3,432 research outputs found

### Extended RPA with ground-state correlations

We propose a time-independent method for finding a correlated ground state of
an extended time-dependent Hartree-Fock theory, known as the time-dependent
density-matrix theory (TDDM). The correlated ground state is used to formulate
the small amplitude limit of TDDM (STDDM) which is a version of extended RPA
theories with ground-state correlations. To demonstrate the feasibility of the
method, we calculate the ground state of 22O and study the first 2+ state and
its two-phonon states using STDDM.Comment: 12 pages, 9 figure

### Double giant dipole resonances in time-dependent density-matrix theory

The strength functions of the DGDRs in 16O and 40Ca are calculated using an
extended version of TDHF known as the time-dependent density-matrix theory
(TDDM). The calculations are done in a self-consistent manner, in which the
same Skyrme force as that used for the mean-field potential is used as the
residual interaction to calculate two-body correlations. It is found that the
DGDR in 16O has a large width due to the Landau damping, although the centroid
energy of the DGDR is close to twice the energy of the GDR calculated in RPA.
The DGDR in 40Ca is found more harmonic than that in 16O.Comment: 4 pages, 3 figure

### Extended RPA with ground-state correlations in a solvable model

The ground states and excited states of the Lipkin model hamiltonian are
calculated using a new theoretical approach which has been derived from an
extended time-dependent Hartree-Fock theory known as the time-dependent
density-matrix theory (TDDM). TDDM enables us to calculate correlated ground
states, and its small amplitude limit (STDDM), which is a version of extended
RPA theories based on a correlated ground state, can be used to calculate
excited states. It is found that this TDDM plus STDDM approach gives much
better results for both the ground states and the excited states than the
Hartree-Fock ground state plus RPA approach.Comment: 4 pages, 4 figure

### Spurious modes in extended RPA theories

Necessary conditions that the spurious state associated with the
translational motion and its double-phonon state have zero excitation energy in
extended RPA (ERPA) theories which include both one-body and two-body
amplitudes are investigated using the small amplitude limit of the
time-dependent density-matrix theory (STDDM). STDDM provides us with a quite
general form of ERPA as compared with other similar theories in the sense that
all components of one-body and two-body amplitudes are taken into account. Two
conditions are found necessary to guarantee the above property of the single
and double spurious states: The first is that no truncation in the
single-particle space should be made. This condition is necessary for the
closure relation to be used and is common for the single and double spurious
states. The second depends on the mode. For the single spurious state all
components of the one-body amplitudes must be included, and for the double
spurious state all components of one-body and two-body amplitudes have to be
included. It is also shown that the Kohn theorem and the continuity equations
for transition densities and currents hold under the same conditions as the
spurious states. ERPA theories formulated using the Hartree-Fock ground state
have a non-hermiticity problem. A method for formulating ERPA with hermiticity
is also proposed using the time-dependent density-matrix formalism.Comment: 15 page

### Asymmetry of the electronic states in hole- and electron-doped cuprates: Exact diagonalization study of the t-t'-t''-J model

We systematically examine the asymmetry of the electronic states in the hole-
and electron-doped cuprates by using the t-t'-t''-J model. Numerically exact
diagonalization method is employed for a 20-site square lattice. We impose
twisted boundary conditions (BC) instead of standard periodic BC. For static
and dynamical correlation functions, averaging procedure over the twisted BC is
used to reduce the finite-size effect. We find that antiferromagnetic spin
correlation remains strong in electron doping in contrast to the case of hole
doping, being similar to the case of the periodic BC. This leads to a
remarkable electron-hole asymmetry in the dynamical spin structure factor and
two-magnon Raman scattering. By changing the twist, the single-particle
spectral function is obtained for all momenta in the Brillouin zone. Examining
the spectral function in detail, we find a gap opening at around the k=(pi,0)
region for 10% doping of holes (the carrier concentration x=0.1), leading to a
Fermi arc that is consistent with experiments. In electron doping, however, a
gap opens at around k=(pi/2,pi/2) and persists up to x=0.2, being correlated
with the strength of the antiferromagnetic correlation. We find that the
magnitude of the gaps is sensitive to t' and t''. A pseudogap is also seen in
the optical conductivity for electron doping, and its magnitude is found to be
the same as that in the spectral function. We compare calculated quantities
with corresponding experimental data, and discuss similarities and differences
between them as well as their implications.Comment: 14 pages, 17 figures, Replaced figures, to be published in Phys. Rev.

### Density-matrix formalism with three-body ground-state correlations

A density-matrix formalism which includes the effects of three-body ground-
state correlations is applied to the standard Lipkin model. The reason to
consider the complicated three-body correlations is that the truncation scheme
of reduced density matrices up to the two-body level does not give satisfactory
results to the standard Lipkin model. It is shown that inclusion of the
three-body correlations drastically improves the properties of the ground
states and excited states. It is pointed out that lack of mean-field effects in
the standard Lipkin model enhances the relative importance of the three-body
ground-state correlations. Formal aspects of the density-matrix formalism such
as a relation to the variational principle and the stability condition of the
ground state are also discussed. It is pointed out that the three-body
ground-state correlations are necessary to satisfy the stability condition

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