Band-structure modulation in carbon nanotube T junctions

We show that the band structure of metallic carbon nanotubes can be dramatically altered by the local electrostatic field. This is realized by coupling chemically functionalized nanotubes to form T junctions. The bar of the T is the conducting channel and the leg of the T is used for local gating. Transport measurements reveal that an energy gap develops upon application of a local electric field in both devices with or without linker molecules at the junction. We propose that the mechanism of the band gap modulation in the T junctions without linker molecules is the field effect, with the linker molecules introducing additional electromechanical and chemical effects.     

Four-fermion production at $\gamma\gamma$ colliders: 1. Lowest-order predictions and anomalous couplings

We have constructed a Monte Carlo generator (the corresponding FORTRAN code can be obtained from the authors upon request) for lowest-order predictions for the processes and in the standard model and extensions thereof by an effective coupling as well as anomalous triple and quartic gauge-boson couplings. Polarization is fully supported, and a realistic photon beam spectrum can be taken into account. For the processes all helicity amplitudes are explicitly given in a compact form. The presented numerical results contain, in particular, a survey of cross sections for representative final states and their comparison to results obtained with the program package Whizard/Madgraph. The impact of a realistic beam spectrum on cross sections and distributions is illustrated. Moreover, the size of various contributions to cross sections, such as from weak charged- or neutral-current, or from strong interactions, is analyzed. Particular attention is paid to W-pair production channels where we investigate the impact of background diagrams, possible definitions of the W-pair signal, and the issue of gauge-invariance violation caused by finite gauge-boson widths. Finally, the effects of triple and quartic anomalous gauge-boson couplings on cross sections as well as the possibility to constrain these anomalous couplings at future colliders are discussed.Received: 19 May 2004, Published online: 16 July 2004     

Phase stability and non-stoichiometry in M-phase solid solutions in the system LiO0.5-NbO2.5-TiO2

Phase relations at 1050°C have been determined for M-phase solid solutions in the LiO_0.5-NbO_2.5-TiO₂ ternary phase system by the quench method. Rietveld analysis has been used to help determine phase boundaries and to study structure composition relations. The M-phases have trigonal structures based on intergrowth of corundum-like layers, [Ti₂O₃]²⁺, with slabs of (N−1) layers of LiNbO₃-type parallel to (0001). Ideal compositions are defined along the pseudobinary join LiNbO₃-Li₄Ti₅O₁₂ by the homologous series formula Li_NNb_N−4Ti₅O_3N, N?4. Homologues with N?10 lie to the low-lithia side of the LiNbO₃-Li₄Ti₅O₁₂ join and show extended single-phase solid solution ranges separated by two-phase regions. The composition variations along the solid solutions are controlled by a major substitution mechanism, Li⁺+3Nb⁵⁺↔4Ti⁴⁺, coupled with a minor substitution 4Li⁺↔Ti⁴⁺+3□, where □=vacancy. The latter substitution results in increasing deviations from the stoichiometric compositions A_2N+1O_3N with increasing Ti substitution. The non-stoichiometry can be reduced by re-equilibration at lower temperatures. Expressions have been developed to describe the compositional changes along the solid solutions.     

Cu2Sc2Ge4O13, a novel germanate isotypic with the quasi-1D compound Cu2Fe2Ge4O13 between 100 and 298 K

The germanate compound Cu₂Sc₂Ge₄O₁₃ has been synthesized by solid-state ceramic sintering techniques between 1173 and 1423 K. The structure was solved from single-crystal data by Patterson methods. The title compound is monoclinic, a=12.336(2) Å, b=8.7034(9) Å, c=4.8883(8) Å, β=95.74(2), space group P2₁/m, Z=4. The compound is isotypic with Cu₂Fe₂Ge₄O₁₃, described very recently. The structure consists of crankshaft-like chains of edge-sharing ScO₆ octahedra running parallel to the crystallographic b-axis. These chains are linked laterally by [Cu₂O₆]⁸⁻ dimers forming a sheet of metal-oxygen-polyhedra within the a-b plane. These sheets are separated along the c-axis by [Ge₄O₁₃]¹⁰⁻ units. Cooling to 100 K does not alter the crystallographic symmetry of Cu₂Sc₂Ge₄O₁₃. While the b, c lattice parameter and the unit cell volume show a positive linear thermal expansion (α=6.4(2)×10⁻⁶, 5.0(2)×10⁻⁶ and 8.3(2)×10⁻⁶ K⁻¹ respectively), the a lattice parameter exhibits a negative thermal expansion (α=−3.0(2)×10⁻⁶ K⁻¹) for the complete T-range investigated. This negative thermal expansion of a is mainly due to the increase of the Cu-Cu interatomic distance, which is along the a-axis. Average bond lengths remain almost constant between 100 and 298 K, whereas individual ones partly show both significant shortages and lengthening.     

