Search Results (13 total)

We analyze the formation and evolution statistics of single-molecule junctions bonded to gold electrodes using amine, methyl sulfide, and dimethyl phosphine link groups by measuring conductance as a function of junction elongation. For each link, the maximum elongation and formation probability increase with molecular length, strongly suggesting that processes other than just metal-molecule bond breakage play a key role in junction evolution under stress. Density functional theory calculations of adiabatic trajectories show sequences of atomic-scale changes in junction structure, including shifts in the attachment point, that account for the long conductance plateau lengths observed.

Substantially more than half of the electromagnetic nuclear physics experiments conducted at the Continuous Electron Beam Accelerator Facility of the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory) require highly polarized electron beams, often at high average current. Spin-polarized electrons are produced by photoemission from various GaAs-based semiconductor photocathodes, using circularly polarized laser light with photon energy slightly larger than the semiconductor band gap. The photocathodes are prepared by activation of the clean semiconductor surface to negative electron affinity using cesium and oxidation. Historically, in many laboratories worldwide, these photocathodes have had short operational lifetimes at high average current, and have often deteriorated fairly quickly in ultrahigh vacuum even without electron beam delivery. At Jefferson Lab, we have developed a polarized electron source in which the photocathodes degrade exceptionally slowly without electron emission, and in which ion back bombardment is the predominant mechanism limiting the operational lifetime of the cathodes during electron emission. We have reproducibly obtained cathode 1/e dark lifetimes over two years, and 1/e charge density and charge lifetimes during electron beam delivery of over 2×10⁵   C/cm² and 200 C, respectively. This source is able to support uninterrupted high average current polarized beam delivery to three experimental halls simultaneously for many months at a time. Many of the techniques we report here are directly applicable to the development of GaAs photoemission electron guns to deliver high average current, high brightness unpolarized beams.

Electronic and structural properties of substitutional group-V donors (N, P, As, Sb) and group-III acceptors (B, Al, Ga, In) in silicon nanocrystals with hydrogen passivation are explored using first-principles calculations based on hybrid density functional theory with complete geometrical optimization. The bonding near the impurity is similar to that found for the impurity in bulk crystalline silicon, with some quantitative differences. The N case shows large local distortions, as it does in the bulk, characteristic of a deep trap. For the other impurities, no evidence is found for a transition to atomic scale localization induced by the small size of the nanocrystal. The chemical trends of the donor and acceptor binding energies and the donor excited state energies in doped nanocrystals are similar to those in the bulk; however, the absolute magnitudes are substantially larger. The increase in the magnitude of the binding energy is mainly due to the quantum confinement effect combined with the reduced screening of the impurity potential in small nanocrystals. The screening of the impurity potential is carefully examined using the self-consistent electrostatic potential from the full calculations. Strong chemical and local-field effects are seen within the radius of the first neighbor bonds to the impurity atom. This explains the large increase in the donor excited state energy level splittings and the relative importance of the central cell contributions to the binding energies. The acceptor and donor orbitals have different atomic character on the impurity site, leading to substantially different acceptor and donor energy level splittings.

Raman spectroscopy demonstrates that ∼5 nm dimension Hf_xZr_1−xO₂ nanocrystals prepared by a nonhydrolytic sol-gel synthesis method are solid solutions of hafnia and zirconia, with no discernable segregation within the individual nanoparticles. Zirconia-rich particles are tetragonal and ensembles of hafnia-rich particles show mixed tetragonal/monoclinic phases. Sintering at 1200 °C produces larger particles (20–30 nm) that are monoclinic. A simple lattice dynamics model with composition-averaged cation mass and scaled force constants is used to understand how the Raman mode frequencies vary with composition in the tetragonal Hf_xZr_1−xO₂ nanoparticles. Background luminescence from these particles is minimized after oxygen treatment, suggesting possible oxygen defects in the as-prepared particles. Raman scattering is also used to estimate composition and the relative fractions of tetragonal and monoclinic phases. In some regimes there are mixed phases, and Raman analysis suggests that in these regimes the tetragonal phase particles are relatively rich in zirconium and the monoclinic phase particles are relatively rich in hafnium.

