1,137 research outputs found
Alloying effects on the optical properties of GeSi nanocrystals from TDDFT and comparison with effective-medium theory
We present the optical spectra of GeSi alloy nanocrystals
calculated with time-dependent density-functional theory in the adiabatic
local-density ap proximation (TDLDA). The spectra change smoothly as a function
of the compositio n . On the Ge side of the composition range, the lowest
excitations at the ab sorption edge are almost pure Kohn-Sham
independent-particle HOMO-LUMO transitio ns, while for higher Si contents
strong mixing of transitions is found. Within T DLDA the first peak is slightly
higher in energy than in earlier independent-par ticle calculations. However,
the absorption onset and in particular its composit ion dependence is similar
to independent-particle results. Moreover, classical depolarization effects are
responsible for a very strong suppression of the abs orption intensity. We show
that they can be taken into account in a simpler way using Maxwell-Garnett
classical effective-medium theory. Emission spectra are in vestigated by
calculating the absorption of excited nanocrystals at their relaxe d geometry.
The structural contribution to the Stokes shift is about 0.5 eV. Th e
decomposition of the emission spectra in terms of independent-particle transit
ions is similar to what is found for absorption. For the emission, very weak
tra nsitions are found in Ge-rich clusters well below the strong absorption
onset.Comment: submitted to Phys. Rev.
Time-dependent Kohn-Sham theory with memory
In time-dependent density-functional theory, exchange and correlation (xc)
beyond the adiabatic local density approximation can be described in terms of
viscoelastic stresses in the electron liquid. In the time domain, this leads to
a velocity-dependent xc vector potential with a memory containing short- and
long-range components. The resulting time-dependent Kohn-Sham formalism
describes the dynamics of electronic systems including decoherence and
relaxation. For the example of collective charge-density oscillations in a
quantum well, we illustrate the xc memory effects, clarify the dissipation
mechanism, and extract intersubband relaxation rates for weak and strong
excitations.Comment: 4 pages, 4 figure
The correlation energy functional within the GW-RPA approximation: exact forms, approximate forms and challenges
In principle, the Luttinger-Ward Green's function formalism allows one to
compute simultaneously the total energy and the quasiparticle band structure of
a many-body electronic system from first principles. We present approximate and
exact expressions for the correlation energy within the GW-RPA approximation
that are more amenable to computation and allow for developing efficient
approximations to the self-energy operator and correlation energy. The exact
form is a sum over differences between plasmon and interband energies. The
approximate forms are based on summing over screened interband transitions. We
also demonstrate that blind extremization of such functionals leads to
unphysical results: imposing physical constraints on the allowed solutions
(Green's functions) is necessary. Finally, we present some relevant numerical
results for atomic systems.Comment: 3 figures and 3 tables, under review at Physical Review
Fast computation of the Kohn-Sham susceptibility of large systems
For hybrid systems, such as molecules grafted onto solid surfaces, the
calculation of linear response in time dependent density functional theory is
slowed down by the need to calculate, in N^4 operations, the susceptibility of
N non interacting Kohn-Sham reference electrons. We show how this
susceptibility can be calculated N times faster within finite precision. By
itself or in combination with previous methods, this should facilitate the
calculation of TDDFT response and optical spectra of hybrid systems.Comment: submitted 25/1/200
Undoing static correlation: Long-range charge transfer in time-dependent density functional theory
Long-range charge transfer excited states are notoriously badly
underestimated in time-dependent density functional theory (TDDFT). We resolve
how {\it exact} TDDFT captures charge transfer between open-shell species: in
particular the role of the step in the ground-state potential, and the severe
frequency-dependence of the exchange-correlation kernel. An expression for the
latter is derived, that becomes exact in the limit that the charge-transfer
excitations are well-separated from other excitations. The exchange-correlation
kernel has the task of undoing the static correlation in the ground state
introduced by the step, in order to accurately recover the physical
charge-transfer states.Comment: 2 figure
Photoelectron spectra of anionic sodium clusters from time-dependent density-functional theory in real-time
We calculate the excitation energies of small neutral sodium clusters in the
framework of time-dependent density-functional theory. In the presented
calculations, we extract these energies from the power spectra of the dipole
and quadrupole signals that result from a real-time and real-space propagation.
For comparison with measured photoelectron spectra, we use the ionic
configurations of the corresponding single-charged anions. Our calculations
clearly improve on earlier results for photoelectron spectra obtained from
static Kohn-Sham eigenvalues
Non-adiabatic electron dynamics in time-dependent density-functional theory
Time-dependent density-functional theory (TDDFT) treats dynamical exchange
and correlation (xc) via a single-particle potential, Vxc(r,t), defined as a
nonlocal functional of the density n(r',t'). The popular adiabatic
local-density approximation (ALDA) for Vxc(r,t) uses only densities at the same
space-time point (r,t). To go beyond the ALDA, two local approximations have
been proposed based on quantum hydrodynamics and elasticity theory: (a) using
the current as basic variable (C-TDDFT) [G. Vignale, C. A. Ullrich, and S.
Conti, Phys. Rev. Lett. 79, 4878 (1997)], (b) working in a co-moving Lagrangian
reference frame (L-TDDFT) [I. V. Tokatly, Phys. Rev. B 71, 165105 (2005)]. This
paper illustrates, compares, and analyzes both non-adiabatic theories for
simple time-dependent model densities in the linear and nonlinear regime, for a
broad range of time and frequency scales. C- and L-TDDFT are identical in
certain limits, but in general exhibit qualitative and quantitative differences
in their respective treatment of elastic and dissipative electron dynamics. In
situations where the electronic density rapidly undergoes large deformations,
it is found that non-adiabatic effects can become significant, causing the ALDA
to break down.Comment: 15 pages, 15 figure
Spin gaps and spin-flip energies in density-functional theory
Energy gaps are crucial aspects of the electronic structure of finite and
extended systems. Whereas much is known about how to define and calculate
charge gaps in density-functional theory (DFT), and about the relation between
these gaps and derivative discontinuities of the exchange-correlation
functional, much less is know about spin gaps. In this paper we give
density-functional definitions of spin-conserving gaps, spin-flip gaps and the
spin stiffness in terms of many-body energies and in terms of single-particle
(Kohn-Sham) energies. Our definitions are as analogous as possible to those
commonly made in the charge case, but important differences between spin and
charge gaps emerge already on the single-particle level because unlike the
fundamental charge gap spin gaps involve excited-state energies. Kohn-Sham and
many-body spin gaps are predicted to differ, and the difference is related to
derivative discontinuities that are similar to, but distinct from, those
usually considered in the case of charge gaps. Both ensemble DFT and
time-dependent DFT (TDDFT) can be used to calculate these spin discontinuities
from a suitable functional. We illustrate our findings by evaluating our
definitions for the Lithium atom, for which we calculate spin gaps and spin
discontinuities by making use of near-exact Kohn-Sham eigenvalues and,
independently, from the single-pole approximation to TDDFT. The many-body
corrections to the Kohn-Sham spin gaps are found to be negative, i.e., single
particle calculations tend to overestimate spin gaps while they underestimate
charge gaps.Comment: 11 pages, 1 figure, 3 table
Rydberg transition frequencies from the Local Density Approximation
A method is given that extracts accurate Rydberg excitations from LDA density
functional calculations, despite the short-ranged potential. For the case of He
and Ne, the asymptotic quantum defects predicted by LDA are in less than 5%
error, yielding transition frequency errors of less than 0.1eV.Comment: 4 pages, 6 figures, submitted to Phys. Rev. Let
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