629 research outputs found
Variational calculation of many-body wave functions and energies from density-functional theory
A generating coordinate is introduced into the exchange-correlation
functional of density-functional theory (DFT). The many-body wave function is
represented as a superposition of Kohn-Sham (KS) Slater determinants arising
from different values of the generating coordinate. This superposition is used
to variationally calculate many-body energies and wave functions from solutions
of the KS equation of DFT. The method works for ground and excited states, and
does not depend on identifying the KS orbitals and energies with physical ones.
Numerical application to the Helium isoelectronic series illustrates the
method's viability and potential.Comment: 4 pages, 2 tables, J. Chem. Phys., accepte
BCS and generalized BCS superconductivity in relativistic quantum field theory. I. formulation
We investigate the BCS and generalized BCS theories in the relativistic
quantum field theory. We select the gauge freedom as U(1), and introduce a
BCS-type effective attractive interaction. After introducing the Gor'kov
formalism and performing the group theoretical consideration of the mean
fields, we solve the relativistic Gor'kov equation and obtain the Green's
functions in analytical forms. We obtain various types of gap equations.Comment: 31 page
The generator coordinate method in time-dependent density-functional theory: memory made simple
The generator coordinate (GC) method is a variational approach to the quantum
many-body problem in which interacting many-body wave functions are constructed
as superpositions of (generally nonorthogonal) eigenstates of auxiliary
Hamiltonians containing a deformation parameter. This paper presents a
time-dependent extension of the GC method as a new approach to improve existing
approximations of the exchange-correlation (XC) potential in time-dependent
density-functional theory (TDDFT). The time-dependent GC method is shown to be
a conceptually and computationally simple tool to build memory effects into any
existing adiabatic XC potential. As an illustration, the method is applied to
driven parametric oscillations of two interacting electrons in a harmonic
potential (Hooke's atom). It is demonstrated that a proper choice of
time-dependent generator coordinates in conjunction with the adiabatic
local-density approximation reproduces the exact linear and nonlinear
two-electron dynamics quite accurately, including features associated with
double excitations that cannot be captured by TDDFT in the adiabatic
approximation.Comment: 10 pages, 13 figure
Interaction Between Superconducting and Ferromagnetic Order Parameters in Graphite-Sulfur Composites
The superconductivity of graphite-sulfur composites is highly anisotropic and
associated with the graphite planes. The superconducting state coexists with
the ferromagnetism of pure graphite, and a continuous crossover from
superconducting to ferromagnetic-like behavior could be achieved by increasing
the magnetic field or the temperature. The angular dependence of the magnetic
moment m(alpha) provides evidence for an interaction between the ferromagnetic
and the superconducting order parameters.Comment: 11 pages, 4 figures, to be published in Phys. Rev.
Exchange-correlation vector potentials and vorticity-dependent exchange-correlation energy densities in two-dimensional systems
We present a new approach how to calculate the scalar exchange-correlation
potentials and the vector exchange-correlation potentials from current-carrying
ground states of two-dimensional quantum dots. From these exchange-correlation
potentials we derive exchange-correlation energy densities and examine their
vorticity (or current) dependence. Compared with parameterizations of
current-induced effects in literature we find an increased significance of
corrections due to paramagnetic current densities.Comment: 5 figures, submitted to PR
Bethe-Ansatz density-functional theory of ultracold repulsive fermions in one-dimensional optical lattices
We present an extensive numerical study of ground-state properties of
confined repulsively interacting fermions on one-dimensional optical lattices.
Detailed predictions for the atom-density profiles are obtained from parallel
Kohn-Sham density-functional calculations and quantum Monte Carlo simulations.
