1,016 research outputs found
Impurity and boundary effects in one and two-dimensional inhomogeneous Heisenberg antiferromagnets
We calculate the ground-state energy of one and two-dimensional spatially
inhomogeneous antiferromagnetic Heisenberg models for spins 1/2, 1, 3/2 and 2.
Our calculations become possible as a consequence of the recent formulation of
density-functional theory for Heisenberg models. The method is similar to
spin-density-functional theory, but employs a local-density-type approximation
designed specifically for the Heisenberg model, allowing us to explore
parameter regimes that are hard to access by traditional methods, and to
consider complications that are important specifically for nanomagnetic
devices, such as the effects of impurities, finite-size, and boundary geometry,
in chains, ladders, and higher-dimensional systems.Comment: 4 pages, 4 figures, accepted by Phys. Rev.
Thermal versus quantum fluctuations of optical-lattice fermions
We show that, for fermionic atoms in a one-dimensional optical lattice, the fraction of atoms in doubly occupied sites is a highly nonmonotonic function of temperature. We demonstrate that this property persists even in the presence of realistic harmonic confinement, and that it leads to a suppression of entropy at intermediate temperatures that offers a route to adiabatic cooling. Our interpretation of the suppression is that such intermediate temperatures are simultaneously too high for quantum coherence and too low for significant thermal excitation of double occupancy thus offering a clear indicator of the onset of quantum fluctuations.Publisher PDFPeer reviewe
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 entanglement of few-particle systems when using the local-density approximation
In this chapter we discuss methods to calculate the entanglement of a system
using density-functional theory. We firstly introduce density-functional theory
and the local-density approximation (LDA). We then discuss the concept of the
`interacting LDA system'. This is characterised by an interacting many-body
Hamiltonian which reproduces, uniquely and exactly, the ground state density
obtained from the single-particle Kohn-Sham equations of density-functional
theory when the local-density approximation is used. We motivate why this idea
can be useful for appraising the local-density approximation in many-body
physics particularly with regards to entanglement and related quantum
information applications. Using an iterative scheme, we find the Hamiltonian
characterising the interacting LDA system in relation to the test systems of
Hooke's atom and helium-like atoms. The interacting LDA system ground state
wavefunction is then used to calculate the spatial entanglement and the results
are compared and contrasted with the exact entanglement for the two test
systems. For Hooke's atom we also compare the entanglement to our previous
estimates of an LDA entanglement. These were obtained using a combination of
evolutionary algorithm and gradient descent, and using an LDA-based
perturbative approach. We finally discuss if the position-space information
entropy of the density---which can be obtained directly from the system density
and hence easily from density-functional theory methods---can be considered as
a proxy measure for the spatial entanglement for the test systems.Comment: 12 pages and 5 figures
Hydrogen effect on fatigue life of a pipe steel
Transport by pipe is the means more used, at the present time, to convey energies of
their point of extraction until their field sites final. To limit any risk of explosion or of escape
and thus to limit the geological problems of pollution and the human risks, it is necessary to
be able to know the mechanical properties of the steels used in the manufacture of these pipes.
With the reduction in oil stocks, it is necessary to find a new energy. Hydrogen is this new
energy vector, it thus will also be necessary to be able to transport it. This study makes it
possible to emphasize the assignment of the lifespan of hydrogen on a pipeline steel normally
used in the transport of gas. The fatigue tests in 3 points bending are carried out on samples
not standards because of dimensions of the tube of origin
Exact and approximate relations for the spin-dependence of the exchange energy in high magnetic fields
The exchange energy of an arbitrary collinear-spin many-body system in an
external magnetic field is a functional of the spin-resolved charge and current
densities, .
Within the framework of density-functional theory (DFT), we show that the
dependence of this functional on the four densities can be fully reconstructed
from either of two extreme limits: a fully polarized system or a completely
unpolarized system. Reconstruction from the limit of an unpolarized system
yields a generalization of the Oliver-Perdew spin scaling relations from
spin-DFT to current-DFT. Reconstruction from the limit of a fully polarized
system is used to derive the high-field form of the local-spin-density
approximation to current-DFT and to magnetic-field DFT.Comment: Int. J. Mod Phys. B, accepted, 2008 (Proceedings of the 18th
International Conference on High Magnetic Fields in Semiconductor Physics and
Nanotechnology). 5 page
Trends to determine fracture intiation and propagation of a pipe under service pressure
A fracture toughness transferability curve has been established for the X52 pipe steels
described by a linear relationship between the notch critical stress intensity factor and the average
value of T stress over the opening stress distribution. This curve is used to determine the fracture
toughness associated with the structure.
the characteristic length of the fracture process. Crack extension modelled by Finite Element
method using CTOA criterion coupled with the the node release technique is used to predict the
crack velocity, the arrest pressure and crack length. This method is compared with the different
Two Curves Methods Batelle, HLP and HLP-Sumitom
Density-functional calculation of ionization energies of current-carrying atomic states
Current-density-functional theory is used to calculate ionization energies of
current-carrying atomic states. A perturbative approximation to full
current-density-functional theory is implemented for the first time, and found
to be numerically feasible. Different parametrizations for the
current-dependence of the density functional are critically compared. Orbital
currents in open-shell atoms turn out to produce a small shift in the
ionization energies. We find that modern density functionals have reached an
accuracy at which small current-related terms appearing in open-shell
configurations are not negligible anymore compared to the remaining difference
to experiment.Comment: 7 pages, 2 tables, accepted by Phys. Rev.
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