Precision measurements of the relative analyzing powers of five electron beam polarimeters, based on Compton, Møller, and Mott scattering, have been performed using the CEBAF accelerator at the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory). A Wien filter in the 100 keV beam line of the injector was used to vary the electron spin orientation exiting the injector. High statistical precision measurements of the scattering asymmetry as a function of the spin orientation were made with each polarimeter. Since each polarimeter receives beam with the same magnitude of polarization, these asymmetry measurements permit a high statistical precision comparison of the relative analyzing powers of the five polarimeters. This is the first time a precise comparison of the analyzing powers of Compton, Møller, and Mott scattering polarimeters has been made. Statistically significant disagreements among the values of the beam polarization calculated from the asymmetry measurements made with each polarimeter reveal either errors in the values of the analyzing power or failure to correctly include all systematic effects. The measurements reported here represent a first step toward understanding the systematic effects of these electron polarimeters. Such studies are necessary to realize high absolute accuracy (ca. 1%) electron polarization measurements, as required for some parity violation measurements planned at Jefferson Laboratory. Finally, a comparison of the value of the spin orientation exiting the injector that provides maximum longitudinal polarization in each experimental hall leads to an independent and very precise (better than 10^-4) absolute measurement of the final electron beam energy.

A direct measurement of the helicity dependence of the total photoabsorption cross section on the proton was carried out at MAMI (Mainz) in the energy range 200<E_γ<800 MeV. The experiment used a 4π detection system, a circularly polarized tagged photon beam, and a frozen spin target. The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward spin polarizability γ₀ determined from the data are 226±5(stat)±12(syst) μb and -187±8(stat)±10(syst)×10^-6 fm⁴, respectively, for 200<E_γ<800 MeV.

The helicity dependence of the single pion photoproduction on the proton has been measured in the energy range from 200 to 450 MeV for the first time. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a circularly polarized, tagged photon beam, and a frozen-spin target. The data obtained provide new information for multipole analyses of pion photoproduction and determine the main contributions to the Gerasimov-Drell-Hearn sum rule and the forward spin polarizability γ₀.

The electric form factor of the neutron G_E,n has been measured in the quasifree ²H(e→,e^′n→)p reaction using the 855 MeV polarized cw electron beam of the Mainz Microtron MAMI. The polarization of the scattered neutrons was analyzed in a polarimeter consisting of two walls of plastic scintillators. The precession of the neutron spin in a magnetic field was used for the first time to circumvent the measurement of the effective analyzing power of the polarimeter and the beam polarization. In this way G_E,n could be determined with little model dependence and experimental uncertainties. The result G_E,n(0.34 GeV ²/c²)  =  0.0611±0.0069_stat({+0.0069}{-0.0055})_syst is larger than previously assumed.

To aid fundamental studies on the polarization of electrons in beta decay, measurements were made of the spin dependence in the scattering of 14 MeV electrons from Pb as a function of scattering angle and foil thickness. The experiment made use of a beam of polarized electrons from a strained GaAsP cathode. A simple theoretical model based on plural scattering explains the observed dependence of the analyzing power on foil thickness. The results extrapolated to infinitely thin targets are in excellent agreement with theory if the finite nuclear size is taken into account.

We have measured the magnetic properties of the cluster compound Ni₂₃Se₁₂(PEt₃)₁₃, where PEt₃ is triethyl phosphine, by dc magnetization (1.5–300 K) and ac susceptibility (0.280–4 K). We observe a small, almost temperature-independent, magnetic moment of only ∼2μ_B/cluster indicating the presence of two unpaired spins in the cluster. Despite the large shape anisotropy of the molecule, we find no preferred magnetic axis. We interpret this as the result of delocalization of the valence electrons due to covalent Ni-Se bonding.

We report the magnetic properties of a new class of materials: Ni₉Te₆ⁿ⁺ and Co₆Te₈ⁿ⁺ with n=0, 1, 2. These cluster compounds, which can be charged by chemical means from neutral to 2+, provide a unique and novel way to change the Fermi level. For most charge states we observe quenching of the spin and orbital moments at low temperatures, accompanied by a large value for the temperature independent susceptibility of the ground state. The generic presence of low-energy magnetic excitations in these compounds indicates that these systems exhibit strong electron correlations and form mesoscopic analogs of the mixed-valence–heavy-fermion compounds.

We use transient optical hole burning and photoluminescence to investigate the static and dynamic electronic properties of 32-Å CdSe quantum dots. We observe a number of discrete electronic transitions, resolve LO-phonon progressions, and obtain homogeneous linewidths and electron–LO-phonon couplings. We find that the band-gap luminescence is not from the exciton state, but from a surface trapped state. Rapid (∼160 fs) trapping into these surface states results in long-lived (∼10–100 ns) bleach and induced-absorption features in pump-probe experiments.

Three sizes of CdSe molecular clusters were studied by ⁷⁷Se NMR spectroscopy. The clusters are synthesized via arrested precipitation in a structured medium and are isolated as a molecular solid after modification of their surfaces with covalently attached organic ligands. While one component of the broad ⁷⁷Se NMR line shape corresponds to the bulk material, increasing proportions appear at shifts upfield as a function of decreasing particle size. The line positions and distribution of chemical shifts can be related to a size dependence of the electronic structure and the local chemical environments.