1,030 research outputs found
Response of the Shockley surface state to an external electrical field: A density-functional theory study of Cu(111)
The response of the Cu(111) Shockley surface state to an external electrical
field is characterized by combining a density-functional theory calculation for
a slab geometry with an analysis of the Kohn-Sham wavefunctions. Our analysis
is facilitated by a decoupling of the Kohn-Sham states via a rotation in
Hilbert space. We find that the surface state displays isotropic dispersion,
quadratic until the Fermi wave vector but with a significant quartic
contribution beyond. We calculate the shift in energetic position and effective
mass of the surface state for an electrical field perpendicular to the Cu(111)
surface; the response is linear over a broad range of field strengths. We find
that charge transfer occurs beyond the outermost copper atoms and that
accumulation of electrons is responsible for a quarter of the screening of the
electrical field. This allows us to provide well-converged determinations of
the field-induced changes in the surface state for a moderate number of layers
in the slab geometry.Comment: 11 pages, 6 figures, 4 tables; accepted for publication by Phys. Rev.
B; changes from v1 in response to referee comments, esp. to Sections I and
V.B (inc. Table 4), with many added references, but no change in results or
conclusion
Improved proton-transfer barriers with van der Waals density functionals: Role of repulsive non-local correlation
Proton-transfer (PT) between organic complexes is a common and important
biochemical process. Unfortunately, PT energy barriers are difficult to
accurately predict using density functional theory (DFT); in particular, the
generalized gradient approximation (GGA) tends to underestimate PT barriers.
Moreover, PT typically occurs in environments where dispersion forces
contribute to the cohesion of the system; thus, a suitable exchange-correlation
functional should accurately describe both dispersion forces and PT barriers.
This paper provides benchmark results for the PT barriers of several density
functionals including several variants of the van der Waals density functional
(vdWDF). The benchmark set comprises small organic molecules with inter- and
intra-molecular PT. The results show that replacing GGA correlation with a
fully non-local vdW-DF correlation increases the PT barriers, making it closer
to the quantum chemical reference values. In contrast, including nonlocal
correlations with the Vydrov-Voorhis (VV) method or dispersion-corrections at
the DFT-D3 or the Tkatchenko-Scheffler (TS) level has barely any impact on the
PT barriers. Hybrid functionals also increase and improve the energies and the
best performance is provided by a hybrid version of the consistent-exchange van
der Waals density functional vdW-DF-cx. For the formic acid dimer PT system, we
analyzed the GGA exchange and non-local correlation contributions. The analysis
shows that the repulsive part of the non-local correlation kernel plays a key
role in the PT energy barriers predicted with vdW-DF
Structure and binding in crystals of cage-like molecules: hexamine and platonic hydrocarbons
In this paper, we show that first-principle calculations using a van der
Waals density functional (vdW-DF), [Phys. Rev. Lett. , 246401
(2004)] permits determination of molecular crystal structure. We study the
crystal structures of hexamine and the platonic hydrocarbons (cubane and
dodecahedrane). The calculated lattice parameters and cohesion energy agree
well with experiments. Further, we examine the asymptotic accounts of the van
der Waals forces by comparing full vdW-DF with asymptotic atom-based pair
potentials extracted from vdW-DF. The character of the binding differ in the
two cases, with vdW-DF giving a significant enhancement at intermediate and
relevant binding separations. We analyze consequences of this result for
methods such as DFT-D, and question DFT-D's transferability over the full range
of separations
Temperature stability of intersubband transitions in AlN/GaN quantum wells
Temperature dependence of intersubband transitions in AlN/GaN multiple
quantum wells grown with molecular beam epitaxy is investigated both by
absorption studies at different temperatures and modeling of conduction-band
electrons. For the absorption study, the sample is heated in increments up to
C. The self-consistent Schr\"odinger-Poisson modeling includes
temperature effects of the band-gap and the influence of thermal expansion on
the piezoelectric field. We find that the intersubband absorption energy
decreases only by meV at C relative to its room temperature
value
Graphene Nanogap for Gate Tunable Quantum Coherent Single Molecule Electronics
We present atomistic calculations of quantum coherent electron transport
through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We
predict that three difficult problems in molecular electronics with single
molecules may be solved by utilizing graphene contacts: (1) a back gate
modulating the Fermi level in the graphene leads facilitate control of the
device conductance in a transistor effect with high on/off current ratio; (2)
the size mismatch between leads and molecule is avoided, in contrast to the
