622 research outputs found
Stacking dependence of carrier transport properties in multilayered black phosphorous
We present the effect of different stacking orders on carrier transport
properties of multi-layer black phosphorous. We consider three different
stacking orders AAA, ABA and ACA, with increasing number of layers (from 2 to 6
layers). We employ a hierarchical approach in density functional theory (DFT),
with structural simulations performed with Generalized Gradient Approximation
(GGA) and the bandstructure, carrier effective masses and optical properties
evaluated with the Meta-Generalized Gradient Approximation (MGGA). The carrier
transmission in the various black phosphorous sheets was carried out with the
non-equilibrium Greens function (NEGF) approach. The results show that ACA
stacking has the highest electron and hole transmission probabilities. The
results show tunability for a wide range of band-gap, carrier effective masses
and transmission with a great promise for lattice engineering (stacking order
and layers) in black phosphorous.Comment: 18 Pages , 10 figure
Towards a simplified description of thermoelectric materials: Accuracy of approximate density functional theory for phonon dispersions
We calculate the phonon-dispersion relations of several two-dimensional
materials and diamond using the density-functional based tight-binding approach
(DFTB). Our goal is to verify if this numerically efficient method provides
sufficiently accurate phonon frequencies and group velocities to compute
reliable thermoelectric properties. To this end, the results are compared to
available DFT results and experimental data. To quantify the accuracy for a
given band, a descriptor is introduced that summarizes contributions to the
lattice conductivity that are available already in the harmonic approximation.
We find that the DFTB predictions depend strongly on the employed repulsive
pair-potentials, which are an important prerequisite of this method. For
carbon-based materials, accurate pair-potentials are identified and lead to
errors of the descriptor that are of the same order as differences between
different local and semi-local DFT approaches
Efficient evaluation of the Fourier Transform over products of Slater-type orbitals on different centers
Using the shift-operator technique, a compact formula for the Fourier
transform of a product of two Slater-type orbitals located on different atomic
centers is derived. The result is valid for arbitrary quantum numbers and was
found to be numerically stable for a wide range of geometrical parameters and
momenta. Details of the implementation are presented together with benchmark
data for representative integrals. We also discuss the assets and drawbacks of
alternative algorithms available and analyze the numerical efficiency of the
new scheme.Comment: Revised versio
A new type of charged defect in amorphous chalcogenides
We report on density-functional-based tight-binding (DFTB) simulations of a
series of amorphous arsenic sulfide models. In addition to the charged
coordination defects previously proposed to exist in chalcogenide glasses, a
novel defect pair, [As4]--[S3]+, consisting of a four-fold coordinated arsenic
site in a seesaw configuration and a three-fold coordinated sulfur site in a
planar trigonal configuration, was found in several models. The
valence-alternation pairs S3+-S1- are converted into [As4]--[S3]+ pairs under
HOMO-to-LUMO electronic excitation. This structural transformation is
accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests
that such transformations could contribute to photo-darkening in these
materials.Comment: 5 pages, 2 figure
GLOBAL LEADERSHIP DEVELOPMENT PLANS: Engaging Students as Agents in Their Own Development
Students need look no further than their immediate surroundings to see global connections to their daily lives. Their clothes are made across the world; their classmates, neighbors, and coworkers represent a wide range of nationalities; their friends are posted overseas in the Peace Corps, the military, and with humanitarian organizations; and issues of human rights at home and abroad are broadcast daily on their TVs and favorite blogs. From business and politics, to environmental activism and social change, issues are seldom contained within the borders of a nation state. They seep and soar across borders and across cultures to impact us in ways we may not even realize. It is within this global context that our students live, so it’s vital that our students have competencies to flourish as the global leaders of tomorrow (or, for that matter, the global leaders of today)
Resonant electron heating and molecular phonon cooling in single C junctions
We study heating and heat dissipation of a single \c60 molecule in the
junction of a scanning tunneling microscope (STM) by measuring the electron
current required to thermally decompose the fullerene cage. The power for
decomposition varies with electron energy and reflects the molecular resonance
structure. When the STM tip contacts the fullerene the molecule can sustain
much larger currents. Transport simulations explain these effects by molecular
heating due to resonant electron-phonon coupling and molecular cooling by
vibrational decay into the tip upon contact formation.Comment: Accepted in Phys. Rev. Let
Influence of copper on the electronic properties of amorphous chalcogenides
We have studied the influence of alloying copper with amorphous arsenic
sulfide on the electronic properties of this material. In our
computer-generated models, copper is found in two-fold near-linear and
four-fold square-planar configurations, which apparently correspond to Cu(I)
and Cu(II) oxidation states. The number of overcoordinated atoms, both arsenic
and sulfur, grows with increasing concentration of copper. Overcoordinated
sulfur is found in trigonal planar configuration, and overcoordinated
(four-fold) arsenic is in tetrahedral configuration. Addition of copper
suppresses the localization of lone-pair electrons on chalcogen atoms, and
localized states at the top of the valence band are due to Cu 3d orbitals.
