Search Results (111 total)

We report the development of a proton identification method for the Super-Kamiokande (SK) detector. This new tool is applied to the search for events with a single proton track, a high purity neutral current sample of interest for sterile neutrino searches. After selection using a neural network, we observe 38 events in the combined SK-I and SK-II data corresponding to 22 85.1 days of exposure, with an estimated signal-to-background ratio of 1.6 to 1. Proton identification was also applied to a direct search for charged-current quasielastic (CCQE) events, obtaining a high precision sample of fully kinematically reconstructed atmospheric neutrinos, which has not been previously reported in water Cherenkov detectors. The CCQE fraction of this sample is 55%, and its neutrino (as opposed to antineutrino) fraction is 91.7±3%. We selected 78μ-like and 47 e-like events in the SK-I and SK-II data set. With this data, a clear zenith angle distortion of the neutrino direction itself is reported in a sub-GeV sample of μ neutrinos where the lepton angular correlation to the incoming neutrino is weak. Our fit to ν_μ→ν_τ oscillations using the neutrino L / E distribution of the CCQE sample alone yields a wide acceptance region compatible with our previous results and excludes the no-oscillation hypothesis at 3-sigma.

We have searched for proton decays via p→e⁺π⁰ and p→μ⁺π⁰ using data from a 91.7  kt·yr exposure of Super-Kamiokande-I and a 49.2  kt·yr exposure of Super-Kamiokande-II. No candidate events were observed with expected backgrounds induced by atmospheric neutrinos of 0.3 events for each decay mode. From these results, we set lower limits on the partial lifetime of 8.2×10³³ and 6.6×10³³ years at 90% confidence level for p→e⁺π⁰ and p→μ⁺π⁰ modes, respectively.

It is shown that the mean-square displacement or the exponent of the Debye-Waller factor of graphene has a singularity except at zero temperature. The zero-temperature values of the mean-square displacement are calculated separately for planar and out-of-plane phonon modes for graphene. These values give the Debye-Waller factor that can be used to model various scattering processes at temperatures much lower than the Debye temperature of graphene. Since the Debye temperature of graphene is about 2300 K for planar modes, the calculated values should provide a useful estimate of the Debye-Waller factor at temperatures of practical interest. Finally, it is shown qualitatively that the singularity can be removed by accounting for the finite size of real graphene crystals.

A parametric interatomic potential is constructed for graphene. The potential energy consists of two parts: a bond energy function and a radial interaction energy function. The bond energy function is based on the Tersoff-Brenner potential model. It includes angular terms and explicitly accounts for flexural deformation of the lattice normal to the plane of graphene. It determines the cohesive energy of graphene and its equilibrium lattice constant. The radial energy function has been chosen such that it does not contribute to the binding energy or the equilibrium lattice constant but contributes to the interatomic force constants. The range of interaction of each atom extends up to its fourth-neighbor atoms in contrast to the Tersoff-Brenner potential, which extends only up to second neighbors. The parameters of the potential are obtained by fitting the calculated values to the cohesive energy, lattice constant, elastic constants, and phonon frequencies of graphene. The values of the force constants between an atom and other atoms that are within its fourth-neighbor distance are calculated. Analytical expressions are given for the elastic constants and the flexural rigidity of graphene. The flexural rigidity of the graphene lattice is found to be 2.13 eV, which is much higher than 0.797 eV calculated earlier using the Tersoff-Brenner potential.

The results of the second phase of the Super-Kamiokande solar neutrino measurement are presented and compared to the first phase. The solar neutrino flux spectrum and time variation as well as oscillation results are statistically consistent with the first phase and do not show spectral distortion. The time-dependent flux measurement of the combined first and second phases coincides with the full period of solar cycle 23 and shows no correlation with solar activity. The measured ⁸B total flux is (2.38±0.05(stat.)_-0.15^+0.16(sys.))×10⁶  cm^-2 s^-1 and the day-night difference is found to be (-6.3±4.2(stat.)±3.7(sys.))%. There is no evidence of systematic tendencies between the first and second phases.

We show analytically that the continuum limit of the lattice-statics Green’s function of a graphene sheet corresponds to the Green’s function for an elastically stable Kirchhoff plate but not the Green’s function for two-dimensional Christoffel equations. This correspondence demonstrates the mechanical stability of graphene in deflection and is necessary for relating its mechanical parameters to its lattice parameters. An explicit expression is derived for relating the continuum flexural rigidity to the force constants of graphene. This relationship can be used to measure flexural rigidity of graphene directly from experimentally observed phonon dispersion curves. The flexural rigidity is predicted to be 0.797 eV by using the Tersoff–Brenner empirical potential. Numerical examples are presented to show the usefulness of the correspondence in bridging the lattice and continuum length scales in graphene.

