Search Results (58 total)

Microwave instability in the low energy ring of KEKB was studied using a broadband impedance model. The model gave excellent descriptions of longitudinal dynamics for both positive and negative momentum compactions. Moreover, it predicted that the threshold of microwave instability was a factor of 2 lower than the machine nominal operating bunch current. The prediction was confirmed by a measurement using the Belle detector. Furthermore, we integrated the longitudinal wakefield into the beam-beam simulation and applied it to study the combined effects in KEKB. As a result, the beam-beam simulation became truly three dimensional with emittance growth in all three dimensions simultaneously as the beam currents increase. In addition, an observed mystery of asymmetry in the horizontal scan could also be explained by our simulations.

We report e⁺e^-→bb̅ cross section measurements by the BABAR experiment performed during an energy scan in the range of 10.54 to 11.20 GeV at the SLAC PEP-II e⁺e^- collider. A total relative error of about 5% is reached in more than 300 center-of-mass energy steps, separated by about 5 MeV. These measurements can be used to derive precise information on the parameters of the Υ(10860) and Υ(11020) resonances. In particular we show that their widths may be smaller than previously measured.

The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q,ω). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers.

The pressure-induced valence transition in TmTe was investigated by resonant inelastic x-ray scattering at the Tm L₃ edge, a powerful probe of the rare-earth valent state. The data are analyzed within the Anderson impurity model which yields key parameters such as the Tm 4f-5d hybridization. In addition to the general tendency of the f electrons towards delocalization, we find a plateau in both the Tm valence and hybridization pressure dependences between 4.3 and 6.5 GPa which is interpreted in terms of an n-channel Kondo (NCK) screening process. This behavior is at odds with the usually continuous, single-channel Kondo-like f delocalization while being supported by the seminal calculations of the NCK temperature in Tm ion by Saso et al. Our study raises the interesting possibility that an NCK effect realized in a compressed mixed-valent f system could impede the concomitant electron delocalization.

We report the results of a search for the bottomonium ground state η_b(1S) in the photon energy spectrum with a sample of (109±1) million of Υ(3S) recorded at the Υ(3S) energy with the BABAR detector at the PEP-II B factory at SLAC. We observe a peak in the photon energy spectrum at E_γ=921.2_-2.8^+2.1(stat)±2.4(syst)  MeV with a significance of 10 standard deviations. We interpret the observed peak as being due to monochromatic photons from the radiative transition Υ(3S)→γη_b(1S). This photon energy corresponds to an η_b(1S) mass of 9388.9_-2.3^+3.1(stat)±2.7(syst)  MeV/c². The hyperfine Υ(1S)-η_b(1S) mass splitting is 71.4_-3.1^+2.3(stat)±2.7(syst)  MeV/c². The branching fraction for this radiative Υ(3S) decay is estimated to be [4.8±0.5(stat)±1.2(syst)]×10^-4.

Near-edge x-ray Raman scattering (XRS) spectra of the silicon L edge have been obtained on SiO₂ glass under compression in a diamond anvil cell up to 74 GPa. Partial densities of states (DOS’s) of electrons, which the spectra reflect, have been calculated. The spectra for the glass do not show significant variations with pressure, whereas distinct differences are observed between quartz and stishovite, providing clear evidence for the nonexistence of sixfold-coordinated silicon by oxygen, that is stishovitelike silicon, in silica glass up to 74 GPa. A post main-edge hump, which is not seen in the XRS spectra and DOS’s of quartz and stishovite, and an edge energy shift suggest that the silicon is in a different coordination environment under pressure. A compression mechanism of SiO₂ glass which involves the formation of fivefold-coordinated silicon following the decrease in Si-O-Si angle is proposed to explain the observed changes up to 74 GPa.

Ab initio study based on density functional theory is performed to study the binding energies of Mg acceptors to single oxygen in AlN and the activation energies of the resultant Mg_n-O complexes (n=2, 3, and 4). It is found that such complexes are energetically favored and have activation energies at least 0.23  eV lower than that of single Mg. The lower activation energies originate from the extra states over the valence band top of AlN induced by the passive Mg-O. By comparing to the well-established case of GaN, it is possible to fabricate Mg:O codoped AlN without MgO precipitates. These results suggest the possibility of achieving higher hole concentration in AlN by Mg:O codoping.

The giant dipole resonance of Ba embedded into the complex Si host lattice structure of Ba₈Si₄₆ has been observed under ambient and high-pressure conditions. The measurements have been accomplished using nonresonant inelastic x-ray scattering for different momentum transfers. The resonance appears as a broad feature between 100- and 150-eV energy loss for low momentum transfer but vanishes for high momentum transfer. Calculations within the time-dependent local-density approximation have been performed by means of a real-space multiple-scattering Green’s-function approach. The results reproduce the shape and the width of the observed resonance. Modulations of the giant resonance spectra are predicted by computations ranging from ambient pressure up to 20  GPa which can be used to study the local environment of the Ba guest. A corresponding experimental setup for high-pressure studies is presented, potential applications to study the phase transitions of Ba clathrates are discussed, and first experimental results are shown.

