1,703 research outputs found
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
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
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
First-Principles Study of Substitutional Metal Impurities in Graphene: Structural, Electronic and Magnetic Properties
We present a theoretical study using density functional calculations of the
structural, electronic and magnetic properties of 3d transition metal, noble
metal and Zn atoms interacting with carbon monovacancies in graphene. We pay
special attention to the electronic and magnetic properties of these
substitutional impurities and found that they can be fully understood using a
simple model based on the hybridization between the states of the metal atom,
particularly the d shell, and the defect levels associated with an
unreconstructed D3h carbon vacancy. We identify three different regimes
associated with the occupation of different carbon-metal hybridized electronic
levels:
(i) bonding states are completely filled for Sc and Ti, and these impurities
are non-magnetic;
(ii) the non-bonding d shell is partially occupied for V, Cr and Mn and,
correspondingly, these impurties present large and localized spin moments;
(iii) antibonding states with increasing carbon character are progressively
filled for Co, Ni, the noble metals and Zn. The spin moments of these
impurities oscillate between 0 and 1 Bohr magnetons and are increasingly
delocalized.
The substitutional Zn suffers a Jahn-Teller-like distortion from the C3v
symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct
position at the border between regimes (ii) and (iii) and shows a more complex
behavior: while is non-magnetic at the level of GGA calculations, its spin
moment can be switched on using GGA+U calculations with moderate values of the
U parameter.Comment: 13 figures, 4 tables. Submitted to Phys. Rev. B on September 26th,
200
Zigzag equilibrium structure in monatomic wires
We have applied first-principles density-functional calculations to the study
of the energetics, and the elastic and electronic properties of monatomic wires
of Au, Cu, K, and Ca in linear and a planar-zigzag geometries.
For Cu and Au wires, the zigzag distortion is favorable even when the linear
wire is stretched, but this is not observed for K and Ca wires.
In all the cases, the equilibrium structure is an equilateral zigzag (bond
angle of 60).
Only in the case of Au, the zigzag geometry can also be stabilized for an
intermediate bond angle of 131.
The relationship between the bond and wire lengths is qualitatively different
for the metallic (Au, Cu and, K) and semiconducting (Ca) wires.Comment: 4 pages with 3 postscript figures. To appear in Surf. Science
(proceedings of the European Conference on Surface Science, ECOSS-19, Madrid
Sept. 2000
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