The formation of nanoscale self-assembled compositional patterns in monolayer bulk-immiscible alloy films is studied from first principles within the framework of a previously proposed hybrid atomistic-continuum model [V. Ozoliņš , Phys. Rev. Lett. 88, 096101 (2002)]. The details surrounding the parametrization of the model from first-principles calculations are described for both hexagonal (0001) and bcc (110) substrates, and we demonstrate how the theoretical model can be employed in Monte Carlo simulations as a predictive framework for modeling the structure and finite-temperature stability of compositional patterns. The methodology is applied in a comparative study of equiatomic FeAg pseudomorphic alloy films on Mo(110) and Ru(0001) substrates. Stripe patterns with periodicities of a few nanometers are predicted to be stable in both systems, which is in good agreement with available experimental data for FeAg/Mo(110). The regularity of the stripe patterns and their stability with respect to disordering are found to be substantially enhanced on the anisotropic Mo(110) substrate relative to the nearly isotropic Ru(0001) surface, despite the slightly stronger ordering energetics in the latter system. A comparison of the results of the present study to the predictions of continuum theories commonly employed to describe pattern formation on crystalline surfaces serves to highlight the limitations of such models in the application to patterns with periodicities with length scales of approximately ten atomic spacings.

We consider ν_μ→ν_τ oscillations in the context of the mass varying neutrino (MaVaN) model, where the neutrino mass can vary depending on the electron density along the flight path of the neutrino. Our analysis assumes a mechanism with dependence only upon the electron density, hence ordinary matter density, of the medium through which the neutrino travels. Fully-contained, partially-contained and upward-going muon atmospheric neutrino data from the Super-Kamiokande detector, taken from the entire SK-I period of 1489 live days, are compared to MaVaN model predictions. We find that, for the case of 2-flavor oscillations, and for the specific models tested, oscillation independent of electron density is favored over density dependence. Assuming maximal mixing, the best-fit case and the density-independent case do not differ significantly.

The peculiar nature of electron scattering in graphene is among many exciting theoretical predictions for the physical properties of this material. To investigate electron scattering properties in a graphene plane, we have created a gate-tunable potential barrier within a single-layer graphene sheet. We report measurements of electrical transport across this structure as the tunable barrier potential is swept through a range of heights. When the barrier is sufficiently strong to form a bipolar junction (n-p-n or p-n-p) within the graphene sheet, the resistance across the barrier sharply increases. We compare these results to predictions for both diffusive and ballistic transport, as the barrier rises on a length scale comparable to the mean free path. Finally, we show how a magnetic field modifies transport across the barrier.

We have investigated the optical conductivity spectra of La_2-2xSr_1+2xMn₂O₇ (0.3≤x≤0.5) systematically and found that for x near 0.4 the charge gap shows enhancement of up to ∼0.3  eV just above the long-range magnetic ordering temperature. From comparison of this charge gap with other experimental results, we conclude that the peculiar x dependence of the charge gap cannot be understood in terms of the charge and lattice correlations only. We suggest that the unusual enhancement originates from the cooperative coupling between the short-range charge or lattice correlations and the quasi-one-dimensional energy band near the Fermi level.

A computationally efficient hybrid Green’s function (GF) technique is developed for multiscale modeling of point defects in a trilayer lattice system that links seamlessly the length scales from lattice (subnanometers) to continuum (bulk). The model accounts for the discrete structure of the lattice including nonlinear effects at the atomistic level and full elastic anisotropy at the continuum level. The model is applied to calculate the discrete core structure of point defects (vacancies and substitutional impurities) in Si-Ge(001) quantum wells (QWs) that are of contemporary technological interest. Numerical results are presented for the short range and long range lattice distortions and strains in the lattice caused by the defects and their formation energy and Kanzaki forces that are basic characteristics of the defects. The continuum and the lattice GFs of the material system are used to link the different length scales, which enables us to model the point defects and extended defects such as the quantum well in a unified formalism. Nonlinear effects in the core of the point defects are taken into account by using an iterative scheme. The Tersoff potential is used to set up the lattice structure, compute the unrelaxed forces and force constants in the lattice, and derive the elastic constants required for the continuum GF. It is found that the overall elastic properties of the material and the properties of defects vary considerably when the material is strained from the bulk to the QW state. This change in the defect properties is very significant and can provide a characteristic signature of the defect. For example, in the case of a single vacancy in Ge, the strain reverses the sign of the relaxation volume. It is also found that the defect properties, such as the defect core structures, change abruptly across a Ge∕Si interface. The transition occurs over a region extending from two to four lattice constants, depending upon the defect species.

