460 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
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
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
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
Universal Magnetic Properties of sp-type Defects in Covalently Functionalized Graphene
Using density-functional calculations, we study the effect of sp-type
defects created by different covalent functionalizations on the electronic and
magnetic properties of graphene. We find that the induced magnetic properties
are {\it universal}, in the sense that they are largely independent on the
particular adsorbates considered. When a weakly-polar single covalent bond is
established with the layer, a local spin-moment of 1.0 always appears
in graphene. This effect is similar to that of H adsorption, which saturates
one orbital in the carbon layer. The magnetic couplings between the
adsorbates show a strong dependence on the graphene sublattice of
chemisorption. Molecules adsorbed at the same sublattice couple
ferromagnetically, with an exchange interaction that decays very slowly with
distance, while no magnetism is found for adsorbates at opposite sublattices.
Similar magnetic properties are obtained if several orbitals are
saturated simultaneously by the adsorption of a large molecule. These results
might open new routes to engineer the magnetic properties of graphene
derivatives by chemical means
First principles study of the adsorption of C60 on Si(111)
The adsorption of C60 on Si(111) has been studied by means of
first-principles density functional calculations.
A 2x2 adatom surface reconstruction was used to simulate the terraces of the
7x7 reconstruction.
The structure of several possible adsorption configurations was optimized
using the ab initio atomic forces, finding good candidates for two different
adsorption states observed experimentally.
While the C60 molecule remains closely spherical, the silicon substrate
appears quite soft, especially the adatoms, which move substantially to form
extra C-Si bonds, at the expense of breaking Si-Si bonds.
The structural relaxation has a much larger effect on the adsorption
energies, which strongly depend on the adsorption configuration, than on the
charge transfer.Comment: 4 pages with 3 postscript figures, to appear in Surf. Science.
(proceedings of the European Conference on Surface Science ECOSS-19, Sept
2000
Electronic structure interpolation via atomic orbitals
We present an efficient scheme for accurate electronic structure
interpolations based on the systematically improvable optimized atomic
orbitals. The atomic orbitals are generated by minimizing the spillage value
between the atomic basis calculations and the converged plane wave basis
calculations on some coarse -point grid. They are then used to calculate the
band structure of the full Brillouin zone using the linear combination of
atomic orbitals (LCAO) algorithms. We find that usually 16 -- 25 orbitals per
atom can give an accuracy of about 10 meV compared to the full {\it ab initio}
calculations. The current scheme has several advantages over the existing
interpolation schemes. The scheme is easy to implement and robust which works
equally well for metallic systems and systems with complex band structures.
Furthermore, the atomic orbitals have much better transferability than the
Shirley's basis and Wannier functions, which is very useful for the
perturbation calculations
Electronic structure and dimerization of a single monatomic gold wire
The electronic structure of a single monatomic gold wire is presented for the
first time. It has been obtained with state-of-the-art ab-initio full-potential
density-functional (DFT) LMTO (linearized muffin-tin orbital) calculations
taking into account relativistic effects. For stretched structures in the
experimentally accessible range the conduction band is exactly half-filled,
whereas the band structures are more complex for the optimized structure. By
studying the total energy as a function of unit-cell length and of a possible
bond-length alternation we find that the system can lower its total energy by
letting the bond lengths alternate leading to a structure containing separated
dimers with bond lengths of about 2.5 \AA, largely independent of the
stretching. However, first for fairly large unit cells (above roughly 7 \AA),
is the total-energy gain upon this dimerization comparable with the energy
costs upon stretching. We propose that this together with band-structure
effects is the reason for the larger interatomic distances observed in recent
experiments. We find also that although spin-orbit couplings lead to
significant effects on the band structure, the overall conclusions are not
altered, and that finite Au_2, Au_4, and Au_6 chains possess electronic
properties very similar to those of the infinite chain.Comment: (14 pages, 5 figures; Elsevier Preprint style elsart.sty
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