341 research outputs found
Prediction of electronic couplings for molecular charge transfer using optimally tuned range-separated hybrid functionals
Electronic coupling matrix elements are important to the theoretical description of electron transfer processes. However, they are notoriously difficult to obtain accurately from time-dependent density functional theory (TDDFT). Here, we use the HAB11 benchmark dataset of coupling matrix elements to assess whether TDDFT using optimally tuned range-separated hybrid functionals, already known to be successful for the description of charge transfer excitation energies, also allows for an improved accuracy in the prediction of coupling matrix elements. We find that this approach outperforms all previous TDDFT calculations, based on semi-local, hybrid or non-tuned range-separated hybrid functionals, with a remaining average deviation as low as ∼12%. We discuss potential sources for the remaining error
Jahn-Teller Distortion and Ferromagnetism in the Dilute Magnetic Semiconductors GaN:Mn
Using first-principles total-energy methods, we investigate Jahn-Teller
distortions in III-V dilute magnetic semiconductors, GaAs:Mn and GaN:Mn in the
cubic zinc blende structure. The results for an isolated Mn impurity on a Ga
site show that there is no appreciable effect in GaAs, whereas, in GaN there is
a Jahn-Teller effect in which the symmetry around the impurity changes from
T to D or to C. The large effect in GaN occurs because of
the localized d character, which is further enhanced by the distortion. The
lower symmetry should be detectable experimentally in cubic GaN with low Mn
concentration, and should be affected by charge compensation (reductions of
holes and conversion of Mn ions to d with no Jahn-Teller effect).
Jahn-Teller effect is greatly reduced because the symmetry at each Mn site is
lowered due to the Mn-Mn interaction. The tendency toward ferromagnetism is
found to be stronger in GaN:Mn than in GaAs:Mn and to be only slightly reduced
by charge compensation.Comment: 6 pages, 3 figure
Role of long-range exact exchange in polaron charge transition levels: The case of MgO
Predicting the degree of localization and calculating the trapping energies of polarons in insulators by density functional theory (DFT) is challenging. Hybrid functionals are often reparametrized to obtain accurate results and the a priori selection of these parameters is still an open question. Here we test the accuracy of several range-separated hybrid functionals, all reparametrized to produce an accurate band gap, by calculating the charge transition levels (CTLs) of experimentally well-studied hole polaron defect centers in MgO. We show that the functional with screened long-range exact exchange is moderately but consistently more accurate than functionals which do not include long-range exact exchange. We provide evidence that the source of the improved accuracy is the eigenvalue associated with the valence band maximum of the bulk material. We discuss the extent to which this accuracy relates to Koopmans' compliance of the defect energy level
Voltage tuning of vibrational mode energies in single-molecule junctions
Vibrational modes of molecules are fundamental properties determined by
intramolecular bonding, atomic masses, and molecular geometry, and often serve
as important channels for dissipation in nanoscale processes. Although
single-molecule junctions have been employed to manipulate electronic structure
and related functional properties of molecules, electrical control of
vibrational mode energies has remained elusive. Here we use simultaneous
transport and surface-enhanced Raman spectroscopy measurements to demonstrate
large, reversible, voltage-driven shifts of vibrational mode energies of C60
molecules in gold junctions. C60 mode energies are found to vary approximately
quadratically with bias, but in a manner inconsistent with a simple vibrational
Stark effect. Our theoretical model suggests instead that the mode shifts are a
signature of bias-driven addition of electronic charge to the molecule. These
results imply that voltage-controlled tuning of vibrational modes is a general
phenomenon at metal-molecule interfaces and is a means of achieving significant
shifts in vibrational energies relative to a pure Stark effect.Comment: 23 pages, 4 figures + 12 pages, 7 figures supporting materia
Photoreflectance and surface photovoltage spectroscopy of beryllium-doped GaAs/AlAs multiple quantum wells