Evidently, these additional Cu states, which are positioned at the same
energies as the states due to ([As4]-)-([S_3]+) pairs, are responsible for
masking photodarkening in Cu chalcogenides
Dynamical generalization of a solvable family of two-electron model atoms with general interparticle repulsion
Holas, Howard and March [Phys. Lett. A {\bf 310}, 451 (2003)] have obtained
analytic solutions for ground-state properties of a whole family of
two-electron spin-compensated harmonically confined model atoms whose different
members are characterized by a specific interparticle potential energy
u(). Here, we make a start on the dynamic generalization of the
harmonic external potential, the motivation being the serious criticism
levelled recently against the foundations of time-dependent density-functional
theory (e.g. [J. Schirmer and A. Dreuw, Phys. Rev. A {\bf 75}, 022513 (2007)]).
In this context, we derive a simplified expression for the time-dependent
electron density for arbitrary interparticle interaction, which is fully
determined by an one-dimensional non-interacting Hamiltonian. Moreover, a
closed solution for the momentum space density in the Moshinsky model is
obtained.Comment: 5 pages, submitted to J. Phys.
Charge-transfer excited states in phosphorescent organo-transition metal compounds: a difficult case for time dependent density functional theory?
Light emitting organo-transition metal complexes have found widespread use in the past. The computational modelling of such compounds is often based on time-dependent density functional theory (TDDFT), which enjoys popularity due to its numerical efficiency and simple black-box character. It is well known, however, that TDDFT notoriously underestimates energies of charge-transfer excited states which are prominent in phosphorescent metal–organic compounds. In this study, we investigate whether TDDFT is providing a reliable description of the electronic properties in these systems. To this end, we compute 0–0 triplet state energies for a series of 17 pseudo-square planar platinum(II) and pseudo-octahedral iridium(III) complexes that are known to feature quite different localization characteristics ranging from ligand-centered (LC) to metal-to-ligand charge transfer (MLCT) transitions. The calculations are performed with conventional semi-local and hybrid functionals as well as with optimally tuned range-separated functionals that were recently shown to overcome the charge transfer problem in TDDFT. We compare our results against low temperature experimental data and propose a criterion to classify excited states based on wave function localization. In addition, singlet absorption energies and singlet–triplet splittings are evaluated for a subset of the compounds and are also validated against experimental data. Our results indicate that for the investigated complexes charge-transfer is much less pronounced than previously believed
Quasiparticle energies for large molecules: a tight-binding GW approach
We present a tight-binding based GW approach for the calculation of
quasiparticle energy levels in confined systems such as molecules. Key
quantities in the GW formalism like the microscopic dielectric function or the
screened Coulomb interaction are expressed in a minimal basis of spherically
averaged atomic orbitals. All necessary integrals are either precalculated or
approximated without resorting to empirical data. The method is validated
against first principles results for benzene and anthracene, where good
agreement is found for levels close to the frontier orbitals. Further, the size
dependence of the quasiparticle gap is studied for conformers of the polyacenes
() up to n = 30.Comment: 10 pages, 5 eps figures submitted to Phys. Rev.
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