Reversible Fluorination of Graphene: towards a Two-Dimensional Wide Bandgap Semiconductor

We report the synthesis and evidence of graphene fluoride, a two-dimensional wide bandgap semiconductor derived from graphene. Graphene fluoride exhibits hexagonal crystalline order and strongly insulating behavior with resistance exceeding 10 G$\Omega$ at room temperature. Electron transport in graphene fluoride is well described by variable-range hopping in two dimensions due to the presence of localized states in the band gap. Graphene obtained through the reduction of graphene fluoride is highly conductive, exhibiting a resistivity of less than 100 k$\Omega$ at room temperature. Our approach provides a new path to reversibly engineer the band structure and conductivity of graphene for electronic and optical applications.Comment: 7 pages, 5 figures, revtex, to appear in PR

Evolution of a fluorinated green fluorescent protein

The fluorescence of bacterial cells expressing a variant (GFPm) of the green fluorescent protein (GFP) was reduced to background levels by global replacement of the leucine residues of GFPm by 5,5,5-trifluoroleucine. Eleven rounds of random mutagenesis and screening via fluorescence-activated cell sorting yielded a GFP mutant containing 20 amino acid substitutions. The mutant protein in fluorinated form showed improved folding efficiency both in vivo and in vitro, and the median fluorescence of cells expressing the fluorinated protein was improved {approx}650-fold in comparison to that of cells expressing fluorinated GFPm. The success of this approach demonstrates the feasibility of engineering functional proteins containing many copies of abiological amino acid constituents

Investigation of the spin-glass regime between the antiferromagnetic and superconducting phases in Fe1+y_{1+y}Sex_xTe1−x_{1-x}

Using bulk magnetization along with elastic and inelastic neutron scattering techniques, we have investigated the phase diagram of Fe$_{1+y}$Se$_{x}$Te$_{1-x}$ and the nature of magnetic correlations in three nonsuperconducting samples of Fe$_{1.01}$Se$_{0.1}$Te$_{0.9}$, Fe$_{1.01}$Se$_{0.15}$Te$_{0.85}$ and Fe$_{1.02}$Se$_{0.3}$Te$_{0.7}$. A cusp and hysteresis in the temperature dependence of the magnetization for the $x=0.15$ and 0.3 samples indicates spin-glass (SG) ordering below $T_{\rm sg} = 23$K. Neutron scattering measurements indicate that the spin-glass behavior is associated with short-range spin density wave (SDW) ordering characterized by a static component and a low-energy dynamic component with a characteristic incommensurate wave vector of ${\bf Q}_m = (0.46, 0, 0.50)$ and an anisotropy gap of $\sim$ 2.5 meV. Our high ${\bf Q}$-resolution data also show that the systems undergo a glassy structural distortion that coincides with the short-range SDW order

Magnetic Structure of Nano-Graphite Moebius Ribbon

We consider the electronic and magnetic properties of nanographite ribbon with zigzag edges under the periodic or Moebius boundary conditions. The zigzag nano-graphite ribbons possess edge localized states at the Fermi level which cause a ferrimagnetic spin polarization localized at the edge sites even in the very weak Coulomb interaction. The imposition of the Moebius boundary condition makes the system non-AB-bipartite lattice, and depress the spin polarization, resulting in the formation of a magnetic domain wall. The width of the magnetic domain depends on the Coulomb interaction and narrows with increasing U/t.Comment: 4 pages; 6 figures; published at J. Phys. Soc. Jpn. Vol. 72 No. 5 pp. 998-1001 (2003

