14,282 research outputs found
Approximate solution of the pairing Hamiltonian in the Berggren basis
We derive the approximate solution for the pairing Hamiltonian in the
Berggren ensemble of single particle states including bound, resonance and
non-resonant scattering states. We show that this solution is reliable in the
limit of a weak pairing interaction
Nuclear three-body problem in the complex energy plane: Complex-Scaling-Slater method
The physics of open quantum systems is an interdisciplinary area of research.
The nuclear "openness" manifests itself through the presence of the many-body
continuum representing various decay, scattering, and reaction channels. As the
radioactive nuclear beam experimentation extends the known nuclear landscape
towards the particle drip lines, the coupling to the continuum space becomes
exceedingly more important. Of particular interest are weakly bound and unbound
nuclear states appearing around particle thresholds. Theories of such nuclei
must take into account their open quantum nature. To describe open quantum
systems, we introduce a Complex Scaling (CS) approach in the Slater basis. We
benchmark it with the complex-energy Gamow Shell Model (GSM) by studying
energies and wave functions of the bound and unbound states of the two-neutron
halo nucleus 6He viewed as an + n + n cluster system. In the CS
approach, we use the Slater basis, which exhibits the correct asymptotic
behavior at large distances. To extract particle densities from the
back-rotated CS solutions, we apply the Tikhonov regularization procedure,
which minimizes the ultraviolet numerical noise. While standard applications of
the inverse complex transformation to the complex-rotated solution provide
unstable results, the stabilization method fully reproduces the GSM benchmark.
We also propose a method to determine the smoothing parameter of the Tikhonov
regularization. The combined suite of CS-Slater and GSM techniques has many
attractive features when applied to nuclear problems involving weakly-bound and
unbound states. While both methods can describe energies, total widths, and
wave functions of nuclear states, the CS-Slater method, if it can be applied,
can provide an additional information about partial energy widths associated
with individual thresholds.Comment: 15 pages, 16 figure
Uniform WKB approximation of Coulomb wave functions for arbitrary partial wave
Coulomb wave functions are difficult to compute numerically for extremely low
energies, even with direct numerical integration. Hence, it is more convenient
to use asymptotic formulas in this region. It is the object of this paper to
derive analytical asymptotic formulas valid for arbitrary energies and partial
waves. Moreover, it is possible to extend these formulas for complex values of
parameters.Comment: 5 pages, 2 figure
Crystal Structures of Polymerized Fullerides AC60, A=K, Rb, Cs and Alkali-mediated Interactions
Starting from a model of rigid interacting C60 polymer chains on an
orthorhombic lattice, we study the mutual orientation of the chains and the
stability of the crystalline structures Pmnn and I2/m. We take into account i)
van der Waals interactions and electric quadrupole interactions between C60
monomers on different chains as well as ii) interactions of the monomers with
the surrounding alkali atoms. The direct interactions i) always lead to an
antiferrorotational structure Pmnn with alternate orientation of the C60 chains
in planes (001). The interactions ii) with the alkalis consist of two parts:
translation-rotation (TR) coupling where the orientations of the chains
interact with displacements of the alkalis, and quadrupolar electronic
polarizability (ep) coupling, where the electric quadrupoles on the C60
monomers interact with induced quadrupoles due to excited electronic d states
of the alkalis. Both interactions ii) lead to an effective
orientation-orientation interaction between the C60 chains and always favor the
ferrorotational structure I2/m where C60 chains have a same orientation. The
structures Pmnn for KC60 and I2/m for Rb- and CsC60 are the result of a
competition between the direct interaction i) and the alkali-mediated
interactions ii). In Rb- and CsC60 the latter are found to be dominant, the
preponderant role being played by the quadrupolar electronic polarizability of
the alkali ions.Comment: J.Chem.Phys., in press, 14 pages, 3 figures, 8 table
