Studies of large scale unstable growth formed during GaAs(001) homoepitaxy

Atomic force and scanning tunneling microscopy studies have been performed on GaAs(001) films grown by molecular beam epitaxy. Multilayered mounds are seen to evolve when the growth conditions favor island nucleation. As the epilayer thickness is increased, these features grow in all dimensions but the angle of inclination remains approximately constant at 1°. The mounding does not occur on surfaces grown in step-flow. We propose that the multi-layered features are due to an unstable growth mode which relies on island nucleation and the presence of a step edge barrier.     

Hydroformylation in fluorous solvents

The hydroformylation of long chain alkenes under fluorous biphase conditions and in neat perfluorocarbon solvents is reviewed. Special emphasis is placed upon the authors' evaluation of the steric and electronic influence of perfluoroalkylation on catalyst activity, regioselectivity and retention in the fluorocarbon phase.     

Sol-Gel Particulate Float Process to Make Vitreous Silica Bodies

Particulate sol-gel technology uses larger particles than that of alkoxide-derived gels and provides larger pore sizes in the gelled object. This allows relatively rapid drying and fabrication of large rods or tubular shapes. However, the formation of more complex or flat shapes, which require extensive surface contact with a mold, is more difficult. The shrinkage during drying, with the significantly greater stress of surface friction due to adhesion, frequently leads to cracking.We have demonstrated a solution to this problem by floating the gel on the surface of a dense liquid. Dry silica panels up to 28 × 40 × 0.7 cm³ were prepared in this way. The use of patterning molds allowed the fabrication of more sophisticated shapes. These bodies were sintered to transparent vitreous silica articles of near net dimensions.     

Formation, isolation and characterization of an AB-biaryl atropisomer of oritavancin

Oritavancin is a semi-synthetic glycopeptide antibiotic which is structurally related to vancomycin. When oritavancin bisphosphate is dried in vacuo with heat, a new compound forms. This new compound is stable only in the solid state and reverts to oritavancin in solution. Highly enriched samples of this compound were obtained by preparative HPLC and the structure of this compound was elucidated by using one and two-dimensional (¹H and ¹³C) NMR spectroscopy in conjunction with computer-assisted molecular modeling. It has been determined that oritavancin adopts a conformation similar to that of vancomycin in solution, while the new compound is the unnatural R-AB-biaryl atropisomer of oritavancin. This is the first observation and isolation of an AB-biaryl atropisomer in an intact member of the vancomycin family of glycopeptide antibiotics.     

Ring resonators with N × M couplers

Multiple-zero multiple-pole optical filter transfer functions may be implemented more efficiently in an integrated optics architecture if higher order N × M optical couplers are utilized. For example, a coherent ring resonator made from two 3×3 couplers offers some advantages over the three mirror Fabry-Perot etalon, which is its analog. To this end we develop the formalism for obtaining the transfer functions and scattering matrices of ring resonators made from two N × M couplers. We then present a methodology for analyzing serial and parallel systems of N × M optical coupler ring resonators.     

Time‐integrated energy domain measurements with synchrotron radiation

A new time integrated method for the study of resonant nuclear scattering of synchrotron radiation in the forward direction or in Bragg directions is introduced. This method gives in principle similar information as the well known time differential method. A brief comparison of both methods is presented. The idea is to excite coherently the nuclei incorporated in two absorbers, one moving with respect to the other. The fields radiated by the nuclei from both absorbers interfere and each time the nuclear energy in one absorber matches, by Doppler modulation, the nuclear energy of the other, an extremum in the time integrated intensity is observed. The results of the first experiments at the Advanced Photon Source at the Argonne National Laboratory will be presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

The completely positive and doubly nonnegative completion problems

We prove the following. Let G be an undirected graph. Every partially specified symmetric matrix, the graph of whose specified entries is G and each of whose fully specified submatrices is completely positive (equal to BB^T for some entrywise nonnegative matrix B), may be completed to a completely positive matrix if and only if G is a block-clique graph (a chordal graph in which distinct maximal cliques overlap in at most one vertex). The same result holds for matrices that are doubly nonnegative (entrywise nonnegative and positive semidefinite).