1,022 research outputs found
New results for the two neutrino double beta decay in deformed nuclei with angular momentum projected basis
Four nuclei which are proved to be emitters (Ge,
Se, Nd, U), and four suspected, due to the corresponding
Q-values, to have this property (Nd, Sm, Gd,
Th), were treated within a proton-neutron quasiparticle random phase
approximation (pnQRPA) with a projected spherical single particle basis. The
advantage of the present procedure over the ones using a deformed Woods Saxon
or Nilsson single particle basis is that the actual pnQRPA states have a
definite angular momentum while all the others provide states having only K as
a good quantum number. The model Hamiltonian involves a mean field term
yielding the projected single particle states, a pairing interaction for alike
nucleons and a dipole-dipole proton-neutron interaction in both the
particle-hole (ph) and particle-particle (pp) channels. The effect of nuclear
deformation on the single beta strength distribution as well as on the double
beta Gamow-Teller transition amplitude (M) is analyzed. The
results are compared with the existent data and with the results from a
different approach, in terms of the process half life T. The case of
different deformations for mother and daughter nuclei is also presented.Comment: 45 pages, 13 figure
Mean field theory for global binding systematics
We review some possible improvements of mean field theory for application to
nuclear binding systematics. Up to now, microscopic theory has been less
successful than models starting from the liquid drop in describing accurately
the global binding systematics. We believe that there are good prospects to
develop a better global theory, using modern forms of energy density
functionals and treating correlation energies systematically by the RPA.Comment: RevTex, 17 pages, 5 eps figures. To be published in Yadernaya Fizika,
special edition for the 90th birthday of Professor A.B. Migda
Effect of magnetic frustration on single-hole spectral function in the t-t'-t''-J model
We examine the effect of the magnetic frustration J' on the single-hole
spectral function in the t-t'-t''-J model. At zero temperature, the exact
diagonalization (ED) and the self-consistent Born approximation (SCBA) methods
are used. We find that the frustration suppresses the quasiparticle (QP) weight
at small momentum k, whereas the QP peak at k=(pi/2,pi/2) remains sharp. We
also show the temperature dependence of the single-hole spectral function by
using the ED method. It is found that the lineshapes at (pi/2,0) and
(pi/2,pi/2) show different temperature dependence. These findings are
consistent with the angle-resolved photoemission data on Sr2CuO2Cl2, and
indicate the importance of the magnetic frustration on the electronic states of
the insulating cuprates.Comment: 5 pages, 3 EPS figures, REVTeX, To be published in Phys. Rev. B, Vol.
59, Num. 3 (15 Jan. 1999
Phenomenological and microscopic cluster models I. The geometric mapping
The geometrical mapping of algebraic nuclear cluster models is investigated
within the coherent state formalism. Two models are considered: the
Semimicroscopic Algebraic Cluster Model (SACM) and the Phenomenological
Algebraic Cluster Model (PACM), which is a special limit of the SACM. The SACM
strictly observes the Pauli exclusion principle while the PACM does not. The
discussion of the SACM is adapted to the coherent state formalism by
introducing the new SO(3) dynamical symmetry limit and third-order interaction
terms in the Hamiltonian. The potential energy surface is constructed in both
models and it is found that the effects of the Pauli principle can be simulated
by higher-order interaction terms in the PACM. The present study is also meant
to serve as a starting point for investigating phase transitions in the two
algebraic cluster models.Comment: 13 pages, 0 figures, part one of a two part wor
The Nucleon Spectral Function at Finite Temperature and the Onset of Superfluidity in Nuclear Matter
Nucleon selfenergies and spectral functions are calculated at the saturation
density of symmetric nuclear matter at finite temperatures. In particular, the
behaviour of these quantities at temperatures above and close to the critical
temperature for the superfluid phase transition in nuclear matter is discussed.
It is shown how the singularity in the thermodynamic T-matrix at the critical
temperature for superfluidity (Thouless criterion) reflects in the selfenergy
and correspondingly in the spectral function. The real part of the on-shell
selfenergy (optical potential) shows an anomalous behaviour for momenta near
the Fermi momentum and temperatures close to the critical temperature related
to the pairing singularity in the imaginary part. For comparison the selfenergy
derived from the K-matrix of Brueckner theory is also calculated. It is found,
that there is no pairing singularity in the imaginary part of the selfenergy in
this case, which is due to the neglect of hole-hole scattering in the K-matrix.
