Search Results (33 total)

Two different adsorption configurations of benzene on the Si(001)-(2×1) surface, the tight-bridge and butterfly structures, were studied using density functional theory. Several exchange and correlation functionals were used, including the recently developed van der Waals density functional (vdW-DF), which accounts for the effect of van der Waals forces. In contrast to the Perdew-Burke-Ernzerhof (PBE), revPBE, and other generalized-gradient approximation functionals, the vdW-DF finds that, for most coverages, the adsorption energy of the butterfly structure is greater than that of the tight-bridge structure.

We measured the lifetime and the mesonic and nonmesonic decay rates of the _Λ⁴He hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K^- beam on a liquid ⁴He target by the reaction ⁴He (K^-,π^-)_Λ⁴He. The _Λ⁴He lifetime was directly measured using protons from Λp→np nonmesonic decay (also referred to as proton-stimulated decay) and was found to have a value of τ=245±24 ps. The mesonic decay rates were determined from the observed numbers of π^-'s and π^○'s as Γ_π^-/Γ_tot=0.270±0.024 and Γ_π^○/Γ_tot=0.564±0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Γ_p/Γ_tot=0.169±0.019 and Γ_n/Γ_tot≤0.032 (95% CL), respectively. The effects of final-state interactions and possible three-body ΛNN decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the nonmesonic branching fractions. The implications of the results of this analysis were considered for the empirical ΔI=1 / 2 rule and the decay rates of the _Λ⁴H hypernucleus.

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q²=0.15, 0.25  (GeV/c)². The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A_n provides a direct probe of the imaginary component of the 2γ exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments.

The structures and energetics of organic molecules adsorbed onto clean and H-passivated Si(001)-(2×1) surfaces have been calculated using density functional theory. For benzene adsorbed on the clean Si surface the tight-bridge structure was found to be stable and the butterfly structure metastable. Both carbonic acid, H₂CO₃, and propane, C₃H₈, dissociate on contact with the surface. Passivation of the Si surface with H atoms has a dramatic effect on the surface properties. The passivated surface is very inert and the binding energy of all the molecules is very weak.

The effect of intermixing at the interface of short period PbTiO₃∕SrTiO₃ superlattices is studied using first-principles density functional theory. The results indicate that interfacial intermixing significantly enhances the polarization within the superlattice. This enhancement is directly related to the off-centering of Pb and Sr cations and can be explained through a discussion of interacting dipoles. This picture should be general for a wide range of multicomponent superlattices and may have important consequences for the design of ferroelectric devices.

The results of photoluminescence measurements on ZnO implanted with stable and radioactive isotopes of Zn and Ga are presented. The donor-related exciton feature I₈ at 3.3600 eV is suggested to be due to bound exciton recombination at Ga donors. The I₁ line at 3.3718 eV is also likely to be due to Ga, and is attributed to ionized Ga donor bound exciton recombination. A feature at 3.3225 eV is observed following transmutation of radioactive Ga into stable Ge, and is attributed to Ge. Finally, a damage-related band is observed in the region of 1.8 eV when the recoil energy of the decay is capable of dislodging the host atoms from their respective lattice sites.

A pioneering experiment in Λ hypernuclear spectroscopy, undertaken at the Thomas Jefferson National Accelerator Facility (JLab), was recently reported. The experiment used the high precision, continuous electron beam at JLab, and a special arrangement of spectrometer magnets to measure the hypernuclear spectrum from C and ⁷Li targets using the (e,e'K⁺) reaction. The _Λ¹²B spectrum found in this investigation was previously published, but is reported here in more detail, with improved resolution. In addition, the results of a _Λ⁷He spectrum also obtained in the experiment, are shown. This latter spectrum indicates the need for a more detailed few-body calculation of the hypernucleus and the reaction process. The success of the experiment demonstrates the potential of the (e,e'K⁺) reaction for high resolution spectroscopy of hypernuclear spectra.

Porous polyethylene oxide–b–polyfluorooctylmethacrylate (PEO-b-PFOMA) diblock copolymer films were drop cast onto substrates from Freon (1,1,2-trichlorotrifluoroethane) in a humid atmosphere. The pores in the films exhibit long range hexagonal order in some cases, depending on the PFOMA-to-PEO molecular weight ratio. Films with the best ordered pores were formed with PFOMA-to-PEO ratios of 70  kDa:2kDa. The pores in the polymer films derive from water droplets that condense as Freon evaporates. The polymer stabilizes the water droplets, or “breath figures,” which act as an immiscible template that molds the porous film. Increased polymer hydrophobicity reduces the water wettability of the air/Freon interface, which in turn decreases water droplet nucleation, thus influencing the final pore size and spatial order in the polymer films. We describe how water droplet nucleation influences the final pore size and packing order in the polymer films.

Artificial PbTiO₃/SrTiO₃ superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO₃ to SrTiO₃ was changed. For PbTiO₃ layer thicknesses larger than the 3-unit cell SrTiO₃ thickness used in the structure, the polarization is found to be reduced as the PbTiO₃ thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO₃ to SrTiO₃ of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.

