949 research outputs found
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
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
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
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
Eigenstates of the time-dependent density-matrix theory
An extended time-dependent Hartree-Fock theory, known as the time-dependent
density-matrix theory (TDDM), is solved as a time-independent eigenvalue
problem for low-lying states in O to understand the foundation of
the rather successful time-dependent approach. It is found that the calculated
strength distribution of the states has physically reasonable behavior
and that the strength function is practically positive definite though the
non-hermitian hamiltonian matrix obtained from TDDM does not guarantee it. A
relation to an extended RPA theory with hermiticity is also investigated. It is
found that the density-matrix formalism is a good approximation to the
hermitian extended RPA theory.Comment: 8 pages, 1 figur
Quadrupole resonances in unstable oxygen isotopes in time-dependent density-matrix formalism
The strength functions of quadrupole modes in the unstable oxygen isotopes
22O and 24O are calculated using an extended version of the time-dependent
Hartree-Fock theory known as the time-dependent density-matrix theory (TDDM).
It is found that TDDM gives the lowest quadrupole states which are
energetically shifted upward and become significantly collective due to the
coupling to two-body configurations. It is pointed out that these features of
the lowest quadrupole states are similar to those obtained in the
quasi-particle random phase approximation.Comment: 6 pages, 6 figure
Effect of nonmagnetic impurities on stripes in high-Tc cuprates
We perform the numerically exact diagonalization study of the t-J model with
nonmagnetic impurities to clarify the relation between Zn impurities and the
stripes. By examining the hole-hole correlation function for a two-hole
\sqrt{18}x\sqrt{18} cluster with a single impurity, we find that the impurity
has a tendency to stabilize vertical charge stripes. This tendency is caused by
the gain of the kinetic energy of holes moving along the stripes that are
formed avoiding the impurity.Comment: 3 pages including 2 figures. Proceedings for ISS2000 (Tokyo, October
2000). To be published in Physica
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