Excitons in a complex organic molecular crystal were studied by inelastic x-ray scattering (IXS) for the first time. The dynamic dielectric response function is measured over a large momentum transfer region, from which an exciton dispersion of 130 meV is observed. Semiempirical quantum chemical calculations reproduce well the momentum dependence of the measured dynamic dielectric responses, and thus unambiguously indicate that the lowest Frenkel exciton is confined within a fraction of the complex molecule. Our results demonstrate that IXS is a powerful tool for studying excitons in complex organic molecular systems. Besides the energy position, the IXS spectra provide a stringent test on the validity of the theoretically calculated exciton wave functions.

Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near K-edge spectra of SiO₂ glass to 51 GPa obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below ∼10 GPa are consistent with those of quartz and coesite, whereas the spectra above ∼22 GPa are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in SiO₂ glass between 10 GPa and 22 GPa. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed SiO₂ glass.

We have investigated the structural stability of MgTe under pressure using a first-principles pseudopotential method within both generalized gradient approximation (GGA) and local density approximation (LDA). The results show that the ground state of MgTe is the wurtzite structure for the GGA and the NiAs structure for the LDA. Which structure on earth is the ground state of MgTe is addressed in this paper. The WZ⇌NiAs transition mechanism of MgTe is presented, that is, the relative shifting of the density-packed Mg atoms layer in the [1̅010]_H or its equivalent directions with the b∕3 relative displacement constraint. Based on this mechanism, the transition barrier is determined to be ∼0.147 eV∕MgTe at the equilibrium transition pressure of 1.1 GPa. The real transition pressure is estimated to be 2.7 GP for WZ⇀NiAs and −0.5 GPa for WZ↽NiAs.

We present a supersymmetric grand unification model based on SO(10) group with S4 flavor symmetry. In this model, the fermion masses are from Yukawa couplings involving 10 and 126̅ Higgs multiplets and the flavor structures of mass matrices of both quarks and leptons are determined by spontaneously broken S4. This model fits all of the masses and mixing angles of the quarks and leptons. For the most general CP-violation scenario, this model gives sin⁡θ₁₃ a wide range of values from zero to the current bound with the most probable values 0.02–0.09. With certain assumptions where leptonic phases have same CP-violation source as CKM phase, one gets a narrower range 0.03–0.06 for sin⁡θ₁₃ with the most probable values 0.04–0.08. This model gives leptonic Dirac CP phase the most probable values 2–4 radians in the general CP-violation case.

First-principles calculations have been conducted to study the structural, vibrational, and dielectric properties of β-Si₃N₄. Calculations of the zone-center optical-mode frequencies (including longitudinal-optical/transverse-optical splittings), Born effective charge tensors for each atom, and dielectric constants, using density functional perturbation theory, are reported. The fully relaxed structural parameters are found to be in good agreement with experimental data. All optic modes are identified and agreement of theory with experiment is excellent. The static dielectric tensor is decomposed into contributions arising from individual infrared-active phonon modes. It is found that high-frequency modes mainly contribute to the lattice dielectric constant.

A sharp feature in the charge-density excitation spectra of single-crystal MgB₂, displaying a remarkable cosinelike, periodic energy dispersion with momentum transfer (q) along the c^* axis, has been observed for the first time by high-resolution nonresonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small-q collective mode residing in the single-particle excitation gap of the B π bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of MgB₂.

In this work we investigate the relevance of phonons for the superconductivity in Na_0.35CoO₂∙1.3H₂O. We measure phonon dispersion in this material as well as in the nonhydrated parent compound Na_0.7CoO₂ by inelastic x-ray scattering. The measured phonon dispersion along the Γ-M direction shows a marked softening of Co-related optical phonon branches close to the Brillouin zone boundary, with hole doping. The phonon spectra, dispersion, and softening of Co-vibration modes are well reproduced by first-principles calculations. The critical temperature estimated from a phonon-mediated mechanism is much lower than the measured one, suggesting a nonconventional superconducting state in this material.

We have investigated the pressure-induced phase transition in BeO using a first-principles pseudopotential method within the generalized gradient approximation. The results show that the enthalpy of BeO with wurtzite structure is lower than that with zinc blende structure at 87 GPa and the same to rocksalt structure at 105 GPa. However, from the point of view of enthalpy barrier, the wurtzite-zinc-blende-rocksalt phase transition sequence will not happen up to 200 GPa but only the wurtzite-rocksalt transition. The phase transition mechanism can be viewed as an orthogonal strain deformation: two strain contractions in [101̅0] and [0001] direction and a strain expansion in [0100] direction.

