lattice sites rather than Nb. The Co_0.05(LiNb)_0.95O_3−δ system exhibits room temperature ferromagnetism of 1.3μ_B∕Co ion with an easy in-plane axis and a high Curie temperature of 710 K.

Ferromagnetic insulators that exhibit strong ferromagnetism at the atomic level are believed to be suitable for magnetic dielectric barriers in spintronic devices and solid-state qubits in quantum computing. Here a giant magnetic moment of 6.1μ_B∕Co and a high Curie temperature T_C of 790 K are observed in (4 at. %) Co-doped ZnO films, which is not carrier mediated, but co-exists with the dielectric state. Direct current reactive magnetron co-sputtering is used to grow Zn_0.96Co_0.04O dilute magnetic insulator on LiNbO₃ (104) substrates at considerably low growth temperature (∼200 °C), which is significant for complementary metal oxide semiconductor technology. X-ray photoelectron spectroscopy and x-ray-absorption spectroscopy reveal a solid solution of cobalt in ZnO, where Co is in the 2+ state substituting for Zn. A supercoupling mechanism in terms of bound magnetic polarons is proposed to discuss the ferromagnetism in the dielectric ground state of Co:ZnO, which would lead to different consideration for the origin of giant magnetic moment and high-temperature ferromagnetism in transition-metal doped oxides.

Structure, magnetization, and magnetoresistance of (Co t_Co nm∕Ru 9.6 nm) multilayers, prepared by vapor deposition, have been studied as a function of Co layer thickness (t_Co), from 1.1 to 6.6 nm. The experimental results indicate that an expansion of average atomic volume would enhance the magnetic moment of Co to 2.11μ_B with decreasing t_Co to 3.3 nm, and then would be reduced by nonmagnetic element alloying. Furthermore, we observe two distinct modes of positive magneto-resistance (PMR) in the Co∕Ru series: In mode 1 the PMR shows consistent variation with the magnetization, producing a narrow full width at half maximum (FWHM), accompanied by ferromagnetic coupling. In mode 2 the FWHM of PMR is wide with a high saturation field, in accordance with antiferromagnetic coupling. The mechanisms responsible for the magnetic-moment enhancement and positive magnetoresistance are discussed in terms of the metastable atomic configuration of Co atoms formed in the films and interface spin-dependent scattering, respectively.