The use of synchrotron X-ray scattering coupled with in situ mechanical testing for studying deformation and structural change in isotactic polypropylene

The mechanical behaviour of semi-crystalline polymers is greatly influenced by the properties of the crystalline and the amorphous phases. As a result this topic has been the subject of extensive research. However, to date, a comprehensive relationship between the structure and mechanical properties for semi-crystalline polymers has yet to be established. This present study concerns the commissioning of a novel method for in situ data collection during the deformation of polymers. This involves the combination of three different techniques into a single experiment, namely tensile testing, synchrotron radiation wide angle X-ray scattering, and optical microscopy. For this current investigation, three isotactic polypropylene samples have been studied, produced using different thermal treatments. This enables the influence of thermal treatment on the mechanical properties and crystallographic structure to be assessed. The results indicate that tensile properties are influenced by thermal treatment via the relative fraction of -phase material in the sample. As the temperature increases at which thermal treatment takes place, iPP ductility decreases due to the greater rigidity of the increasing -phase content. Differences in crystal strain between the different iPP crystal phases are also observed although the reasons for such differences remain unclear.     

Direct measurement of entropic forces induced by rigid rods

We present the first direct depletion potential measurements for a single colloidal sphere close to a wall in a suspension of rigid colloidal rods. Since all particle interactions are as good as hard-core-like, the depletion potentials are of entirely entropic origin. We developed a density functional theory approach that accurately accounts for this experimental situation. The depletion potentials calculated for different rod number densities are in very good quantitative agreement with the experimental results.     

Measurements of electron-transfer rates of charge-storage molecular monolayers on Si(100). Toward hybrid molecular/semiconductor information storage devices

Redox kinetics were measured for two electroactive molecules attached to Si(100) surfaces, a ferrocene (Fc-BzOH) and a Zn(II) trimesitylporphyrin (Por-BzOH). Each molecule was derivatized with a benzyl alcohol linker for attachment to the Si surface via the formation of a Si-O bond. A complete protocol was developed for the preparation of stable Si(100) surfaces derivatized with the electroactive molecules. The redox-kinetic measurements were performed on the resulting Fc-BzOH and Por-BzOH monolayers to probe (1) the rate of electron transfer (k0) for oxidation in the presence of applied potentials and (2) the rate of charge dissipation after the applied potential is disconnected (in the form of a charge-retention half-life t1/2). The k0 values for the two types of monolayers were found to be similar to one another as were the t1/2 values. Perhaps more importantly, the electron-transfer rates for both the Fc-BzOH and the Por-BzOH monolayers differ from the charge-dissipation rates by approximately 6 orders of magnitude and are strongly dependent on the surface concentration of the electroactive species. For the Por-BzOH monolayers on Si(100), the k0 and t1/2 values and their trends as a function of surface coverage were determined to be similar to those previously measured for the analogous thiol-derivatized molecule assembled on Au(111). In contrast, the Fc-BzOH monolayers on Si(100) were found to exhibit much slower electron-transfer and charge-dissipation rates than those in the corresponding thiol-Au(111) case. Two alternative hypotheses are advanced to explain both the diminution in rates with increased surface coverage and the contrasting behavior with the analogous thiols on Au, one based on space-charge effects at the monolayer-solution interface, and a second relying on changes in distance of the redox centers from the surface as modulated by the orientation of the linking chains. Collectively, the ability to prepare and study stable, electroactive molecular media on Si(100) is likely to be key in the development of hybrid molecular/semiconductor devices.     

Direct continuous supercritical fluid extraction as a novel method of wine analysis. Comparison with conventional indirect extraction and implications for wine variety identification

Direct supercritical fluid extraction (SFE) of wines with carbon dioxide was compared to SFE of the sorbent used for solid-phase extraction of the same wine samples (SPE-SFE). Compared to SPE-SFE, the direct SFE results in amore specific and representative gas chromatographic fingerprint of the wine sample. The multivariate statistical processing of the direct SFE-GC data provides a clear-cut and sharp discrimination among the individual wine varieties while the discrimination based on the SPE-SFE-GC data is relatively poor. This finding reflects the adverse effects of additional analyte-sorbent interactions and sorption/desorption steps involved in SPE-SFE.     

Maximally aligned states in 99Ag

Excited states of ⁹⁹Ag were populated via the ⁵⁰Cr + ⁵⁸Ni (261 MeV) reaction using the NORDBALL detector array equipped with charged-particle and neutron detector systems for reaction channel separation. On the basis of the measured γγ-coincidence relations and angular distribution ratios a significantly extended level scheme has been constructed up to E _x ∼ 7.8 MeV and I = 35/2. The experimental results were described within the framework of the shell model. Candidates for states fully aligned in the πg _9/2 ^-3ν(d _5/2, g _7/2)² valence configuration space were found at 4109 and 6265 keV. Received: 18 June 2002 / Accepted: 11 October 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: sohler@atomki.hu Communicated by J. ?yst?     

On support points of univalent functions and a disproof of a conjecture of Bombieri

We consider the linear functional for on the set of normalized univalent functions in the unit disk and use the result to disprove a conjecture of Bombieri.