Optical spectra of a double-layered perovskite ruthenate Ca₃Ru₂O₇ show a pseudogap opening around 200  cm^-1 below 50 K, which is attributable to the partial k-space gap opening due to the density wave instability. Unlike most other density wave materials, Ca₃Ru₂O₇ has spectral weight redistributions, not near the energy gap region, but at a much higher energy region around 800  cm^-1. As a possible origin of these intriguing features, we discuss the orbital flip excitation in the density wave ground state.

The duration of the x-ray pulse generated at a synchrotron light source is typically tens of picoseconds. Shorter pulses are highly desired by the users. In electron storage rings, the vertical beam size is usually orders of magnitude less than the bunch length due to radiation damping; therefore, a shorter pulse can be obtained by slitting the vertically tilted bunch. Zholents proposed tilting the bunch using rf deflection. We found that tilted bunches can also be generated by a dipole magnet kick. A vertical tilt is developed after the kick in the presence of nonzero chromaticity. The tilt was successfully observed and a 4.2-ps pulse was obtained from a 27-ps electron bunch at the Advanced Photon Source. Based on this principle, we propose a short-pulse generation scheme that produces picosecond x-ray pulses at a repetition rate of 1–2 kHz, which can be used for pump-probe experiments.

A method for producing short electron bunches in an electron storage ring using pulsed phase modulation has been demonstrated. A simple theoretical model was validated using the particle tracking code elegant, and the bunch compression process was observed experimentally in the Advanced Photon Source storage ring using a visible light streak camera. Compression to 54% of the initial bunch length was achieved.

Very high energy resolution photoemission experiments on high quality samples of optimally doped Bi₂Sr₂CaCu₂O_8+δ show new features in the low-energy electronic excitations. A marked change in the binding energy and temperature dependence of the near-nodal scattering rates is observed near the superconducting transition temperature, T_C. The temperature slope of the scattering rate measured at low energy shows a discontinuity at T_C. In the superconducting state, coherent excitations are found with the scattering rates showing a cubic dependence on frequency and temperature. The superconducting gap has a d-wave magnitude with negligible contribution from higher harmonics. Further, the bilayer splitting has been found to be finite at the nodal point.

We present an experimental realization of a robust quantum communication scheme [Phys. Rev. Lett. 93, 220501 (2004)] using pairs of photons entangled in polarization and time. Our method overcomes errors due to collective rotation of the polarization modes (e.g., birefringence in optical fiber or misalignment), is insensitive to the phase’s fluctuation of the interferometer, and does not require any shared reference frame including time reference, except the need to label different photons. The practical robustness of the scheme is further shown by implementing a variation of the Bennett-Brassard 1984 quantum key distribution protocol over 1 km optical fiber.

We propose a novel alignment technique utilizing the x-ray beam of an undulator in conjunction with pinholes and position-sensitive detectors for positioning components of the accelerator, undulator, and beam line in an x-ray free-electron laser. Two retractable pinholes at each end of the undulator define a stable and reproducible x-ray beam axis (XBA). Targets are precisely positioned on the XBA using a pinhole camera technique. Position-sensitive detectors responding to both x-ray and electron beams enable direct transfer of the position setting from the XBA to the electron beam. This system has the potential to deliver superior alignment accuracy (1–3   μm) for target pinholes in the transverse directions over a long distance (200 m or longer). It can be used to define the beam axis of the electron-beam–based alignment, enabling high reproducibility of the latter. This x-ray–based concept should complement the electron-beam–based alignment and the existing survey methods to raise the alignment accuracy of long accelerators to an unprecedented level. Further improvement of the transverse accuracy using x-ray zone plates will be discussed. We also propose a concurrent measurement scheme during accelerator operation to allow real-time feedback for transverse position correction.

The formation and ordering of Si nanocrystals in dewetting and agglomeration of the thin single crystalline Si layer in silicon-on-insulator has been investigated using low-energy electron microscopy. The evolution of the Si dewetting and agglomeration is captured in real time, revealing the detailed process for the formation and ordering of the nanocrystals. A surface faceting mechanism governing this self-organized process is proposed and supported with first-principles calculations.

