396 research outputs found
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,
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First-principles study of the atomic and electronic structure of the Si(111)-(5x2-Au surface reconstruction
We present a systematic study of the atomic and electronic structure of the
Si(111)-(5x2)-Au reconstruction using first-principles electronic structure
calculations based on the density functional theory. We analyze the structural
models proposed by Marks and Plass [Phys. Rev. Lett.75, 2172 (1995)], those
proposed recently by Erwin [Phys. Rev. Lett.91, 206101 (2003)], and a
completely new structure that was found during our structural optimizations. We
study in detail the energetics and the structural and electronic properties of
the different models. For the two most stable models, we also calculate the
change in the surface energy as a function of the content of silicon adatoms
for a realistic range of concentrations. Our new model is the energetically
most favorable in the range of low adatom concentrations, while Erwin's "5x2"
model becomes favorable for larger adatom concentrations. The crossing between
the surface energies of both structures is found close to 1/2 adatoms per 5x2
unit cell, i.e. near the maximum adatom coverage observed in the experiments.
Both models, the new structure and Erwin's "5x2" model, seem to provide a good
description of many of the available experimental data, particularly of the
angle-resolved photoemission measurements
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
Effect of electron and hole doping on the structure of C, Si, and S nanowires
We use ab initio density functional calculations to study the effect of
electron and hole doping on the equilibrium geometry and electronic structure
of C, Si, and S monatomic wires. Independent of doping, all these nanowires are
found to be metallic. In absence of doping, C wires are straight, whereas Si
and S wires display a zigzag structure. Besides two preferred bond angles of 60
deg and 120 deg in Si wires, we find an additional metastable bond angle of 90
deg in S wires. The equilibrium geometry and electronic structure of these
nanowires is shown to change drastically upon electron and hole doping.Comment: 5 pages including 5 figure
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
Electronic stopping power in a narrow band gap semiconductor from first principles
© 2015 American Physical Society. The direction and impact parameter dependence of electronic stopping power, along with its velocity threshold behavior, is investigated in a prototypical small-band-gap semiconductor. We calculate the electronic stopping power of H in Ge, a semiconductor with relatively low packing density, using time-evolving time-dependent density-functional theory. The calculations are carried out in channeling conditions with different impact parameters and in different crystal directions for projectile velocities ranging from 0.05 to 0.6 atomic units. The satisfactory comparison with available experiments supports the results and conclusions beyond experimental reach. The calculated electronic stopping power is found to differ in different crystal directions; however, strong impact parameter dependence is observed only in one of these directions. The distinct velocity threshold observed in experiments is well reproduced, and its nontrivial relation with the band gap follows a perturbation theory argument surprisingly well. This simple model is also successful in explaining why different density functionals give the same threshold even with substantially different band gaps.We are thankful to M. A. Zeb, A. Arnau, J. I. Juaristi, J. M. Pitarke, P. Bauer, D. Roth, and A. Correa for useful discussions. The financial support from MINECO-Spain through Plan Nacional Grant No. FIS2012-37549-C05-01, FPI Ph.D. Fellowship Grant No. BES-2013-063728, and Grant No. MAT2013-46593-C6-2-P along with the EU Grant “ElectronStopping” in the Marie Curie CIG Program is duly acknowledged. SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) support is gratefully acknowledged.
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
Structural models for the Si(553)-Au atomic chain reconstruction
Recent photoemission experiments on the Si(553)-Au reconstruction show a
one-dimensional band with a peculiar ~1/4 filling. This band could provide an
opportunity for observing large spin-charge separation if electron-electron
interactions could be increased. To this end, it is necessary to understand in
detail the origin of this surface band. A first step is the determination of
the structure of the reconstruction. We present here a study of several
structural models using first-principles density functional calculations. Our
models are based on a plausible analogy with the similar and better known
Si(557)-Au surface, and compared against the sole structure proposed to date
for the Si(553)-Au system [Crain JN et al., 2004 Phys. Rev. B 69 125401 ].
Results for the energetics and the band structures are given. Lines for the
future investigation are also sketched
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