900 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
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
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
Temperature and Dimensionality Dependences of Optical Absorption Spectra in Mott Insulators
We investigate the temperature dependence of optical absorption spectra of
one-dimensional (1D) and two-dimensional (2D) Mott insulators by using an
effective model in the strong-coupling limit of a half-filed Hubbard model. In
the numerically exact diagonalization calculations on finite-size clusters, we
find that in 1D the energy position of the absorption edge is almost
independent of temperature, while in 2D the edge position shifts to lower
energy with increasing temperature. The different temperature dependence
between 1D and 2D is attributed to the difference of the coupling of the charge
and spin degrees of freedom. The implications of the results on experiments are
discussed in terms of the dimensionality dependence.Comment: 5 pages, 4 figure
Numerical approach to low-doping regime of the t-J model
We develop an efficient numerical method for the description of a single-hole
motion in the antiferromagnetic background. The method is free of finite-size
effects and allows calculation of physical properties at an arbitrary
wavevector. Methodical increase of the functional space leads to results that
are valid in the thermodynamic limit. We found good agreement with cumulant
expansion, exact- diagonalization approaches on finite lattices as well as
self-consistent Born approximations. The method allows a straightforward
addition of other inelastic degrees of freedom, such as lattice effects. Our
results confirm the existence of a finite quasiparticle weight near the band
minimum for a single hole and the existence of string-like peaks in the
single-hole spectral function.Comment: 6 pages, 6 figures, accepted for publication in PR
Density matrix renormalization group study of optical conductivity in the one-dimensional Mott insulator Sr_2CuO_3
Applying newly developed dynamical density matrix renormalization group
techniques at zero and finite temperatures to a Hubbard-Holstein model at
half-filling, we examine the optical conductivity of a typical one-dimensional
Mott insulator Sr_2CuO_3. We find a set of parameters in the Hubbard-Holstein
model, which can describe optical conductivity for both Mott-gap excitation in
the high-energy region and phonon-assisted spin excitation in the low-energy
region. We also find that electron-phonon interaction gives additional
broadening in the temperature dependence of the Mott-gap excitation.Comment: 5 pages, 3figure
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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