Jeff Baker

Isomeric Character of the Lowest Observed 4+ State in S 44

Previous experiments observed a 4^{+} state in the N=28 nucleus ^{44}S and suggested that this state may exhibit a hindered E2-decay rate, inconsistent with being a member of the collective ground state band. We populate this state via two-proton knockout from a beam of exotic ^{46}Ar projectiles and measure its lifetime using the recoil distance method with the GRETINA γ-ray spectrometer. The result, 76(14)_{stat}(20)_{syst}  ps, implies a hindered transition of B(E2;4^{+}→2_{1}^{+})=0.61(19) single-particle or Weisskopf units strength and supports the interpretation of the 4^{+} state as a K=4 isomer, the first example of a high-K isomer in a nucleus of such low mass.

Strain relaxation in thin films of Cu grown on Ni(0 0 1)

Surface X-ray diffraction and kinematical model calculations are used to determine the strain relaxation of embedded wedges with internal (1 1 1) facets formed in thin Cu films when grown on Ni(0 0 1). We show the wedges to be inhomogenously strained with a large lateral relaxation near the Cu/Ni interface which decays rapidly away from the interface.

High resolution helium atom scattering from an epitaxially grown one monolayer film of KCN on KBr(001)

Abstract High resolution helium atom diffraction and single phonon inelastic scattering experiments were performed on a single monolayer KCN film epitaxially grown onto a KBr(001) substrate. Although the cleaved surface of single crystal KCN appears disordered, the one monolayer film was stabilized by the substrate. The measurements from this film in the 〈100〉 direction gave dispersion curves with a Rayleigh mode, a dispersionless mode near 12 meV, a possible crossing mode, and a suggestion of a weak interface mode near 2 meV. The Debye temperature of the film was found to be 127 K, which was very close to that found for the cleaved single crystal KCN. A kink in the Debye-Waller factor near 150–160 K may be associated with a surface phase transition.

Surface lattice dynamics of KBr epitaxially grown on RbCl(0 0 1)

Abstract High resolution He atom scattering experiments were performed on epitaxially grown layers of KBr on a RbCl(0 0 1) substrate for coverages of one, two, and three monolayers. The in situ growth of the KBr was monitored by following the intensity of the specularly scattered He beam versus coverage. The inelastic scattering was measured for each deposited layer to determine the surface phonon dispersion curves as a function of coverage in both the Γ M ¯ and Γ X ¯ high symmetry directions of the surface Brillouin zone. Fits to the data from the KBr/RbCl system, which is nearly perfectly lattice matched, were obtained from a shell model calculation whose parameters are found to provide not only a good match to the measured dynamics of these deposited films, but in addition, to the dispersion of bulk and clean surfaces of KBr, RbCl, KCl, and RbBr.

Continuous Monte Carlo Simulation of Surfaces and of Interfaces of Mismatched Crystals

The relaxation phenomena encountered when an otherwise perfect crystal is terminated either by vacuum - i.e. at a surface, or by another crystal i.e. at an interface, is the topic of this study. The interface could be a grain boundary between two differently oriented crystals of the same material or between two different materials with different structures and/or lattice constants. In both cases there is an often incommensurate and large mismatch between the optimal atomic positions at the interface. The energy cost of this can be minimized by relaxing the atoms from the perfect positions in a region near the interface and sometimes in addition by a rotation of the adjacent crystals by a small epitaxial angle away from high symmetry relative orientation.