We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12≤Q²≤1.0  GeV². These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q² dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.

The crystal structure and local spontaneous polarization of (BaTiO₃)_m∕(SrTiO₃)_n superlattices are calculated using first-principles density-functional theory. The in-plane lattice constant is constrained to be 1% larger than the SrTiO₃ (ST) substrate to take into account the in-plane expansion observed in recent experiments. The symmetry is lowered to monoclinic space group Cm allowing for polarization along the [110] and [001] directions. The polarization component in the [110] direction is found to develop only in the SrTiO₃ layers and falls to zero in the BaTiO₃ (BT) layers, whereas the polarization in the [001] direction is approximately uniform throughout the superlattice. These findings are consistent with recent experimental data and first-principles results for epitaxially strained BT and ST.

(1×1) and (2×1) reconstructions of the (001) SrTiO₃ surface were studied using the first-principles full-potential linear muffin-tin orbital method. Surface energies were calculated as a function of TiO₂ chemical potential, oxygen partial pressure p_O2and temperature. The (1×1) unreconstructed surfaces were found to be energetically stable for many of the conditions considered. Under conditions of very low oxygen partial pressure the (2×1) Ti₂O₃ reconstruction [Martin R. Castell, Surf. Sci. 505, 1 (2002)] is stable. The question as to why STM images of the (1×1) surfaces have not been obtained was addressed by calculating charge densities for each surface. These suggest that the (2×1) reconstructions would be easier to image than the (1×1) surfaces. The possibility that the presence of oxygen vacancies would destabilise the (1×1) surfaces was also investigated. If the (1×1) surfaces are unstable then there exists the further possibility that the (2×1) DL-TiO₂ reconstruction [Natasha Erdman Nature (London) 419, 55 (2002)] is stable in a TiO₂-rich environment and for p_O2>10⁻¹⁸ atm.

In air, or vacuum environments, liquid polystyrene (PS) thin films (thickness, h<100  nm) supported by SiO_x∕Si substrates are structurally metastable or unstable, depending on film thickness. They rupture and eventually form droplets on the SiO_x∕Si substrates (dewet) due to the influence of destabilizing long-ranged van der Waals dispersion forces. We used scanning force microscopy to examine the structural stability of liquid PS films in the thickness range 5  nm<h<100  nm in liquid and in supercritical carbon dioxide (CO₂) environments. All films in this thickness range were metastable; holes developed throughout the films and over time these holes grew, impinged, and eventually formed droplets. The rate of destabilization is controlled by three factors: film thickness, temperature, and CO₂ pressure (which dictates CO₂ volume fraction in the films). Calculations of the effective interface potentials suggest that the energy barrier for nucleation and growth of holes in CO₂ is larger than that in air, and in the limit of vanishingly low PS volume fraction the films should be stable.

The glass transition temperature is known to increase with decreasing film thickness h for sufficiently thin poly(methyl methacrylate) films supported by silicon oxide substrates. We show that this system undergoes a CO₂ pressure-induced devitrification transition, P_g, which is film thickness dependent, P_g(h)=ΔP_g+P_g^bulk. P_g^bulk is the bulk glass transition and ΔP_g can be positive or negative depending on T and P. The phenomenon of retrograde vitrification, wherein the polymer exhibits a rubbery-to-glassy-to-rubbery transition upon changing temperature isobarically, is also shown to occur in this system and it is film thickness dependent.

High-energy, cw electron beams at new accelerator facilities allow electromagnetic production and precision study of hypernuclear structure, and we report here on the first experiment demonstrating the potential of the (e,e^′K⁺) reaction for hypernuclear spectroscopy. This experiment is also the first to take advantage of the enhanced virtual photon flux available when electrons are scattered at approximately zero degrees. The observed energy resolution was found to be ≈900  keV for the _Λ¹²B spectrum, and is substantially better than any previous hypernuclear experiment using magnetic spectrometers. The positions of the major excitations are found to be in agreement with a theoretical prediction and with a previous binding energy measurement, but additional structure is also observed in the core excited region, underlining the future promise of this technique.

We report the finding of an electron-paramagnetic-resonance (EPR) center, labeled O4, which contains cobalt on a site in the diamond lattice. O4 is observed at low temperatures (<30 K) in high temperature and pressure synthetic diamonds grown with a cobalt containing metal-solvent catalyst after a high-temperature anneal at 1800 °C. The center has S=1/2, monoclinic I symmetry and a hyperfine splitting from a nucleus with I=7/2. Analysis of the measured hyperfine interaction in terms of 3d⁵ gives the unpaired electronic wave function on the cobalt atom, and correctly predicts the measured g matrix. The wide linewidth of the EPR absorption suggests a small hyperfine interaction arising from a nearby nitrogen atom. Based on this evidence, a model for O4 is presented in terms of a CoN complex.

