Metastable tight knots in a wormlike polymer

Based on an estimate of the knot entropy of a wormlike chain we predict that the interplay of bending energy and confinement entropy will result in a compact metastable configuration of the knot that will diffuse, without spreading, along the contour of the semiflexible polymer until it reaches one of the chain ends. Our estimate of the size of the knot as a function of its topological invariant (ideal aspect ratio) agrees with recent experimental results of knotted dsDNA. Further experimental tests of our ideas are proposed.     

Development of Ferromagnetic Materials Containing Co2P,Fe2P Phases from Organometallic Dendrimers Precursors

The development of synthesis methods to access advanced materials, such as magnetic materials that combine multimetallic phosphide phases, remains a worthy research challenge. The most widely used strategies for the synthesis of magnetic transition metal phosphides (TMPs) are organometallic approaches. In this study, Fe-containing homometallic dendrimers and Fe/Co-containing heterometallic dendrimers were used to synthesize magnetic materials containing multimetallic phosphide phases. The crystalline nature of the nearly aggregated particles was indicated for both designed magnetic samples. In contrast to heterometallic samples, homometallic samples showed dendritic effects on their magnetic properties. Specifically, saturation magnetization (M_s) and coercivity (H_c) decrease as dendritic generation increases. Incorporating cobalt into the homometallic dendrimers to prepare the heterometallic dendrimers markedly increases the magnetic properties of the magnetic materials from 60 to 75 emu/g. Ferromagnetism in homometallic and heterometallic particles shows different responses to temperature changes. For example, heterometallic samples were less sensitive to temperature changes due to the presence of Co₂P in contrast to the homometallic ones, which show an abrupt change in their slopes at a temperature close to 209 K, which appears to be related to the Fe₂P ratios. This study presents dendrimers as a new type of precursor for the assembly of magnetic materials containing a mixture of iron- and cobalt-phosphides phases with tunable magnetism, and provides an opportunity to understand magnetism in such materials.     

DNA capture into a nanopore: interplay of diffusion and electrohydrodynamics

We present a detailed analysis of the process of voltage driven capture of DNA molecules by nanopores. We show that ionic current generates a nonuniform electric field that acts on both the DNA and on its counterions and that the response of DNA to the electric field is affected by its electroosmotic coupling to the mobile counterions. We calculate the voltage and molecular mass dependence of the radius of capture and of the capture rate in the diffusion limited regime. We argue that electroosmotic flow through the DNA coil is suppressed in the vicinity of the pore and present a tentative estimate of the capture rate in the barrier limited regime.     

Application of ion-impact energy measurement to electrospray ionization mass spectrometry of proteins and protein mixtures

We have used a normal metal-insulator-superconductor (NIS) microcalorimeter to measure the impact energy of protein ions produced by electrospray ionization (ESI) in a magnetic-sector mass spectrometer (MS). We have used these measurements to resolve spectral ambiguities and to analyze protein mixtures. Energy measurement may be useful for the direct MS analysis of complex biopolymer mixtures that normally would confound ESI-MS deconvolution algorithms.     

Enhancement of weak anti-localization signatures in the magneto-resistance of bismuth anti-dot thin films

Anti-dot array thin films of bismuth were prepared by the e-beam deposition of this semi-metal on nano-porous substrates. The magneto-resistance measurements of bismuth thin films deposited under identical conditions on various substrates displayed signatures from both classical magneto-resistance and weak anti-localization effects. The relative intensity of the two effects could be altered by the choice of the substrate, with the anti-dot array morphology suppressing the classical magneto-resistance, and enhancing the weak anti-localization contribution to the measured magneto-resistance. As a result, the weak anti-localization effect can be traced to higher magnetic field strengths and higher temperatures than is possible in non-patterned films, improving the accuracy of the parameters extracted from the data. PACS 73.20.Fz; 73.23-b; 73.61.At     

The influence of CO on the surface plasmon absorption band of small silver clusters (D≤20 Å)

The surface plasmon absorption band of small Ag particles (diameter≤100 Å) in the gasphase as well as for clusters embedded in noble gas matrices is known to shift to higher energies and to broaden with decreasing diameter. This size effect depends sensitively on the microscopic surface properties of the clusters, which can be influenced by the interaction with the embedding matrix. Previous experiments with silver clusters in the size range 20 Å≤D≤100 Å proved that for SiO₂ and CO the matrix influence is strong enough to supersede the intrinsic size effect as observed in the noble gas matrices. The investigation of the influence of a CO matrix on smaller clusters with D≤20 Å reveals a novel feature: the position as well as the width of the surface plasmon absorption band become independent of the cluster size.     

Fluorescence spectroscopy of silver clusters formed in rare gas droplets

The Pick Up technique allows continuous formation of high density metal cluster beams in the otherwise hardly accessible size range Me₂ to about Me₅₀. With an apparatus based on the use of cryo-condensation pumps, Ar as inert gas for production of the host clusters by adiabatic expansion and silver as metal we demonstrate how the various source parameters influence the composition and mean size of the resulting cluster distributions. The analysis in the gas phase by means fluorescence spectroscopy allows to conclude that an Ar shell encapsulates the metal clusters. Received 30 November 2000     

Supertori are algebraic curves

Super Riemann surfaces of genus 1, with arbitrary spin structures, are shown to be the sets of zeroes of certain polynomial equations in projective superspace. We conjecture that the same is true for arbitrary genus. Properties of superelliptic functions and super theta functions are discussed. The boundary of the genus 1 super moduli space is determined.Research partially supported by the DOE (DE-AC02-82-ER-40073) and NSF (PHY-85-21588)     

Methylphenylphosphinate complexes with tri- and tetra-positive metal ions

Summary A series of methylphenylphosphinate {mpp; [-O-P-Ph(Me)-O-]^–} complexes of the forms M(mpp)₃ (M = Al, Ga, In, Sc, Y, Dy, Ti, V, Cr and Fe) and M(mpp)₄ (M = Th or U) were made from the appropriate metal chloride and a large excess of MeO(Me)PhP(O) at elevated temperatures. Structures involving linear, ligand-bridged polymeric species are suggested on the basis of the i.r. and magnetic data.