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Mitsuru Tohyama

Publications and source records attributed to Mitsuru Tohyama.

At least 19 recordsLinked to original sources
arXiv2023

Is perturbative study of ground-state correlations valid?

Perturbative approaches have often been used to include the effects of ground-state correlations in extended theories of the random-phase approximation. Validity of such approaches is investigated for a solvable model where comparison with exact solutions can be made. It is pointed out that there is a case where perturbative approaches give good results in spite of the fact that interaction strength is far beyond a perturbative region.

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arXiv2023

Number conservation in odd-particle number random phase approximation and extensions

The number conservation law in the odd-particle number random-phase approximation (oRPA) and its extension (EoRPA) is studied by applying them to a pairing model and $^{16}$O. It is found in the application to $^{16}$O that the number conservation law is not fulfilled in oRPA and EoRPA and that it is drastically improved in EoRPA due to the inclusion of ground-state correlation effects.

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arXiv2022

Effects of ground-state correlations on damping of giant dipole resonaces in $LS$ closed shell nuclei

The effects of ground-state correlations on the damping of isovector giant dipole resonances in $LS$ closed shell nuclei $^{16}$O and $^{40}$Ca are studied using extended random-phase-approximation (RPA) approaches derived from the time-dependent density-matrix theory. It is pointed out that unconventional two-body amplitudes of one particle--three hole and three particle--one hole types which are neglected in most extended RPA theories play an important role in the fragmentation of isovector dipole strength.

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arXiv2021

Properties of Skyrme force as a residual interaction in beyond mean-field theories

In an effort to find an effective interaction which can consistently be used for both the mean-field part and the residual part in beyond mean-field theories, properties of the Skyrme interactions as a residual interaction are investigated. The time-dependent density-matrix theory (TDDM) is used as a beyond mean-field theory and the ground states of $^{16}$O and $^{40}$Ca are calculated using the five standard parametrizations of the Skyrme interaction which differs in density and momentum dependence. It is found that the Skyrme interaction which has strong density dependence and weak momentum dependence induces substantial ground-state correlations comparable to the results of other theoretical calculations.

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arXiv2020

Effects of ground-state correlations on magnetic dipole excitations in $^{40}$Ca

The effects of ground-state correlations on the magnetic dipole excitations in $^{40}$Ca are studied using an extended random phase approximation (ERPA) derived from the time-dependent density-matrix theory. Comparison is made with other extended RPA approaches, the renormalized RPA, the self-consistent RPA and the extended second RPA which also include the effects of ground-state correlations. It is pointed out that direct excitations from two particle - two hole space which are properly treated in ERPA cause strong magnetic dipole transitions in $^{40}$Ca.

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arXiv2019

Truncation scheme of time-dependent density-matrix approach III

The time-dependent density-matrix theory (TDDM) where the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy for reduced density matrices is truncated by approximating a three-body density matrix with one-body and two-body density matrices is applied to the Lipkin model. It is shown that in the large $N$ limit the ground state in TDDM approaches the exact solution. Various truncation schemes for the three-body density matrix are also tested for an extended three-level Lipkin model.

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arXiv2017

Effects of ground-state correlations on dipole and quadrupole excitations of $^{40}$Ca and $^{48}$Ca

The effects of ground-state correlations on the dipole and quadrupole excitations are studied for $^{40}$Ca and $^{48}$Ca using the extended random phase approximation (ERPA) derived from the time-dependent density-matrix theory. Large effects of the ground-state correlations are found in the fragmentation of the giant quadrupole resonance in $^{40}$Ca and in the low-lying dipole strength in $^{48}$Ca. It is discussed that the former is due to a mixing of different configurations in the ground state and the latter is from the partial occupation of the neutron single-particle states. The dipole and quadrupole strength distributions below 10 MeV calculated in ERPA are in qualitatively agreement with experiment.

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arXiv2017

Role of small-norm components in extended random-phase approximation

The role of the small-norm amplitudes in extended RPA theories such as the particle-particle and hole-hole components of one-body amplitudes and the two-body amplitudes other than two particle - two holes components are investigated for the one-dimensional Hubbard model using an extended RPA derived from the time-dependent density-matrix theory. It is found that these amplitudes cannot be neglected in strongly interacting regions where the effects of ground-state correlations are significant.

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arXiv2017

Truncation scheme of time-dependent density-matrix approach II

A truncation scheme of the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy for reduced density matrices, where a three-body density matrix is approximated by two-body density matrices, is improved to take into account a normalization effect. The truncation scheme is tested for the Lipkin model. It is shown that the obtained results are in good agreement with the exact solutions.

