Electrostatic potentials and average electron densities of bioisosteres in methylsquarate and acetic acid

© 2016 Future Science Ltd. Background: The bioisosterism in -CO2H and -C4HO3 is exploited using the quantum theory of atoms in molecules and molecular electrostatic potentials (ESP). Results & discussion: Bioisosteres in methylsquarate and acetic acid, in the neutral/anionic forms, have average electron densities that differ by less than 2% (i.e., ∼0.01 atomic units) while irrespective of the capping group. The topography of the ESP reveals similarities in the case of the neutral species but not in the anionic forms. Conclusion: The nonclassical bioisosteres in methylsquarate and acetic acid have average electron densities that are similar and relatively insensitive to the ionization state (neutral or anionic) or its studied capping group (H, CH3, Cl or phenyl). The ESP reveals similarities in the topography of neutral molecules

Effects of Intense Electric Fields on the Double Proton Transfer in the Watson-Crick Guanine-Cytosine Base Pair

Copyright © 2018 American Chemical Society. The double proton transfer reaction in the guanine-cytosine (GC) base pair is studied, using density functional theory, to understand the chances of mutations under the effect of uniform electric fields in the order of 108 to 109 V m-1. On the basis of potential energy surfaces, reaction Gibbs energies, equilibrium constants, imaginary frequencies, forward and reverse barrier heights, tunneling-corrected rate constants, half-lives of the forward and reverse reactions, percent tautomerization, and Boltzmann distributions, it was found that fields ≥+3.60 × 109 V m-1 facilitate the mutation in the GC base pair and reduce the rectification of point mutations. Fields applied along the double proton transfer in the -x (defined in the C to G direction) direction favor the canonical over the rare tautomers. Tunneling-corrected rate constants of the forward reaction increase exponentially with stronger fields in the -x direction and follow a Gaussian curve for the reverse reaction

Magnetic properties of exchange biased and of unbiased oxide/permalloy thin layers: a ferromagnetic resonance and Brillouin scattering study

Microstrip ferromagnetic resonance and Brillouin scattering are used to provide a comparative determination of the magnetic parameters of thin permalloy layers interfaced with a non-magnetic (Al2O3) or with an antiferromagnetic oxide (NiO). It is shown that the perpendicular anisotropy is monitored by an interfacial surface energy term which is practically independent of the nature of the interface. In the investigated interval of thicknesses (5-25 nm) the saturation magnetisation does not significantly differ from the reported one in bulk permalloy. In-plane uniaxial anisotropy and exchange-bias anisotropy are also derived from this study of the dynamic magnetic excitations and compared to our independent evaluations using conventional magnetometryComment: 7 pages, 6 figures, submited to Journal of Physics: Condensed Matte

On the rotational dynamics of the Rattleback

The Rattleback is a very popular science toy shown to students all over the world to demonstrate the non-triviality of rotational motion. When spun on a horizontal table, this boat-shaped object behaves in a peculiar way. Although the object appears symmetric, the dynamics of its motion seem very asymmetric. When spun in the preferred direction, it spins smoothly, whereas in the other direction it starts to oscillate wildly. The oscillation soon dies out and the rattleback starts to spin in the preferred way. We will construct and go through an analytical model capable of explaining this behaviour in a simple and intelligible way. Although we aim at a semi-pedagogical treatise, we will study the details only when they are necessary to understand the calculation. After presenting the calculations we will discuss the physical validity of our assumptions and take a look at more sophisticated models requiring numerical analysis. We will then improve our model by assuming a simple friction force.Comment: 17 pages and 2 figures, typos corrected, some minor additions and rewording

Gamma ray production cross sections in proton induced reactions on natural Mg, Si and Fe targets over the proton energy range 30 up to 66 MeV

Gamma-ray excitation functions have been measured for 30, 42, 54 and 66 MeV proton beams accelerated onto C + O (Mylar), Mg, Si, and Fe targets of astrophysical interest at the separate-sector cyclotron of iThemba LABS in Somerset West (Cape Town, South Africa). A large solid angle, high energy resolution detection system of the Eurogam type was used to record Gamma-ray energy spectra. Derived preliminary results of Gamma-ray line production cross sections for the Mg, Si and Fe target nuclei are reported and discussed. The current cross section data for known, intense Gamma-ray lines from these nuclei consistently extend to higher proton energies previous experimental data measured up to Ep ~ 25 MeV at the Orsay and Washington tandem accelerators. Data for new Gamma-ray lines observed for the first time in this work are also reported.Comment: 11 pages, 6 figures. IOP Institute of Physics Conference Nuclear Physics in Astrophysics VII, 28th EPF Nuclear Physics Divisional Conference, May 18-22 2015, York, U

Room temperature chiral magnetic skyrmion in ultrathin magnetic nanostructures

Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometer size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetisation at the nanoscale. To date, chiral skyrmion structures have been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films and under external magnetic field or at low temperature. Here, we report on the observation of stable skyrmions in sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero applied magnetic field. We use high lateral resolution X-ray magnetic circular dichroism microscopy to image their chiral N\'eel internal structure which we explain as due to the large strength of the Dzyaloshinskii-Moriya interaction as revealed by spin wave spectroscopy measurements. Our results are substantiated by micromagnetic simulations and numerical models, which allow the identification of the physical mechanisms governing the size and stability of the skyrmions.Comment: Submitted version. Extended version to appear in Nature Nanotechnolog