175 research outputs found
Cosmological implications of conformal field theory
Requiring all massless elementary fields to have conformal scaling symmetry
removes a conflict between gravitational theory and the quantum theory of
elementary particles and fields. Extending this postulate to the scalar field
of the Higgs model, dynamical breaking of both gauge and conformal symmetries
determines parameters for the interacting fields. In uniform isotropic geometry
a modified Friedmann cosmic evolution equation is derived with nonvanishing
cosmological constant. Parameters determined by numerical solution are
consistent with empirical data for redshifts , including
luminosity distances for observed type Ia supernovae and peak structure ratios
in the cosmic microwave background (CMB). The theory does not require dark
matter.Comment: 8 pages Conclusions about the early universe which must be reexamined
have been removed. Manuscript revised and reformatted. Accepted for
publication in Modern Physics Letters A (2011
Cohesive energies of cubic III-V semiconductors
Cohesive energies for twelve cubic III-V semiconductors with zincblende
structure have been determined using an ab-initio scheme. Correlation
contributions, in particular, have been evaluated using the coupled-cluster
approach with single and double excitations (CCSD). This was done by means of
increments obtained for localized bond orbitals and for pairs and triples of
such bonds. Combining these results with corresponding Hartree-Fock data, we
recover about 92 \% of the experimental cohesive energies.Comment: 16 pages, 1 figure, late
The Hilbert-Schmidt Theorem Formulation of the R-Matrix Theory
Using the Hilbert-Schmidt theorem, we reformulate the R-matrix theory in
terms of a uniformly and absolutely convergent expansion. Term by term
differentiation is possible with this expansion in the neighborhood of the
surface. Methods for improving the convergence are discussed when the
R-function series is truncated for practical applications.Comment: 16 pages, Late
Resonance Lifetimes from Complex Densities
The ab-initio calculation of resonance lifetimes of metastable anions
challenges modern quantum-chemical methods. The exact lifetime of the
lowest-energy resonance is encoded into a complex "density" that can be
obtained via complex-coordinate scaling. We illustrate this with one-electron
examples and show how the lifetime can be extracted from the complex density in
much the same way as the ground-state energy of bound systems is extracted from
its ground-state density
Extension of continuum time-dependent Hartree-Fock method to proton states
This paper deals with the solution of the spherically symmetric time-dependent Hartree-Fock approximation applied to nuclear giant monopole resonances in the small amplitude regime. The problem is spatially unbounded as the resonance state is in the continuum. The practical requirement to perform the calculation in a finite-sized spatial region yields an artificial boundary, which is not present physically. The question of how to ensure the boundary does not interfere with the internal solution, while keeping the overall calculation time low is studied. Here we propose an absorbing boundary condition scheme to handle the conflict. The derivation, via a Laplace transform method, and implementation is described. An inverse Laplace transform required by the absorbing boundaries is calculated using a method of non-linear least squares. The accuracy and efficiency of the scheme is tested and results presented to support the case that they are a effective way of handling the artificial boundary
Spin-transfer in an open ferromagnetic layer: from negative damping to effective temperature
Spin-transfer is a typical spintronics effect that allows a ferromagnetic
layer to be switched by spin-injection. Most of the experimental results about
spin transfer are described on the basis of the Landau-Lifshitz-Gilbert
equation of the magnetization, in which additional current-dependent damping
factors are added, and can be positive or negative. The origin of the damping
can be investigated further by performing stochastic experiments, like one shot
relaxation experiments under spin-injection in the activation regime of the
magnetization. In this regime, the N\'eel-Brown activation law is observed
which leads to the introduction of a current-dependent effective temperature.
In order to justify the introduction of these counterintuitive parameters
(effective temperature and negative damping), a detailed thermokinetic analysis
of the different sub-systems involved is performed. We propose a thermokinetic
description of the different forms of energy exchanged between the electric and
the ferromagnetic sub-systems at a Normal/Ferromagnetic junction. The
corresponding Fokker Planck equations, including relaxations, are derived. The
damping coefficients are studied in terms of Onsager-Casimir transport
coefficients, with the help of the reciprocity relations. The effective
temperature is deduced in the activation regime.Comment: 65 pages, 10 figure
Low-lying continuum structures in B8 and Li8 in a microscopic model
We search for low-lying resonances in the B8 and Li8 nuclei using a
microscopic cluster model and a variational scattering method, which is
analytically continued to complex energies. After fine-tuning the
nucleon-nucleon interaction to get the known 1+ state of B8 at the right
energy, we reproduce the known spectra of the studied nuclei. In addition, our
model predicts a 1+ state at 1.3 MeV in B8, relative to the Be7+p threshold,
whose corresponding pair is situated right at the Li7+n threshold in Li8.
Lacking any experimental evidence for the existence of such states, it is
presently uncertain whether these structures really exist or they are spurious
resonances in our model. We demonstrate that the predicted state in B8, if it
exists, would have important consequences for the understanding of the
astrophysically important Be7(p,gamma)B8 reaction.Comment: 6 pages with 1 figure. The postscript file and more information are
available at http://nova.elte.hu/~csot
Non-Hermitian Rayleigh-Schroedinger Perturbation Theory
We devise a non-Hermitian Rayleigh-Schroedinger perturbation theory for the
single- and the multireference case to tackle both the many-body problem and
the decay problem encountered, for example, in the study of electronic
resonances in molecules. A complex absorbing potential (CAP) is employed to
facilitate a treatment of resonance states that is similar to the
well-established bound-state techniques. For the perturbative approach, the
full CAP-Schroedinger Hamiltonian, in suitable representation, is partitioned
according to the Epstein-Nesbet scheme. The equations we derive in the
framework of the single-reference perturbation theory turn out to be identical
to those obtained by a time-dependent treatment in Wigner-Weisskopf theory. The
multireference perturbation theory is studied for a model problem and is shown
to be an efficient and accurate method. Algorithmic aspects of the integration
of the perturbation theories into existing ab initio programs are discussed,
and the simplicity of their implementation is elucidated.Comment: 10 pages, 1 figure, RevTeX4, submitted to Physical Review
Ferromagnetic behavior of ultrathin manganese nanosheets
Ferromagnetic behaviour has been observed experimentally for the first time
in nanostructured Manganese. Ultrathin ( 0.6 nm) Manganese nanosheets
have been synthesized inside the two dimensional channels of sol-gel derived
Na-4 mica. The magnetic properties of the confined system are measured within
2K-300K temperature range. The confined structure is found to show a
ferromagnetic behaviour with a nonzero coercivity value. The coercivity value
remains positive throughout the entire temperature range of measurement. The
experimental variation of susceptibility as a function of temperature has been
satisfactorily explained on the basis of a two dimensional system with a
Heisenberg Hamiltonian involving direct exchange interaction.Comment: 13 pages, 9 figure
Identification of the Beutler-Fano formula in eigenphase shifts and eigentime delays near a resonance
Eigenphase shifts and eigentime delays near a resonance for a system of one
discrete state and two continua are shown to be functionals of the Beutler-
Fano formulas using appropriate dimensionless energy units and line profile
indices. Parameters responsible for the avoided crossing of eigenphase shifts
and eigentime delays are identified. Similarly, parameters responsible for the
eigentime delays due to a frame change are identified. With the help of new
parameters, an analogy with the spin model is pursued for the S matrix and time
delay matrix. The time delay matrix is shown to comprise three terms, one due
to resonance, one due to a avoided crossing interaction, and one due to a frame
change. It is found that the squared sum of time delays due to the avoided
crossing interaction and frame change is unity.Comment: 17 pages, 3 figures, RevTe
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