A search for ν_μ→ν_e oscillations has been conducted at the Los Alamos Meson Physics Facility using ν_μ from π⁺ decay in flight. An excess in the number of beam-related events from the ν_e C→e^- X inclusive reaction is observed. The excess is too large to be explained by normal ν_e contamination in the beam at a confidence level greater than 99%. If interpreted as an oscillation signal, the observed oscillation probability of (2.6±1.0±0.5)×10^-3 is consistent with the previously reported ν̅ _μ→ν̅ _e oscillation evidence from LSND.

A search for ν_μ→ν_e oscillations has been conducted with the LSND apparatus using ν_μ from π⁺ decay in flight. Two analyses observe a total of 40 beam-on high-energy (60–200 MeV) electron events consistent with the ν_e C→e^- X inclusive reaction. This number is significantly above the 21.9±2.1 events expected from the ν_e contamination in the beam and the beam-off background. If interpreted as an oscillation signal, the observed oscillation probability of (2.6±1.0±0.5)×10^-3 is consistent with the previously reported ν̅ _μ→ν̅ _e oscillation evidence from LSND.

We have observed a clear peak below the Σ⁺-production threshold in the ⁴He(K^-,π^-) reaction at 600 MeV/c and θ_Kπ  =  4^∘. This is confirmation of the existence of the bound state of _Σ⁴He, which was reported in the ⁴He(stoppedK^-,π^-) reaction. As in the case of stopped kaons, no such peak was found in the ⁴He(K^-,π⁺) spectrum. Quantitatively reliable parameters for this level have been established. The binding energy and the width of the bound state are 4.4±0.3(stat)±1(syst) MeV and 7.0±0.7(stat)_-0.0^+1.2(syst) MeV, respectively.

Charged current scattering of ν_μ on ¹²C has been studied using a π⁺ decay-in-flight ν_μ beam at the Los Alamos Meson Physics Facility. A sample of 56.8 ±9.6 events satisfying criteria for the exclusive reaction ¹²C(ν_μ,μ^-)¹²N_g.s. was obtained using a large liquid scintillator neutrino detector. The observed flux-averaged cross section (6.6±1.0±1.0)×10^-41 cm² agrees well with reliable theoretical expectations. A measurement was also obtained for the inclusive cross section to all accessible ¹²N states ¹²C(ν_μ,μ^-)X. This flux-averaged cross section is (11.2±0.3±1.8)×10^-40 cm², which is approximately half of that given by a recent continuum random-phase approximation calculation.

Charged current reactions of ν_e on ¹²C have been studied using a μ⁺ decay-at-rest ν_e beam from the Los Alamos Meson Physics Facility. More than 500 events from the exclusive reaction ¹²C(ν_e,e^-)¹²N_g.s. were measured in a large Liquid Scintillator Neutrino Detector (LSND). The observed energy dependence of the cross section and the angular distribution of the outgoing electron agree well with theoretical expectations. Measurements are also presented for inclusive transitions to ¹²N excited states, ¹²C(ν_e,e^-)¹²N^* and compared with theoretical expectations. Results are consistent with a recent continuum random phase approximation (CRPA) calculation.

The attenuation cross sections measured in transmission experiments at the alternating-gradient synchrotron for K⁺ on ⁶Li, C, Si, and Ca at p_L = 488, 531, 656, and 714 MeV/c are reanalyzed in order to derive total (σ_T) and reaction (σ_R) cross sections. The effect of plural (Molière) scattering is properly accounted for, leading to revised values of σ_T. We demonstrate the model dependence of these values, primarily due to the choice of K⁺ nuclear optical potential used to generate the necessary Coulomb-nuclear and nuclear elastic corrections. Values of σ_R are also derived, for the first time, from the same data and exhibit a remarkable degree of model independence. The derived values of σ_T and σ_R exceed those calculated by the first-order tρ optical potential for C, Si, and Ca, but not for ⁶Li, particularly at 656 and 714 MeV/c where the excess is 10–25%. Relative to ⁶Li, this excess is found to be nearly energy independent and its magnitude of 15–25% is not reproduced by any nuclear medium effect studied so far.

A search for ν¯_μ→ν¯_e oscillations has been conducted at the Los Alamos Meson Physics Facility using ν¯_μ from μ⁺ decay at rest. The ν¯_e are detected via the reaction ν¯_ep→e⁺n, correlated with the 2.2 MeV γ from np→dγ. The use of tight cuts to identify e⁺ events with correlated γ rays yields 22 events with e⁺ energy between 36 and 60 MeV and only 4.6±0.6 background events. The probability that this excess is due entirely to a statistical fluctuation is 4.1×10^-8. A χ² fit to the entire e⁺ sample results in a total excess of 51.0_-19.5^+20.2±8.0 events with e⁺ energy between 20 and 60 MeV. If attributed to ν¯_μ→ν¯_e oscillations, this corresponds to an oscillation probability (averaged over the experimental energy and spatial acceptance) of (0.31±0.12±0.05)%. © 1996 The American Physical Society.