From the selfenergy the spectral function and the occupation numbers for finite
temperatures are calculated.Comment: LaTex, 23 pages, 21 PostScript figures included (uuencoded), uses
prc.sty, aps.sty, revtex.sty, psfig.sty (last included
Variational Monte Carlo Study of Electron Differentiation around Mott Transition
We study ground-state properties of the two-dimensional Hubbard model at half
filling by improving variational Monte Carlo method and by implementing
quantum-number projection and multi-variable optimization. The improved
variational wave function enables a highly accurate description of the Mott
transition and strong fluctuations in metals. We clarify how anomalous metals
appear near the first-order Mott transition. The double occupancy stays nearly
constant as a function of the on-site Coulomb interaction in the metallic phase
near the Mott transition in agreement with the previous unbiased results. This
unconventional metal at half filling is stabilized by a formation of
``electron-like pockets'' coexisting with an arc structure, which leads to a
prominent differentiation of electrons in momentum space. An abrupt collapse of
the ``pocket'' and ``arc'' drives the first-order Mott transition.Comment: 4 pages, 3 figure
Spectrum generating algebras for position-dependent mass oscillator Schrodinger equations
The interest of quadratic algebras for position-dependent mass Schr\"odinger
equations is highlighted by constructing spectrum generating algebras for a
class of d-dimensional radial harmonic oscillators with and a
specific mass choice depending on some positive parameter . Via some
minor changes, the one-dimensional oscillator on the line with the same kind of
mass is included in this class. The existence of a single unitary irreducible
representation belonging to the positive-discrete series type for and
of two of them for d=1 is proved. The transition to the constant-mass limit
is studied and deformed su(1,1) generators are constructed.
These operators are finally used to generate all the bound-state wavefunctions
by an algebraic procedure.Comment: 21 pages, no figure, 2 misprints corrected; published versio
Shell model description of normal parity bands in odd-mass heavy deformed nuclei
The low-energy spectra and B(E2) electromagnetic transition strengths of
159Eu, 159Tb and 159Dy are described using the pseudo SU(3) model. Normal
parity bands are built as linear combinations of SU(3) states, which are the
direct product of SU(3) proton and neutron states with pseudo spin zero (for
even number of nucleons) and pseudo spin 1/2 (for odd number of nucleons). Each
of the many-particle states have a well-defined particle number and total
angular momentum. The Hamiltonian includes spherical Nilsson single-particle
energies, the quadrupole-quadrupole and pairing interactions, as well as three
rotor terms which are diagonal in the SU(3) basis. The pseudo SU(3) model is
shown to be a powerful tool to describe odd-mass heavy deformed nuclei.Comment: 11 pages, 2 figures, Accepted to be published in Phys. Rev.
Superconductivity in Ultrasmall Metallic Grains
We develop a theory of superconductivity in ultrasmall (nm-scale) metallic
grains having a discrete electronic eigenspectrum with a mean level spacing of
order of the bulk gap. The theory is based on calculating the eigenspectrum
using a generalized BCS variational approach, whose applicability has been
extensively demonstrated in studies of pairing correlations in nuclear physics.
We discuss how conventional mean field theory breaks down with decreasing
sample size, how the so-called blocking effect weakens pairing correlations in
states with non-zero total spin, and how this affects the discrete
eigenspectrum's behavior in a magnetic field, which favors non-zero total spin.
In ultrasmall grains, spin magnetism dominates orbital magnetism, just as in
thin films in a parallel field; but whereas in the latter the magnetic-field
induced transition to a normal state is known to be first-order, we show that
in ultrasmall grains it is softened by finite size effects. Our calculations
qualitatively reproduce the magnetic-field dependent tunneling spectra for
individual aluminum grains measured recently by Ralph, Black and Tinkham. We
argue that previously-discussed parity effects for the odd-even ground state
energy difference are presently not observable for experimental reasons, and
propose an analogous parity effect for the pair-breaking energy that should be
observable provided that the grain size can be controlled sufficiently well.
Finally, experimental evidence is pointed out that the dominant role played by
time-reversed pairs of states, well-established in bulk and in dirty
superconductors, persists also in ultrasmall grains.Comment: 21 pages RevTeX, 12 EPS figures included, uses epsf.st
Dissociation of vertical semiconductor diatomic artificial molecules
We investigate the dissociation of few-electron circular vertical
semiconductor double quantum dot artificial molecules at 0 T as a function of
interdot distance. Slight mismatch introduced in the fabrication of the
artificial molecules from nominally identical constituent quantum wells induces
localization by offsetting the energy levels in the quantum dots by up to 2
meV, and this plays a crucial role in the appearance of the addition energy
spectra as a function of coupling strength particularly in the weak coupling
limit.Comment: Accepted for publication in Phys. Rev. Let
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