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arXiv2016

Progress in Many Body Theory with the Equation of Motion method. Time dependent Density Matrix meets Self-Consistent RPA. Applications to solvable Models

The Bogoliubov-Born-Green-Kirkwood-Yvon or Time-Dependent Density Matrix (TDDM) hierarchy of equations for higher density matrices is truncated at the three body level in approximating the three body correlation function by a quadratic form of two body ones, closing the equations in this way. The procedure is discussed in detail and it is shown in non-trivial model cases that the approximate inclusion of three body correlation functions is very important to obtain precise results. A small amplitude approximation of this time dependent nonlinear equation for the two body correlation function is performed (STDDM*-b) and it is shown that the one body sector of this generalised non-linear second RPA equation is equivalent to the Self-Consistent RPA (SCRPA) approach which had been derived previously by different techniques. It is discussed in which way SCRPA also contains the three body correlations. TDDM and SCRPA are tested versus exactly solvable model cases.

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arXiv2014

Extension of time-dependent Hartree-Fock-Bogoliubov equations

An extension of the time-dependent Hartree-Fock-Bogoliubov theory (ETDHFB) which includes higher-order effects such as screening of the pairing correlation is proposed. ETDHFB is applied to a fermion system trapped in a harmonic potential to test its feasability by comparison with the exact solution. With the use of perturbative expressions for the pairing tensor and the two-body density matrix derived from ETDHFB, the screening effect is investigated for atomic fermion systems and isotopes of tin nuclei. It is found that the screening effect on the pairing correlation is not significant.

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arXiv2013

Truncation scheme of time-dependent density-matrix approach

A truncation scheme of the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy for reduced density matrices, where a three-body density matrix is approximated by the antisymmetrized products of two-body density matrices, is proposed. This truncation scheme is tested for three model hamiltonians. It is shown that the obtained results are in good agreement with the exact solutions.

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arXiv2013

Magnetization and collective excitations of a magnetic dipole fermion gas

The ground states and collective excitations of trapped Fermion gases consisting of atoms with magnetic dipole moment are studied using a time-dependent density-matrix approach. The advantages of the density-matrix approach are that one-body and two-body observables are directly calculated using one-body and two-body density matrices and that it has a clear relation to the Hartree-Fock (HF) and time-dependent HF theory. The HF calculations show the magnetization of the gases when the dipole-dipole interaction is strong. It is shown that the tensor properties of the dipole-dipole interaction are revealed in the excitation modes associated with spin degrees of freedom.

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arXiv2013

Effects of ground-state correlations on collective excitations of $^{16}$O

The effects of the correlations in the ground state of $^{16}$O on the octupole and dipole excitations are studied using the extended random phase approximation (ERPA) derived from the time-dependent density-matrix theory. It is found that the ground-state correlation effects are significant especially in the octupole excitation. It is shown that the first $3^-$ state calculated in the random phase approximation (RPA) is shifted upward when the self-energy contributions are included in particle - hole pairs. The coupling to the two particle - two hole states plays a role in shifting the first $3^-$ state down to the right position. It is also found that the dipole strength is fragmented due to the partial occupation of the single-particle states and that the peak position of the giant dipole resonance calculated in ERPA is little changed from that in RPA due to the above-mentioned competing effects: the increase in particle - hole energy and the coupling to two particle - two hole configurations.

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arXiv2013

Odd-particle number random phase approximation and extensions: Applications to particle and hole states around $^{16}$O

The hole-state random phase approximation (hRPA) and the particle-state random phase approximation (pRPA) for systems like odd $A$ nuclei are discussed. These hRPA and pRPA are formulated based on the Hartree-Fock ground state. An extension of hRPA and pRPA based on a correlated ground state is given using time-dependent density-matrix theory. Applications to the single-particle states around $^{16}$O are presented. It is shown that inclusion of ground-state correlation affects appreciably the results of hRPA and pRPA. The question of the coupling of the center of mass motion of the core to the particle (hole) is also discussed.

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arXiv2011

Fragmentation of electric dipole strength in N=82 isotones

Fragmentation of the dipole strength in the N=82 isotones 140Ce, 142Nd and 144Sm is calculated using the second random-phase approximation (SRPA). In comparison with the result of the random-phase approximation (RPA), the SRPA provides the additional damping of the giant dipole resonance and the redistribution of the low-energy dipole strength. Properties of the low-energy dipole states are significantly changed by the coupling to two-particle-two-hole (2p2h) states, which are also sensitive to the correlation among the 2p2h states. Comparison with available experimental data shows a reasonable agreement for the low-energy E1 strength distribution.

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arXiv2010

Scissors mode of trapped dipolar gases

We study the scissors modes of dipolar boson and fermion gases trapped in a spherically symmetric potential. We use the harmonic oscillator states to solve the time-dependent Gross-Pitaevskii equation for bosons and the time-dependent Hartree-Fock equation for fermions. It is pointed out that the scissors modes of bosons and fermions can be of quite different nature.

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