Access to improve the muon mass and magnetic moment anomaly via the bound-muon factor
A theoretical description of the factor of a muon bound in a nuclear
potential is presented. One-loop self-energy and multi-loop vacuum polarization
corrections are calculated, taking into account the interaction with the
binding potential exactly. Nuclear effects on the bound-muon factor are
also evaluated. We put forward the measurement of the bound-muon factor via
the continuous Stern-Gerlach effect as an independent means to determine the
free muons magnetic moment anomaly and mass. The scheme presented enables to
increase the accuracy of the mass by more than an order of magnitude
Accretion of Ghost Condensate by Black Holes
The intent of this letter is to point out that the accretion of a ghost
condensate by black holes could be extremely efficient. We analyze steady-state
spherically symmetric flows of the ghost fluid in the gravitational field of a
Schwarzschild black hole and calculate the accretion rate. Unlike minimally
coupled scalar field or quintessence, the accretion rate is set not by the
cosmological energy density of the field, but by the energy scale of the ghost
condensate theory. If hydrodynamical flow is established, it could be as high
as tenth of a solar mass per second for 10MeV-scale ghost condensate accreting
onto a stellar-sized black hole, which puts serious constraints on the
parameters of the ghost condensate model.Comment: 5 pages, 3 figures, REVTeX 4.0; discussion expande
Chaotic motion of charged particles in toroidal magnetic configurations
We study the motion of a charged particle in a tokamak magnetic field and
discuss its chaotic nature. Contrary to most of recent studies, we do not make
any assumption on any constant of the motion and solve numerically the
cyclotron gyration using Hamiltonian formalism. We take advantage of a
symplectic integrator allowing us to make long-time simulations. First
considering an idealized magnetic configuration, we add a non generic
perturbation corresponding to a magnetic ripple, breaking one of the invariant
of the motion. Chaotic motion is then observed and opens questions about the
link between chaos of magnetic field lines and chaos of particle trajectories.
Second, we return to a axi-symmetric configuration and tune the safety factor
(magnetic configuration) in order to recover chaotic motion. In this last
setting with two constants of the motion, the presence of chaos implies that no
third global constant exists, we highlight this fact by looking at variations
of the first order of the magnetic moment in this chaotic setting. We are
facing a mixed phase space with both regular and chaotic regions and point out
the difficulties in performing a global reduction such as gyrokinetics
Dipole-active optical phonons in YTiO_3: ellipsometry study and lattice-dynamics calculations
The anisotropic complex dielectric response was accurately extracted from
spectroscopic ellipsometry measurements at phonon frequencies for the three
principal crystallographic directions of an orthorhombic (Pbnm) YTiO_3 single
crystal. We identify all twenty five infrared-active phonon modes allowed by
symmetry, 7B_1u, 9B_2u, and 9B_3u, polarized along the c-, b-, and a-axis,
respectively. From a classical dispersion analysis of the complex dielectric
functions \tilde\epsilon(\omega) and their inverses -1/\tilde\epsilon(\omega)
we define the resonant frequencies, widths, and oscillator strengths of the
transverse (TO) and longitudinal (LO) phonon modes. We calculate
eigenfrequencies and eigenvectors of B_1u, B_2u, and B_3u normal modes and
suggest assignments of the TO phonon modes observed in our ellipsometry spectra
by comparing their frequencies and oscillator strengths with those resulting
from the present lattice-dynamics study. Based on these assignments, we
estimate dynamical effective charges of the atoms in the YTiO_3 lattice. We
find that, in general, the dynamical effective charges in YTiO_3 lattice are
typical for a family of perovskite oxides. By contrast to a ferroelectric
BaTiO_3, the dynamical effective charge of oxygen related to a displacement
along the c-axis does not show the anomalously large value. At the same time,
the dynamical effective charges of Y and ab-plane oxygen exhibit anisotropy,
indicating strong hybridization along the a-axis.Comment: 8 pages, 7 figure
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