5,935 research outputs found
Conserving Approximations in Time-Dependent Density Functional Theory
In the present work we propose a theory for obtaining successively better
approximations to the linear response functions of time-dependent density or
current-density functional theory. The new technique is based on the
variational approach to many-body perturbation theory (MBPT) as developed
during the sixties and later expanded by us in the mid nineties. Due to this
feature the resulting response functions obey a large number of conservation
laws such as particle and momentum conservation and sum rules. The quality of
the obtained results is governed by the physical processes built in through
MBPT but also by the choice of variational expressions. We here present several
conserving response functions of different sophistication to be used in the
calculation of the optical response of solids and nano-scale systems.Comment: 11 pages, 4 figures, revised versio
Wick Theorem for General Initial States
We present a compact and simplified proof of a generalized Wick theorem to
calculate the Green's function of bosonic and fermionic systems in an arbitrary
initial state. It is shown that the decomposition of the non-interacting
-particle Green's function is equivalent to solving a boundary problem for
the Martin-Schwinger hierarchy; for non-correlated initial states a one-line
proof of the standard Wick theorem is given. Our result leads to new
self-energy diagrams and an elegant relation with those of the imaginary-time
formalism is derived. The theorem is easy to use and can be combined with any
ground-state numerical technique to calculate time-dependent properties.Comment: 9 pages, 5 figure; extended version published in Phys. Rev.
Correlation effects in bistability at the nanoscale: steady state and beyond
The possibility of finding multistability in the density and current of an
interacting nanoscale junction coupled to semi-infinite leads is studied at
various levels of approximation. The system is driven out of equilibrium by an
external bias and the non-equilibrium properties are determined by real-time
propagation using both time-dependent density functional theory (TDDFT) and
many-body perturbation theory (MBPT). In TDDFT the exchange-correlation effects
are described within a recently proposed adiabatic local density approximation
(ALDA). In MBPT the electron-electron interaction is incorporated in a
many-body self-energy which is then approximated at the Hartree-Fock (HF),
second-Born (2B) and GW level. Assuming the existence of a steady-state and
solving directly the steady-state equations we find multiple solutions in the
HF approximation and within the ALDA. In these cases we investigate if and how
these solutions can be reached through time evolution and how to reversibly
switch between them. We further show that for the same cases the inclusion of
dynamical correlation effects suppresses bistability.Comment: 13 pages, 12 figure
Hipparcos open clusters and stellar evolution
By relying on recently improved Hipparcos parallaxes for the Hyades, Pleiades
and Ursa Major clusters we find that stellar models with updated physical
inputs nicely reproduce the location in the color magnitude diagram of main
sequence stars of different metallicities. Stars in the helium burning phase
are also discussed, showing that the luminosity of giants in the Hyades,
Praesepe and Ursa Major clusters appears to be in reasonable agreement with
theoretical predictions. A short discussion concerning the current evolutionary
scenarios closes the paper.Comment: 5 pages, 6 Postscript figures, accepted by MNRA
Normal modes of a quasi-one-dimensional multi-chain complex plasma
We studied equally charged particles, suspended in a complex plasma, which
move in a plane and interact with a screened Coulomb potential (Yukawa type)
and with an additional external confining parabolic potential in one direction,
that makes the system quasi-one-dimensional (Q1D). The normal modes of the
system are studied in the presence of dissipation. We also investigated how a
perpendicular magnetic field couples the phonon modes with each other. Two
different ways of exciting the normal modes are discussed: 1) a uniform
excitation of the Q1D lattice, and 2) a local forced excitation of the system
in which one particle is driven by e.g. a laser. Our results are in very good
agreement with recent experimental findings on a finite single chain system
(Phys. Rev. Lett. {\bf 91}, 255003 (2003)). Predictions are made for the normal
modes of multi-chain structures in the presence of damping.Comment: 15 pages, 14 figures, accepted for publication on PR
Ensemble v-representable ab-initio density functional calculation of energy and spin in atoms: atest of exchange-correlation approximations
The total energies and the spin states for atoms and their first ions with Z
= 1-86 are calculated within the the local spin-density approximation (LSDA)
and the generalized-gradient approximation (GGA) to the exchange-correlation
(xc) energy in density-functional theory. Atoms and ions for which the
ground-state density is not pure-state v-representable, are treated as ensemble
v- representable with fractional occupations of the Kohn-Sham system. A newly
developed algorithm which searches over ensemble v-representable densities [E.
Kraisler et al., Phys. Rev. A 80, 032115 (2009)] is employed in calculations.
It is found that for many atoms the ionization energies obtained with the GGA
are only modestly improved with respect to experimental data, as compared to
the LSDA. However, even in those groups of atoms where the improvement is
systematic, there remains a non-negligible difference with respect to the
experiment. The ab-initio electronic configuration in the Kohn-Sham reference
system does not always equal the configuration obtained from the spectroscopic
term within the independent-electron approximation. It was shown that use of
the latter configuration can prevent the energy-minimization process from
converging to the global minimum, e.g. in lanthanides. The spin values
calculated ab-initio fit the experiment for most atoms and are almost
unaffected by the choice of the xc-functional. Among the systems with
incorrectly obtained spin there exist some cases (e.g. V, Pt) for which the
result is found to be stable with respect to small variations in the
xc-approximation. These findings suggest a necessity for a significant
modification of the exchange-correlation functional, probably of a non-local
nature, to accurately describe such systems. PACS numbers: 31.15.
Levels of self-consistency in the GW approximation
We perform calculations on atoms and diatomic molecules at different
levels of self-consistency and investigate the effects of self-consistency on
total energies, ionization potentials and on particle number conservation. We
further propose a partially self-consistent scheme in which we keep the
correlation part of the self-energy fixed within the self-consistency cycle.
This approximation is compared to the fully self-consistent results and to
the and the approximations. Total energies, ionization
potentials and two-electron removal energies obtained with our partially
self-consistent approximation are in excellent agreement with fully
self-consistent results while requiring only a fraction of the
computational effort. We also find that self-consistent and partially
self-consistent schemes provide ionization energies of similar quality as the
values but yield better total energies and energy differences.Comment: 11 pages, 3 figures, 3 table
Anomalous Negative Magnetoresistance Caused by Non-Markovian Effects
A theory of recently discovered anomalous low-field magnetoresistance is
developed for the system of two-dimensional electrons scattered by hard disks
of radius randomly distributed with concentration For small magnetic
fields the magentoresistance is found to be parabolic and inversely
proportional to the gas parameter, With increasing field the magnetoresistance becomes linear
in a good agreement with the
experiment and numerical simulations.Comment: 4 pages RevTeX, 5 figure
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