Modulational instability and exact soliton solutions for a twist-opening model of DNA dynamics

We study the nonlinear dynamics of DNA which takes into account the twist-opening interactions due to the helicoidal molecular geometry. The small amplitude dynamics of the model is shown to be governed by a solution of a set of coupled nonlinear Schrödinger equations. We analyze the modulational instability and solitary wave solution in the case. On the basis of this system, we present the condition for modulation instability occurrence and attention is paid to the impact of the backbone elastic constant K. It is shown that high values of K extend the instability region. Through the Jacobian elliptic function method, we derive a set of exact solutions of the twist-opening model of DNA. These solutions include, Jacobian periodic solution as well as kink and kink-bubble solitons.     

Photoproduction of η -mesons off nuclei for Eγ ? 2.2 GeV

The photoproduction of η -mesons off ¹²C , ⁴⁰Ca , ⁹³Nb , and ^nat Pb nuclei has been measured with a tagged photon beam with energies between 0.6 and 2.2GeV. The experiment was performed at the Bonn ELSA accelerator with the combined setup of the Crystal Barrel and TAPS calorimeters. It aimed at the in-medium properties of the S ₁₁(1535) nucleon resonance and the study of the absorption properties of nuclear matter for η -mesons. Careful consideration was given to contributions from ηπ final states and secondary production mechanisms of η -mesons, e.g. from inelastic πN reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross-section for η -mesons is constant over a wide range of the η momentum. The comparison of the excitation functions to data off the deuteron and to calculations in the framework of a BUU model show no unexplained in-medium modifications of the S ₁₁(1535) .     

Current control in periodically driven quantum dots using nonadiabatic double crossing

The purpose of this Letter is to propose a method for controlling electron currents on quantum dots driven by an oscillating electric field. The effects of nonadiabatic transition on time-averaged currents are theoretically studied in dot systems where energy levels exhibit a double crossing within one period of the driving field. The current is enhanced or suppressed as a result of the constructive or destructive interference between different transition paths at a double crossing. The current also depends on the number of dots because of the presence of dot-lead coupling.     

Line strength and collisional broadening studies of hydrogen sulphide in the 1.58 μm region using diode laser spectroscopy

High resolution diode laser spectroscopy has been applied to the detection of hydrogen sulphide at ppm levels utilizing different transitions within the region of the ν ₁+ν ₂+ν ₃ and 2ν ₁+ν ₂ combination bands around 1.58 μm. Suitable lines in this spectral region have been identified, and absolute absorption cross sections have been determined through single-pass absorption spectroscopy and confirmed in the Doppler linewidth regime using cavity enhanced absorption spectroscopy (CEAS). The desire for a sensitive system potentially applicable to H₂S sensing at atmospheric pressure has led to an investigation on suitable transitions using wavelength modulation spectroscopy (WMS). The set-up sensitivity has been calculated as 1.73×10⁻⁸ cm⁻¹ s^1/2, and probing the strongest line at 1576.29 nm a minimum detectable concentration of 700 ppb under atmospheric conditions has been achieved. Furthermore, pressure broadening coefficients for a variety of buffer gasses have been measured and correlated to the intermolecular potentials governing the collision process; the H₂S–H₂S dimer well depth is estimated to be 7.06±0.09 kJ mol⁻¹.     

Open-system density matrix description of an STM-driven atomic switch: H on Si(100)

Previous experiments indicate that an STM (scanning tunnelling microscope) can be used to switch a hydrogen atom at a partially hydrogen-covered Si(100)-2×1 surface, from one Si atom of a Si dimer to a neighbouring, empty Si site [U.J. Quaade et al., Surf. Sci. 415, L1037, 1998]. It has been suggested that the switching occurs via a transient positive ion resonance state. In an earlier paper, we have examined the switching process for the “above threshold” regime when the bias is large enough to directly populate the positive ion resonance. In the present paper we study the “below threshold” regime instead, where the switching is more appropriately modelled as a ladder climbing over the barrier, in the ground electronic state. For this purpose we solve the Liouville–von Neumann equation in Lindblad form, describing a switching H atom on a Si dimer. STM-induced transition rates between vibrational levels are estimated from cluster calculations, assuming contributions both from a dipole and a resonance scattering mechanism. Vibrational relaxation is also included, as well as finite temperature and field effects. The switching rate in a current regime of about 1 to 10 nA scales highly non-linearly with current, and it is found to be governed by vibrational “ladder climbing” and subsequent tunnelling through the top of the ground state barrier. Multi-phonon processes also play a role. As a result of tunnelling, pronounced isotope effects are observed when replacing H with D. It is further argued that resonance-mediated inelastic scattering dominates over dipole excitation, and that the STM switch is stable also at room temperature.     

<Emphasis Type="Italic">Trans</Emphasis> to <Emphasis Type="Italic">cis</Emphasis> isomerization of an azobenzene derivative on a Cu(100) surface

We biosynthesized iron oxide nanoparticles with four different pH in the solution to see its influence in the oxides obtained. This method allowed for generating aggregates of 1–10 nm, and under optimal conditions (pH=10) we could control the size in the range of 1–4 nm. With the purpose to analyze the biosynthesized iron oxide clusters we employed electron transmission microscopy techniques. Because the biosynthetic method with alfalfa has been used, the presence of the biomass, which is dense and within which are contained the nanoparticles, makes their observation difficult. Using the HAADF (Z contrast) technique it is possible to locate the nanoparticles, which are then characterized using EDS and HRTEM. PACS 61.46.-w; 68.37.Lp; 81.07.-b; 81.16.Be     

Bistability of single 1,5 cyclooctadiene molecules on Si(001)

The adsorption and current-induced bistability of single 1,5 cyclooctadiene molecules on Si(001) were studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). After a dosage of ≈0.05 L at room temperature followed by cooling to the measuring temperature of 7 K, we find that the cyclic alkene molecule preferably adsorbs in the bridge structure with both C=C double bonds reacting with two adjacent Si dimers via [2+2] cycloaddition reaction. The time-dependent current measured upon tunneling through the adsorbed molecule at fixed STM tip height displays a switching between two current levels with the same mean residence time in each level. Higher bias and/or reduced tip height—and therefore higher current—increase the switching rate, suggesting that the reversible switching is due to inelastic electron tunneling. The observed bistability is interpreted as a dynamic interconversion between two degenerate conformations of the adsorbed molecule.     

Stabilization of catalytically active gold species in Fe-modified zeolites

The influence of iron additive on redox, electronic and catalytic properties of gold incorporated into zeolite catalysts has been studied by means of TEM, XPS, XRD, TPR, ICP and AES. The interaction of gold with iron modifier was observed in Y-zeolites and mordenites with different cations and method of Fe incorporation (impregnation or ion exchange). This interaction leads to mutual influence on redox properties of Fe and Au ionic species and facilitates their reduction. Limited diffusion of Au precursor after Fe species deposition in narrow mordenite channels does not permit to incorporate Au in adequate concentration, while in large super-cage of Y-zeolites this limitation is absent. The structure of Y-zeolites favors formation of active gold species. Catalytic tests in CO oxidation show that Fe additive stabilizes the gold active species active at low-temperature (partly charged clusters) and makes them insensitive to redox treatments.