The Evolution in the Faint-End Slope of the Quasar Luminosity Function

(Abridged) Based on numerical simulations of galaxy mergers that incorporate black hole (BH) growth, we predict the faint end slope of the quasar luminosity function (QLF) and its evolution with redshift. Our simulations have yielded a new model for quasar lifetimes where the lifetime depends on both the instantaneous and peak quasar luminosities. This motivates a new interpretation of the QLF in which the bright end consists of quasars radiating at nearly their peak luminosities, but the faint end is mostly made up of quasars in less luminous phases of evolution. The faint-end QLF slope is then determined by the faint-end slope of the quasar lifetime for quasars with peak luminosities near the observed break. We determine this slope from the quasar lifetime as a function of peak luminosity, based on a large set of simulations spanning a wide variety of host galaxy, merger, BH, and ISM gas properties. Brighter peak luminosity (higher BH mass) systems undergo more violent evolution, and expel and heat gas more rapidly in the final stages of quasar evolution, resulting in a flatter faint-end slope (as these objects fall below the observed break in the QLF more rapidly). Therefore, as the QLF break luminosity moves to higher luminosities with increasing redshift, implying a larger typical quasar peak luminosity, the faint-end QLF slope flattens. From the quasar lifetime as a function of peak luminosity and this interpretation of the QLF, we predict the faint-end QLF slope and its evolution with redshift in good agreement with observations. Although BHs grow anti-hierarchically (with lower-mass BHs formed primarily at lower redshifts), the observed change in slope and differential or luminosity dependent density evolution in the QLF is completely determined by the luminosity-dependent quasar lifetime and physics of quasar feedback.Comment: 13 pages, 4 figures, submitted to ApJ (Replacement with minor revisions and changed sign convention

Free Energy Self-Averaging in Protein-Sized Random Heteropolymers

Current theories of heteropolymers are inherently macrpscopic, but are applied to folding proteins which are only mesoscopic. In these theories, one computes the averaged free energy over sequences, always assuming that it is self-averaging -- a property well-established only if a system with quenched disorder is macroscopic. By enumerating the states and energies of compact 18, 27, and 36mers on a simplified lattice model with an ensemble of random sequences, we test the validity of the self-averaging approximation. We find that fluctuations in the free energy between sequences are weak, and that self-averaging is a valid approximation at the length scale of real proteins. These results validate certain sequence design methods which can exponentially speed up computational design and greatly simplify experimental realizations.Comment: 4 pages, 3 figure

Novel electronic wave interference patterns in nanographene sheets

Superperiodic patterns with a long distance in a nanographene sheet observed by STM are discussed in terms of the interference of electronic wave functions. The period and the amplitude of the oscillations decrease spatially in one direction. We explain the superperiodic patterns with a static linear potential theoretically. In the k-p model, the oscillation period decreases, and agrees with experiments. The spatial difference of the static potential is estimated as 1.3 eV for 200 nm in distance, and this value seems to be reasonable in order that the potential difference remains against perturbations, for example, by phonon fluctuations and impurity scatterings. It turns out that the long-distance oscillations come from the band structure of the two-dimensional graphene sheet.Comment: Published as a LETTER in J. Phys.: Condens. Matter; 8 pages; 6 figures; Online version at http://www.iop.org/EJ/S/3/1256/0hJAmc5sCL6d.7sOO.BtLw/abstract/0953-8984/14/3 6/10

Numerical Galaxy Catalog -I. A Semi-analytic Model of Galaxy Formation with N-body simulations

We construct the Numerical Galaxy Catalog ($\nu$GC), based on a semi-analytic model of galaxy formation combined with high-resolution N-body simulations in a $\Lambda$-dominated flat cold dark matter ($\Lambda$CDM) cosmological model. The model includes several essential ingredients for galaxy formation, such as merging histories of dark halos directly taken from N-body simulations, radiative gas cooling, star formation, heating by supernova explosions (supernova feedback), mergers of galaxies, population synthesis, and extinction by internal dust and intervening HI clouds. As the first paper in a series using this model, we focus on basic photometric, structural and kinematical properties of galaxies at present and high redshifts. Two sets of model parameters are examined, strong and weak supernova feedback models, which are in good agreement with observational luminosity functions of local galaxies in a range of observational uncertainty. Both models agree well with many observations such as cold gas mass-to-stellar luminosity ratios of spiral galaxies, HI mass functions, galaxy sizes, faint galaxy number counts and photometric redshift distributions in optical pass-bands, isophotal angular sizes, and cosmic star formation rates. In particular, the strong supernova feedback model is in much better agreement with near-infrared (K'-band) faint galaxy number counts and redshift distribution than the weak feedback model and our previous semi-analytic models based on the extended Press-Schechter formalism. (Abridged)Comment: 26 pages including 27 figures, accepted for publication in ApJ, full-resolution version is available at http://grape.astron.s.u-tokyo.ac.jp/~yahagi/nugc