The density-functional calculations employ a Bethe-Ansatz-based local-density
approximation for the correlation energy, which accounts for Luttinger-liquid
and Mott-insulator physics. Semi-analytical and fully numerical formulations of
this approximation are compared with each other and with a cruder
Thomas-Fermi-like local-density approximation for the total energy. Precise
quantum Monte Carlo simulations are used to assess the reliability of the
various local-density approximations, and in conjunction with these allow to
obtain a detailed microscopic picture of the consequences of the interplay
between particle-particle interactions and confinement in one-dimensional
systems of strongly correlated fermions.Comment: 14 pages, 11 figures, 1 table, submitte
Assessing does not mean threatening : the purpose of assessment as a key determinant of girls' and boys' performance in a science class
International audienceDans le domaine scientifique, plusieurs travaux pointent les performances plus faibles des filles, en comparaison de celles obtenues par les garçons, surtout lorsque la situation évaluative active le stéréotype négatif supposé à propos de leurs capacités. L'originalité de cette recherche a été de ne pas répliquer un effet de menace du stéréotype, cette étude a testé l'efficacité d'une évaluation orientée vers la maîtrise en gardant le caractère évaluatif de la situation et surtout a permis de prouver que cette situation offrait un environnement d'apprentissage équitable pour les filles et les garçons
Spin currents and spin dynamics in time-dependent density-functional theory
We derive and analyse the equation of motion for the spin degrees of freedom
within time-dependent spin-density-functional theory (TD-SDFT). Results are (i)
a prescription for obtaining many-body corrections to the single-particle spin
currents from the Kohn-Sham equation of TD-SDFT, (ii) the existence of an
exchange-correlation (xc) torque within TD-SDFT, (iii) a prescription for
calculating, from TD-SDFT, the torque exerted by spin currents on the spin
magnetization, (iv) a novel exact constraint on approximate xc functionals, and
(v) the discovery of serious deficiencies of popular approximations to TD-SDFT
when applied to spin dynamics.Comment: now includes discussion of OEP and GGA; to appear in Phys. Rev. Let
Segmented flow coil equilibrator coupled to a proton-transfer-reaction mass spectrometer for measurements of a broad range of volatile organic compounds in seawater
We present a technique that utilises a segmented flow coil equilibrator coupled to a proton-transferreaction mass spectrometer to measure a broad range of dissolved volatile organic compounds. Thanks to its relatively large surface area for gas exchange, small internal volume, and smooth headspace-water separation, the equilibrator is highly efficient for gas exchange and has a fast response time (under 1 min). The system allows for both continuous and discrete measurements of volatile organic compounds in seawater due to its low sample water flow (100 cm3 min-1) and the ease of changing sample intake. The equilibrator setup is both relatively inexpensive and compact. Hence, it can be easily reproduced and installed on a variety of oceanic platforms, particularly where space is limited. The internal area of the equilibrator is smooth and unreactive. Thus, the segmented flow coil equilibrator is expected to be less sensitive to biofouling and easier to clean than membrane-based equilibration systems. The equilibrator described here fully equilibrates for gases that are similarly soluble or more soluble than toluene and can easily be modified to fully equilibrate for even less soluble gases. The method has been successfully deployed in the Canadian Arctic. Some example data from underway surface water and Niskin bottle measurements in the sea ice zone are presented to illustrate the efficacy of this measurement system
Systematic investigation of a family of gradient-dependent functionals for solids
Eleven density functionals are compared with regard to their performance for
the lattice constants of solids. We consider standard functionals, such as the
local-density approximation and the Perdew-Burke-Ernzerhof (PBE)
generalized-gradient approximation (GGA), as well as variations of PBE GGA,
such as PBEsol and similar functionals, PBE-type functionals employing a
tighter Lieb-Oxford bound, and combinations thereof. Several of these
variations are proposed here for the first time. On a test set of 60 solids we
perform a system-by-system analysis for selected functionals and a full
statistical analysis for all of them. The impact of restoring the gradient
expansion and of tightening the Lieb-Oxford bound is discussed, and confronted
with previous results obtained from other codes, functionals or test sets. No
functional is uniformly good for all investigated systems, but surprisingly,
and pleasingly, the simplest possible modifications to PBE turn out to have the
most beneficial effect on its performance. The atomization energy of molecules
was also considered and on a testing set of six molecules, we found that the
PBE functional is clearly the best, the others leading to strong overbinding
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