We present an optical study of beryllium delta-doped GaAs/AlAs multiple quantum well (QW) structures designed for sensing terahertz (THz) radiation. Photoreflectance (PR), surface photovoltage (SPV), and wavelength-modulated differential surface photovoltage (DSPV) spectra were measured in the structures with QW widths ranging from 3 to 20 nm and doping densities from 2×10(10) to 5×10(12) cm(–2) at room temperature. The PR spectra displayed Franz-Keldysh oscillations which enabled an estimation of the electric-field strength of ~20 kV/cm at the sample surface. By analyzing the SPV spectra we have determined that a buried interface rather than the sample surface mainly governs the SPV effect. The DSPV spectra revealed sharp features associated with excitonic interband transitions which energies were found to be in a good agreement with those calculated including the nonparabolicity of the energy bands. The dependence of the exciton linewidth broadening on the well width and the quantum index has shown that an average half monolayer well width fluctuations is mostly predominant broadening mechanism for QWs thinner than 10 nm. The line broadening in lightly doped QWs, thicker than 10 nm, was found to arise from thermal broadening with the contribution from Stark broadening due to random electric fields of the ionized impurities in the structures. We finally consider the possible influence of strong internal electric fields, QW imperfections, and doping level on the operation of THz sensors fabricated using the studied structures. © 2005 American Institute of Physic
Hybridization and Bond-Orbital Components in Site-Specific X-Ray Photoelectron Spectra of Rutile TiO\u3csub\u3e2\u3c/sub\u3e
We have determined the Ti and O components of the rutile TiO2 valence band using the method of sitespecific x-ray photoelectron spectroscopy. Comparisons with calculations based on pseudopotentials within the local density approximation reveal the hybridization of the Ti 3d, 4s, and 4p states, and the O 2s and 2p states on each site. These chemical effects are observed due to the large differences between the angular-momentum dependent matrix elements of the photoelectron process
Electronic structure and magnetism of Mn doped GaN
Mn doped semiconductors are extremely interesting systems due to their novel
magnetic properties suitable for the spintronics applications. It has been
shown recently by both theory and experiment that Mn doped GaN systems have a
very high Curie temperature compared to that of Mn doped GaAs systems. To
understand the electronic and magnetic properties, we have studied Mn doped GaN
system in detail by a first principles plane wave method. We show here the
effect of varying Mn concentration on the electronic and magnetic properties.
For dilute Mn concentration, states of Mn form an impurity band completely
separated from the valence band states of the host GaN. This is in contrast to
the Mn doped GaAs system where Mn states in the gap lie very close to the
valence band edge and hybridizes strongly with the delocalized valence band
states.
To study the effects of electron correlation, LSDA+U calculations have been
performed.
Calculated exchange interaction in (Mn,Ga)N is short ranged in contrary to
that in (Mn,Ga)As where the strength of the ferromagnetic coupling between Mn
spins is not decreased substantially for large Mn-Mn separation. Also, the
exchange interactions are anisotropic in different crystallographic directions
due to the presence or absence of connectivity between Mn atoms through As
bonds.Comment: 6 figures, submitted to Phys. Rev.
On-site Coulomb interaction and the magnetism of (GaMn)N and (GaMn)As
We use the local density approximation (LDA) and LDA+U schemes to study the
magnetism of (GaMn)As and (GaMn)N for a number of Mn concentrations and varying
number of holes. We show that for both systems and both calculational schemes
the presence of holes is crucial for establishing ferromagnetism. For both
systems, the introduction of increases delocalization of the holes and,
simultaneously, decreases the p-d interaction. Since these two trends exert
opposite influences on the Mn-Mn exchange interaction the character of the
variation of the Curie temperature (T) cannot be predicted without direct
calculation. We show that the variation of T is different for two systems.
For low Mn concentrations we obtain the tendency to increasing T in the
case of (GaMn)N whereas an opposite tendency to decreasing T is obtained
for (GaMn)As. We reveal the origin of this difference by inspecting the
properties of the densities of states and holes for both systems. The main body
of calculations is performed within a supercell approach. The Curie
temperatures calculated within the coherent potential approximation to atomic
disorder are reported for comparison. Both approaches give similar qualitative
behavior. The results of calculations are related to the experimental data.Comment: to appear in Physical Review
Structure and properties of small sodium clusters
We have investigated structure and properties of small metal clusters using
all-electron ab initio theoretical methods based on the Hartree-Fock
approximation and density functional theory, perturbation theory and compared
results of our calculations with the available experimental data and the
results of other theoretical works. We have systematically calculated the
optimized geometries of neutral and singly charged sodium clusters having up to
20 atoms, their multipole moments (dipole and quadrupole), static
polarizabilities, binding energies per atom, ionization potentials and
frequencies of normal vibration modes. Our calculations demonstrate the great
role of many-electron correlations in the formation of electronic and ionic
structure of small metal clusters and form a good basis for further detailed
study of their dynamic properties, as well as structure and properties of other
atomic cluster systems.Comment: 47 pages, 16 figure
- …