traditional metal contacts; (3) as a consequence, distinct features in charge
flow patterns throughout the device are directly detectable by scanning
techniques. We show that moderate graphene edge disorder is unimportant for the
transistor function.Comment: 8 pages, 6 figure
Theory of Polar Corrections to Donor Binding
We calculate the optical phonon correction to the binding energy of electrons
to donors in cubic materials. Previous theories calculated the Rydberg energy
reduced by the effective mass and the static dielectric function. They omitted
an important energy term from the long-range polarization of the ionized donor,
which vanishes for the neutral donor. They also omitted the donor-phonon
interaction. Including these terms yields a new formula for the donor binding
energy
Understanding adhesion at as-deposited interfaces from ab initio thermodynamics of deposition growth: thin-film alumina on titanium carbide
We investigate the chemical composition and adhesion of chemical vapour
deposited thin-film alumina on TiC using and extending a recently proposed
nonequilibrium method of ab initio thermodynamics of deposition growth (AIT-DG)
[Rohrer J and Hyldgaard P 2010 Phys. Rev. B 82 045415]. A previous study of
this system [Rohrer J, Ruberto C and Hyldgaard P 2010 J. Phys.: Condens. Matter
22 015004] found that use of equilibrium thermodynamics leads to predictions of
a non-binding TiC/alumina interface, despite the industrial use as a
wear-resistant coating. This discrepancy between equilibrium theory and
experiment is resolved by the AIT-DG method which predicts interfaces with
strong adhesion. The AIT-DG method combines density functional theory
calculations, rate-equation modelling of the pressure evolution of the
deposition environment and thermochemical data. The AIT-DG method was
previously used to predict prevalent terminations of growing or as-deposited
surfaces of binary materials. Here we extent the method to predict surface and
interface compositions of growing or as-deposited thin films on a substrate and
find that inclusion of the nonequilibrium deposition environment has important
implications for the nature of buried interfaces.Comment: 8 pages, 6 figures, submitted to J. Phys.: Condens. Matte
Spin Signature of Nonlocal Correlation Binding in Metal-Organic Frameworks
We develop a proper nonempirical spin-density formalism for the van der Waals density functional (vdW-DF) method. We show that this generalization, termed svdW-DF, is firmly rooted in the single-particle nature of exchange and we test it on a range of spin systems. We investigate in detail the role of spin in the nonlocal correlation driven adsorption of H-2 and CO2 in the linear magnets Mn-MOF74, Fe-MOF74, Co-MOF74, and Ni-MOF74. In all cases, we find that spin plays a significant role during the adsorption process despite the general weakness of the molecular-magnetic responses. The case of CO2 adsorption in Ni-MOF74 is particularly interesting, as the inclusion of spin effects results in an increased attraction, opposite to what the diamagnetic nature of CO2 would suggest. We explain this counterintuitive result, tracking the behavior to a coincidental hybridization of the O p states with the Ni d states in the down-spin channel. More generally, by providing insight on nonlocal correlation in concert with spin effects, our nonempirical svdW-DF method opens the door for a deeper understanding of weak nonlocal magnetic interactions
Characterisation of the mantle transcriptome and biomineralisation genes in the blunt-gaper clam, Mya truncata
Members of the Myidae family are ecologically and economically important, but there is currently very little molecular data on these species. The present study sequenced and assembled the mantle transcriptome of Mya truncata from the North West coast of Scotland and identified candidate biomineralisation genes. RNA-Seq reads were assembled to create 20,106 contigs in a de novo transciptome, 18.81% of which were assigned putative functions using BLAST sequence similarity searching (cuttoff E-value 1E − 10). The most highly expressed genes were compared to the Antarctic clam (Laternula elliptica) and showed that many of the dominant biological functions (muscle contraction, energy production, biomineralisation) in the mantle were conserved. There were however, differences in the constitutive expression of heat shock proteins, which were possibly due to the M. truncata sampling location being at a relatively low latitude, and hence relatively warm, in terms of the global distribution of the species. Phylogenetic analyses of the Tyrosinase proteins from M. truncata showed a gene expansion which was absent in L. elliptica. The tissue distribution expression patterns of putative biomineralisation genes were investigated using quantitative PCR, all genes showed a mantle specific expression pattern supporting their hypothesised role in shell secretion. The present study provides some preliminary insights into how clams from different environments – temperate versus polar – build their shells. In addition, the transcriptome data provides a valuable resource for future comparative studies investigating biomineralisation
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