1,493 research outputs found
van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions
The theoretical description of sparse matter attracts much interest, in
particular for those ground-state properties that can be described by density
functional theory (DFT). One proposed approach, the van der Waals density
functional (vdW-DF) method, rests on strong physical foundations and offers
simple yet accurate and robust functionals. A very recent functional within
this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89,
035412] stands out in its attempt to use an exchange energy derived from the
same plasmon-based theory from which the nonlocal correlation energy was
derived. Encouraged by its good performance for solids, layered materials, and
aromatic molecules, we apply it to several systems that are characterized by
competing interactions. These include the ferroelectric response in PbTiO,
the adsorption of small molecules within metal-organic frameworks (MOFs), the
graphite/diamond phase transition, and the adsorption of an aromatic-molecule
on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well
suited to tackle these challenging systems. In addition to being a competitive
density functional for sparse matter, the vdW-DF-cx construction presents a
more robust general purpose functional that could be applied to a range of
materials problems with a variety of competing interactions
Effect of uniaxial strain on plasmon excitations in graphene
Uniaxial strain is known to modify significantly the electronic properties of
graphene, a carbon single layer of atomic width. Here, we study the effect of
applied strain on the composite excitations arising from the coupling of charge
carriers and plasmons in graphene, i.e. the plasmarons. Specifically, we
predict that the plasmaron energy dispersion, which has been recently observed
experimentally in unstrained graphene, is shifted and broadened by applied
uniaxial strain. Thus, strain constitutes an additional parameter which may be
useful to tune graphene properties in plasmaronic devices.Comment: Invited oral lecture at the 23rd AIRAPT International Conference on
"High Pressure Science and Technology", Mumbai (India), September 25-30,
2011. To be published in J. Phys.: Conf. Series (2012
Atomic and molecular adsorption on transition-metal carbide (111) surfaces from density-functional theory: A trend study of surface electronic factors
This study explores atomic and molecular adsorption on a number of early
transition-metal carbides (TMC's) by means of density-functional theory
calculations. Trend studies are conducted with respect to both period and group
in the periodic table, choosing the substrates ScC, TiC, VC, ZrC, NbC,
delta-MoC, TaC, and WC and the adsorbates H, B, C, N, O, F, NH, NH2, and NH3.
Trends in adsorption strength are explained in terms of surface electronic
factors, by correlating the calculated adsorption energy values with the
calculated surface electronic structures. The results are rationalized with use
of a concerted-coupling model (CCM), which has previously been applied
succesfully to the description of adsorption on TiC(111) and TiN(111) surfaces
[Solid State Commun. 141, 48 (2007)]. First, the clean TMC(111) surfaces are
characterized by calculating surface energies, surface relaxations, Bader
charges, and surface-localized densities of states (DOS's). Detailed
comparisons between surface and bulk DOS's reveal the existence of
transition-metal localized SR's (TMSR's) in the pseudogap and of several
C-localized SR's (CSR's) in the upper valence band on all considered TMC(111)
surfaces. Then, atomic and molecular adsorption energies, geometries, and
charge transfers are presented. An analysis of the adsorbate-induced changes in
surface DOS's reveals a presence of both adsorbate--TMSR and adsorbate--CSR's
interactions, of varying strengths depending on the surface and the adsorbate.
These variations are correlated to the variations in adsorption energies. The
results are used to generalize the content and applications of the previously
proposed CCM to this larger class of substrates and adsorbates. Implications
for other classes of materials, for catalysis, and for other surface processes
are discussed
Nature of Versatile Chemisorption on TiC(111) and TiN(111) Surfaces
Density-functional calculations on the polar TiX(111) (X = C, N) surfaces
show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR's) at
the Fermi level and X2p-derived SR's deep in the upper valence band and (ii)
for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies,
peaking at oxygen (9 eV). A concerted-coupling model, where adatom states
couple to both kinds of SR's in a concerted way, describes the adsorption. The
chemisorption versatility and the general nature of the model indicate
ramifications and predictive abilities in, e.g., growth and catalysis.Comment: 5 pages, 4 figures, submitted to Physical Review Letters (2006
Electron self-energy in A3C60 (A=K, Rb): Effects of t1u plasmon in GW approximation
The electron self-energy of the t1u states in A3C60 (A=K, Rb) is calculated
using the so-called GW approximation. The calculation is performed within a
model which considers the t1u charge carrier plasmon at 0.5 eV and takes into
account scattering of the electrons within the t1u band. A moderate reduction
(35 %) of the t1u band width is obtained.Comment: 4 pages, revtex, 1 figure more information at
http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
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