We report on resonant inelastic x-ray scattering studies of a one-dimensional quantum spin system Sr₂CuO₃. Data were taken at the Cu 1s→3d edge, which is expected to provide dd excitation spectra. The energy loss spectra show the lowest peak near 2.0 eV, smaller by ≈0.6 eV than the lowest energy peak obtained at the 1s→4p edge. The feature has a total dispersion of about 0.2 eV with a periodicity of π. While a comparison with optical measurements supports the interpretation of the data as being from dd excitations, the dispersion is apparently incompatible with theoretical results.

The dynamics of doped charge in an antiferromagnetic lattice is central to the description of the insulator-metal transition that occurs on doping the parent high T_c compounds. In this work we use high resolution resonant inelastic x-ray scattering to investigate the dynamics of the charge-transfer exciton by measuring its energy dispersion in two prototype compounds, La₂CuO₄ and La₂NiO₄. We show that this behavior is radically different in the cuprate with respect to a system known to exhibit strong polaronic behavior, namely, the nickelate: the exciton is mobile in the cuprate while it is well localized in the nickelate. Using a simple Wannier-Mott model we can estimate the total hole plus electron effective mass in the cuprate to be 3.5±0.3m_e which would exclude strong localization in the undoped cuprate.

Femtosecond explosive processes of argon clusters irradiated by linearly chirped ultraintense laser pulses have been investigated by 90° side spectral scattering. The spectral redshift and blueshift, which correlate with the cluster explosion processes have been measured for negatively and positively chirped driving laser pulses, respectively. The evolution of the heated-cluster polarizability indicates that the core of the cluster is shielded from the laser field in the beginning of the explosion and enhanced scattering occurs after the fast explosion initiates. Evidence of resonant heating is found from the coincidence of enhanced scattering with enhanced absorption measured using the transmitted spectra. Anomalously large-size clusters with very low gas density have been observed in this way and can be used as clean and important cluster targets.

Ultrathin SiO₂ films on Mo(112) were synthesized using different preparation procedures and characterized with ultraviolet photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES), and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS). By correlating the vibrational and electronic data, an assignment of the prominent spectral features are made. The physical properties of SiO₂ films near one monolayer are influenced by the Mo substrate due to the Si-O-Mo linkages, whereas films greater than two monolayers show properties comparable to bulklike silica samples. The electronic and vibrational properties of the SiO₂ thin films are strongly coverage dependent. The data show that highly ordered SiO₂ films can be grown up to one monolayer, whereas films with a thickness of greater than one monolayer are amorphous.

The electronic structure of thulium monochalcogenides TmX (X=S,Se,Te) is investigated by x-ray absorption spectroscopy (XAS) in the partial fluorescence yield mode (PFY-XAS) and resonant x-ray emission spectroscopy around and far below the edge. These three resonant inelastic x-ray scattering (RIXS)-derived techniques yield consistent values of the thulium valency of 2.67, 2.65, and 2.70, respectively, in the mixed-valent compound TmSe. These values are in closer agreement with the valency derived from the lattice parameter measurement compared to previous x-ray photoemission spectroscopy and XAS studies. The study demonstrates that RIXS is a powerful and accurate probe of mixed-valent states in f-electron systems.

We have studied charge excitations in graphite using inelastic x-ray scattering. The spectra were measured as a function of the momentum transfer, q, along the [100] (k_x), [210] (k_y), and [001] (k_z) axes on a high-quality single-crystal sample. Anisotropic and periodic features representing the semimetallic electronic structure have been observed. These features are interpreted based on band structure obtained from the local density approximation. On the q‖k_x and q‖k_y spectra, periodic signals exhibiting the gap opening and closing of the π bands have been observed. On the other hand, on q‖k_z spectra, a forbidden excitation between the π bands has been detected in the high q region.

We theoretically study ghost imaging with incoherent and partially coherent light radiation by using classical optical coherence theory. A Gaussian thin lens equation is derived for the ghost image. The equation depends on both paths. The quality and visibility of the ghost image are influenced by the source’s transverse size, coherence width, and object characteristics. The differences between ghost imaging formed with incoherent light radiation and with entangled photon pairs are discussed.

The near K-edge structure of oxygen in liquid water and ices III, II, and IX at 0.25 GPa and several low temperatures down to 4 K has been studied using inelastic x-ray scattering at 9884.7 eV with a total energy resolution of 305 and 175 meV. A marked decrease of the preedge intensity from the liquid phase and ice III to ices II and IX is attributed to ordering of the hydrogen bonds in the proton-ordered lattice of the latter phases. Density functional theory calculations including the influence of the Madelung potential of the ice IX crystal correctly account for the remaining preedge feature. Furthermore, we obtain spectroscopic evidence suggesting a possible new phase of ice at temperatures between 4 and 50 K.