We report a systematic angle-resolved photoemission study on Na_xCoO₂ for a wide range of Na concentrations (0.3≤x≤0.72). In all the metallic samples at different x, we observed (i) only a single holelike Fermi surface centered around Γ and (ii) its area changes with x according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The e_g^′ band and the associated small Fermi surface pockets near the K points predicted by band calculations are found to sink below the Fermi energy in a manner almost independent of the doping and temperature.

We report free-space distribution of entangled photon pairs over a noisy ground atmosphere of 13 km. It is shown that the desired entanglement can still survive after both entangled photons have passed through the noisy ground atmosphere with a distance beyond the effective thickness of the aerosphere. This is confirmed by observing a spacelike separated violation of Bell inequality of 2.45±0.09. On this basis, we exploit the distributed entangled photon source to demonstrate the Bennett-Brassard 1984 quantum cryptography scheme. The distribution distance of entangled photon pairs achieved in the experiment is for the first time well beyond the effective thickness of the aerosphere, hence presenting a significant step towards satellite-based global quantum communication.

The perturbation of nonspherical symmetrical acoustic pressure is added to the equation governing the spherical stability of sonoluminescing bubbles. The numerical calculations of the shape instability of sonoluminescing bubbles with the modified equation are conducted and the results are illustrated accordingly in the p_a‐R₀ phase diagrams. The calculated results indicate that the stability region vanishes as the amplitude of the driving acoustic pressure p_a arrives at the upper threshold (∼1.6  atm) due to the perturbation of a small nonspherical symmetrical acoustic pressure (about a few Pa), which is in consistence with the experimental observations.

We performed high-resolution angle-resolved photoemission spectroscopy on Nd_1.87Ce_0.13CuO₄, which is located at the boundary of the antiferromagnetic (AF) and the superconducting phase. We observed that the quasiparticle (QP) effective mass around (π,0) is strongly enhanced due to the opening of the AF gap. The QP mass and the AF gap are found to be anisotropic, with the largest value near the intersecting point of the Fermi surface and the AF zone boundary. In addition, we observed that the QP peak disappears around the Néel temperature (T_N) while the AF pseudogap is gradually filled up at much higher temperatures, possibly due to the short-range AF correlation.

We report angle-resolved photoelectron spectroscopy results of the Fermi surface of Ca_1.5Sr_0.5RuO₄, which is at the boundary of magnetic/orbital instability in the phase diagram of the Ca-substituted Sr ruthenates. Three t_2g energy bands and the corresponding Fermi surface sheets are observed, which are also present in the Ca-free Sr₂RuO₄. We find that while the Fermi surface topology of the α,β (d_yz,zx) sheets remains almost the same in these two materials, the γ (d_xy) sheet exhibits a holelike Fermi surface in Ca_1.5Sr_0.5RuO₄ in contrast to being electronlike in Sr₂RuO₄. Our observation of all three volume conserving Fermi surface sheets clearly demonstrates the absence of orbital-selective Mott transition, which was proposed theoretically to explain the unusual transport and magnetic properties in Ca_1.5Sr_0.5RuO₄.

The electronic structure of single crystals Na_0.6CoO₂, which are closely related to the superconducting Na_0.3CoO₂·yH₂O (T_c∼5   K), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface (FS) is consistent with the large FS enclosing the Γ point from the band theory, the predicted small FS pockets near the K points are absent. In addition, the band dispersion is found to be highly renormalized, and anisotropic along the two principal axes (Γ-K, Γ-M). Our measurements also indicate that an extended flatband is formed slightly above E_F along Γ-K.

The bulk-representative low-energy spectrum of Sr₂RuO₄ can be directly measured by angle-resolved photoemission. We find that the quasiparticle spectral line shape of Sr₂RuO₄ is sensitive to both temperature and momentum. Along the (0,0)-(π,0) direction, both γ and β bands develop a sharp quasiparticle peak near k_F at low temperatures, but as the temperature increases the spectra quickly lose coherent weight and become broad backgrounds above ∼130  K, which is the metal-nonmetal crossover temperature, T_M, in the c-axis resistivity. However, spectra along the (0,0)-(π,π) direction evolve smoothly across T_M. A simple transport model can describe both in-plane and c-axis resistivity in terms of the quasiparticle line shape. Comparisons are also made to the cuprates, with implications for two dimensionality, magnetic fluctuations, and superconductivity.