Empirical distributions of galactic λ\lambda spin parameters from the SDSS

Using simple dimensional arguments for both spiral and elliptical galaxies, we present formulas to derive an estimate of the halo spin parameter $\lambda$ for any real galaxy, in terms of common observational parameters. This allows a rough estimate of $\lambda$, which we apply to a large volume limited sample of galaxies taken from the SDSS data base. The large numbers involved (11,597) allow the derivation of reliable $\lambda$ distributions, as signal adds up significantly in spite of the errors in the inferences for particular galaxies. We find that if the observed distribution of $\lambda$ is modeled with a log-normal function, as often done for this distribution in dark matter halos that appear in cosmological simulations, we obtain parameters $\lambda_{0}=0.04 \pm 0.005$ and $\sigma_{\lambda}=0.51 \pm 0.05$, interestingly consistent with values derived from simulations. For spirals, we find a good correlation between empirical values of $\lambda$ and visually assigned Hubble types, highlighting the potential of this physical parameter as an objective classification tool.Comment: 8 pages, 6 figures, expanded final version, MNRAS (in press

Edge-functionalized and substitutional doped graphene nanoribbons: electronic and spin properties

Graphene nanoribbons are the counterpart of carbon nanotubes in graphene-based nanoelectronics. We investigate the electronic properties of chemically modified ribbons by means of density functional theory. We observe that chemical modifications of zigzag ribbons can break the spin degeneracy. This promotes the onset of a semiconducting-metal transition, or of an half-semiconducting state, with the two spin channels having a different bandgap, or of a spin-polarized half-semiconducting state -where the spins in the valence and conduction bands are oppositely polarized. Edge functionalization of armchair ribbons gives electronic states a few eV away from the Fermi level, and does not significantly affect their bandgap. N and B produce different effects, depending on the position of the substitutional site. In particular, edge substitutions at low density do not significantly alter the bandgap, while bulk substitution promotes the onset of semiconducting-metal transitions. Pyridine-like defects induce a semiconducting-metal transition.Comment: 12 pages, 5 figure

Interplay between edge states and simple bulk defects in graphene nanoribbons

We study the interplay between the edge states and a single impurity in a zigzag graphene nanoribbon. We use tight-binding exact diagonalization techniques, as well as density functional theory calculations to obtain the eigenvalue spectrum, the eigenfunctions, as well the dependence of the local density of states (LDOS) on energy and position. We note that roughly half of the unperturbed eigenstates in the spectrum of the finite-size ribbon hybridize with the impurity state, and the corresponding eigenvalues are shifted with respect to their unperturbed values. The maximum shift and hybridization occur for a state whose energy is inverse proportional to the impurity potential; this energy is that of the impurity peak in the DOS spectrum. We find that the interference between the impurity and the edge gives rise to peculiar modifications of the LDOS of the nanoribbon, in particular to oscillations of the edge LDOS. These effects depend on the size of the system, and decay with the distance between the edge and the impurity.Comment: 10 pages, 15 figures, revtex

Optical properties and charge-transfer excitations in edge-functionalized all-graphene nanojunctions

We investigate the optical properties of edge-functionalized graphene nanosystems, focusing on the formation of junctions and charge transfer excitons. We consider a class of graphene structures which combine the main electronic features of graphene with the wide tunability of large polycyclic aromatic hydrocarbons. By investigating prototypical ribbon-like systems, we show that, upon convenient choice of functional groups, low energy excitations with remarkable charge transfer character and large oscillator strength are obtained. These properties can be further modulated through an appropriate width variation, thus spanning a wide range in the low-energy region of the UV-Vis spectra. Our results are relevant in view of designing all-graphene optoelectronic nanodevices, which take advantage of the versatility of molecular functionalization, together with the stability and the electronic properties of graphene nanostructures.Comment: J. Phys. Chem. Lett. (2011), in pres

DNA nucleotide-specific modulation of \mu A transverse edge currents through a metallic graphene nanoribbon with a nanopore

We propose two-terminal devices for DNA sequencing which consist of a metallic graphene nanoribbon with zigzag edges (ZGNR) and a nanopore in its interior through which the DNA molecule is translocated. Using the nonequilibrium Green functions combined with density functional theory, we demonstrate that each of the four DNA nucleotides inserted into the nanopore, whose edge carbon atoms are passivated by either hydrogen or nitrogen, will lead to a unique change in the device conductance. Unlike other recent biosensors based on transverse electronic transport through DNA nucleotides, which utilize small (of the order of pA) tunneling current across a nanogap or a nanopore yielding a poor signal-to-noise ratio, our device concept relies on the fact that in ZGNRs local current density is peaked around the edges so that drilling a nanopore away from the edges will not diminish the conductance. Inserting a DNA nucleotide into the nanopore affects the charge density in the surrounding area, thereby modulating edge conduction currents whose magnitude is of the order of \mu A at bias voltage ~ 0.1 V. The proposed biosensor is not limited to ZGNRs and it could be realized with other nanowires supporting transverse edge currents, such as chiral GNRs or wires made of two-dimensional topological insulators.Comment: 6 pages, 6 figures, PDFLaTe

Electronic Structures of Porous Nanocarbons

We use large scale ab-initio calculations to describe electronic structures of graphene, graphene nanoribbons, and carbon nanotubes periodically perforated with nanopores. We disclose common features of these systems and develop a unified picture that permits us to analytically predict and systematically characterize metal-semiconductor transitions in nanocarbons with superlattices of nanopores of different sizes and types. These novel materials with highly tunable band structures have numerous potential applications in electronics, light detection, and molecular sensing.Comment: 7 pages, 8 figure

The role of spin in the formation and evolution of galaxies

Using the SDSS spectroscopic sample, we estimate the dark matter halo spin parameter lambda for ~53,000 disk galaxies for which MOPED star formation histories are available. We investigate the relationship between spin and total stellar mass, star formation history, and environment. First, we find a clear anti-correlation between stellar mass and spin, with low mass galaxies generally having high dark matter spins. Second, galaxies which have formed more than ~5% of their stars in the last 0.2 Gyr have more broadly distributed and typically higher spins (including a significant fraction with lambda > 0.1) than galaxies which formed a large fraction of their stars more than 10 Gyr ago. Finally, we find little or no correlation between the value of spin of the dark halo and environment as determined both by proximity to a new cluster catalog and a marked correlation study. This agrees well with the predictions from linear hierarchical torquing theory and numerical simulations.Comment: Accepted to MNRAS after moderate revisio

Chemically-induced Mobility Gaps in Graphene Nanoribbons: A Route for Upscaling Device Performances

We report a first-principles based study of mesoscopic quantum transport in chemically doped graphene nanoribbons with a width up to 10 nm. The occurrence of quasibound states related to boron impurities results in mobility gaps as large as 1 eV, driven by strong electron-hole asymmetrical backscattering phenomena. This phenomenon opens new ways to overcome current limitations of graphene-based devices through the fabrication of chemically-doped graphene nanoribbons with sizes within the reach of conventional lithography.Comment: Nano Letters (in press