1,733 research outputs found
Transport properties of armchair graphene nanoribbon junctions between graphene electrodes
The transmission properties of armchair graphene nanoribbon junctions between
graphene electrodes are investigated by means of first-principles quantum
transport calculations. First the dependence of the transmission function on
the size of the nanoribbon has been studied. Two regimes are highlighted: for
small applied bias transport takes place via tunneling and the length of the
ribbon is the key parameter that determines the junction conductance; at higher
applied bias resonant transport through HOMO and LUMO starts to play a more
determinant role, and the transport properties depend on the details of the
geometry (width and length) of the carbon nanoribbon. In the case of the
thinnest ribbon it has been verified that a tilted geometry of the central
phenyl ring is the most stable configuration. As a consequence of this rotation
the conductance decreases due to the misalignment of the orbitals between
the phenyl ring and the remaining part of the junction. All the computed
transmission functions have shown a negligible dependence on different
saturations and reconstructions of the edges of the graphene leads, suggesting
a general validity of the reported results
Calculation of the optical response of C60 and Na8 using time-dependent density functional theory and local orbitals
We report on a general method for the calculation of the frequency-dependent
optical response of clusters based upon time-dependent density functional
theory (TDDFT). The implementation is done using explicit propagation in the
time domain and a self-consistent program that uses a linear combination of
atomic orbitals (LCAO). Our actual calculations employ the SIESTA program,
which is designed to be fast and accurate for large clusters. We use the
adiabatic local density approximation to account for exchange and correlation
effects. Results are presented for the imaginary part of the linear
polarizability, Im [\alpha(w)], and the dipole strength function, S(w), of C60
and Na8, compared to previous calculations and to experiment. We also show how
to calculate the integrated frequency-dependent second order non-linear
polarizability for the case of a step function electric field,
\gamma_{step}(w), and present results for C60.Comment: 11 pages with 6 postscript figures. Submitted for publicatio
Role of the spin-orbit splitting and the dynamical fluctuations in the Si(557)-Au surface
Our it ab initio calculations show that spin-orbit coupling is crucial to
understand the electronic structure of the Si(557)-Au surface. The spin-orbit
splitting produces the two one-dimensional bands observed in photoemission,
which were previously attributed to spin-charge separation in a Luttinger
liquid. This spin splitting might have relevance for future device
applications. We also show that the apparent Peierls-like transition observed
in this surface by scanning tunneling microscopy is a result of the dynamical
fluctuations of the step-edge structure, which are quenched as the temperature
is decreased
Study of star-forming galaxies in SDSS up to redshift 0.4: I. Metallicity evolution
The chemical composition of the gas in galaxies versus cosmic time provides a
very important tool for understanding galaxy evolution. Although there are many
studies at high redshift, they are rather scarce at lower redshifts. However,
low redshift studies can provide important clues about the evolution of
galaxies, furnishing the required link between local and high redshift
universe. In this work we focus on the metallicity of the gas of star-forming
galaxies at low redshift, looking for signs of chemical evolution.
To analyze the metallicity contents star-forming galaxies of similar
luminosities and masses at different redshifts. With this purpose, we present a
study of the metallicity of relatively massive (log(M_star/M_sun)>10.5) star
forming galaxies from SDSS--DR5 (Sloan Digital Sky Survey--Data Release 5),
using different redshift intervals from 0.04 to 0.4.
We used data processed with the STARLIGHT spectral synthesis code, correcting
the fluxes for dust extinction, estimating metallicities using the R_23 method,
and segregating the samples with respect to the value of the
[NII]6583/[OII]3727 line ratio in order to break the R_23 degeneracy selecting
the upper branch. We analyze the luminosity and mass-metallicity relations, and
the effect of the Sloan fiber diameter looking for possible biases.
By dividing our redshift samples in intervals of similar magnitude and
comparing them, significant signs of metallicity evolution are found.
Metallicity correlates inversely with redshift: from redshift 0 to 0.4 a
decrement of ~0.1 dex in 12+log(O/H) is found.Comment: 11 pages, 9 figures, Accepted for publication in A&
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