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Yoshifumi R. Shimizu

Publications and source records attributed to Yoshifumi R. Shimizu.

36 records · Page 2Linked to original sources
arXiv2010

Improved microscopic-macroscopic approach incorporating the effects of continuum states

The Woods-Saxon-Strutinsky method (the microscopic-macroscopic method) combined with Kruppa's prescription for positive energy levels, which is necessary to treat neutron rich nuclei, is studied to clarify the reason for its success and to propose improvements for its shortcomings. The reason why the plateau condition is met for the Nilsson model but not for the Woods-Saxon model is understood in a new interpretation of the Strutinsky smoothing procedure as a low-pass filter. Essential features of Kruppa's level density is extracted in terms of the Thomas-Fermi approximation modified to describe spectra obtained from diagonalization in truncated oscillator bases. A method is proposed which weakens the dependence on the smoothing width by applying the Strutinsky smoothing only to the deviations from a reference level density. The BCS equations are modified for the Kruppa's spectrum, which is necessary to treat the pairing correlation properly in the presence of continuum. The potential depth is adjusted for the consistency between the microscopic and macroscopic Fermi energies. It is shown, with these improvements, that the microscopic-macroscopic method is now capable to reliably calculate binding energies of nuclei far from stability.

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arXiv2008

Microscopic calculation of the wobbling excitations by using the Woods-Saxon potential as a nuclear mean-field

The wobbling excitations of the triaxial superdeformed (TSD) bands in the Lu and Hf region are studied by the microscopic framework of the cranked mean-field and the random-phase approximation (RPA). In contrast to the previous works, where the Nilsson potential was used, the more realistic Woods-Saxon potential is employed as a nuclear mean-field. The wobbling-like RPA solutions have been found systematically in the nuclei studied and their characteristic properties are investigated in details. This confirms the wobbling phonon excitations in TSD nuclei from the microscopic calculations. The result of $B(E2)$ values indicates that the triaxial deformation is increasing as a function of spin in the observed TSD bands in $^{163}$Lu.

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arXiv2007

Parametrizations of triaxial deformation and E2 transitions of the wobbling band

By the very definition the triaxial deformation parameter $γ$ is related to the expectation values of the K=0 and K=2 components of the intrinsic quadrupole tensor operator. On the other hand, using the same symbol "$γ$", various different parametrizations of triaxial deformation have been employed, which are suitable for various types of the mean-field potentials. It is pointed out that the values of various "$γ$" are quite different for the same actual triaxial deformation, especially for the large deformation; for example, the difference can be almost a factor two for the case of the triaxial superdeformed bands recently observed in the Hf and Lu nuclei. In our previous work, we have studied the wobbling band in Lu nuclei by using the microscopic framework of the cranked Nilsson mean-field and the random phase approximation, where the most serious problem is that the calculated B(E2) value is about factor two smaller. It is shown that the origin of this underestimation can be mainly attributed to the small triaxial deformation; if is used the same triaxial deformation as in the analysis of the particle-rotor model, the calculated B(E2) increases and gives correct magnitude compared with the experimental data.

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arXiv2006

Wobbling motion in triaxial superdeformed nuclei

We discuss some characteristic features of the wobbling motion excited on the triaxial superdeformed Lu nucleus. We show how these features are connected to the moments of inertia microscopically calculated by means of the quasiparticle RPA in the rotating frame.

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arXiv2006

Microscopic Description of Nuclear Wobbling Motion -- Rotation of traxially deformed nuclei --

The nuclear wobbling motion in the Lu region is studied by the microscopic cranked mean-field plus RPA method. The Woods-Saxon potential is used as a mean-field with a new parameterization which gives reliable description of rapidly rotating nuclei. The prescription of symmetry-preserving residual interaction makes the calculation of the RPA step parameter-free, and we find the wobbling-like RPA solution if the triaxial deformation of the mean-field is suitably chosen. It is shown that the calculated out-of-band $B(E2)$ of the wobbling-like solution depends on the triaxial deformation in the same way as in the macroscopic rotor model, and can be used to probe the triaxiality of the nuclear mean-field.

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arXiv2005

Precession Mode on High-K Configurations: Non-Collective Axially-Symmetric Limit of Wobbling Motion

We have studied the precession mode, the rotational excitation built on the high-$K$ isomeric state, in comparison with the recently identified wobbling mode. The random-phase-approximation (RPA) formalism, which has been developed for the nuclear wobbling motion, is invoked and the precession phonon is obtained by the non-collective axially-symmetric limit of the formalism. The excitation energies and the electromagnetic properties of the precession bands in $^{178}$W are calculated, and it is found that the results of RPA calculations well correspond to those of the rotor model; the correspondence can be understood by an adiabatic approximation to the RPA phonon. As a by-product, it is also found that the problem of too small out-of-band $B(E2)$ in our previous RPA wobbling calculations can be solved by a suitable choice of the triaxial deformation which corresponds to the one used in the rotor model.

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arXiv2004

High-K Precession modes: Axially symmetric limit of wobbling motion

The rotational band built on the high-K multi-quasiparticle state can be interpreted as a multi-phonon band of the precession mode, which represents the precessional rotation about the axis perpendicular to the direction of the intrinsic angular momentum. By using the axially symmetric limit of the random-phase-approximation (RPA) formalism developed for the nuclear wobbling motion, we study the properties of the precession modes in $^{178}$W; the excitation energies, B(E2) and B(M1) values. We show that the excitations of such a specific type of rotation can be well described by the RPA formalism, which gives a new insight to understand the wobbling motion in the triaxial superdeformed nuclei from a microscopic view point.

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arXiv2004

Dynamical moments of inertia associated with wobbling motion in the triaxial superdeformed nucleus

The three moments of inertia associated with the wobbling mode built on the triaxial superdeformed states in Lu--Hf region are investigated by means of the cranked shell model plus random-phase approximation to the configurations with aligned quasiparticle(s). The result indicates that it is crucial to take into account the direct contribution to the moments of inertia from the aligned quasiparticle(s) so as to realize \mathcal{J}_x > \mathcal{J}_y in positive-γshapes.

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arXiv2004

Microscopic Study of Wobbling Motions in Hf and Lu Nuclei

One of the most striking findings in the recent high-spin spectroscopy is the discovery of one-phonon, and possibly double-phonon, excitation of the nuclear wobbling rotational bands. In this talk, we first review the properties of observed wobbling motions, and discuss the failure and success of the possible interpretation in terms of the simple rotor model. Then, we further present results of our microscopic study in Hf and Lu nuclei by means of the theoretical framework, the cranked mean-field and the random phase approximation.

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arXiv2004

Nuclear moments of inertia and wobbling motions in triaxial superdeformed nuclei

The wobbling motion excited on triaxial superdeformed nuclei is studied in terms of the cranked shell model plus random phase approximation. Firstly, by calculating at a low rotational frequency the γ-dependence of the three moments of inertia associated with the wobbling motion, the mechanism of the appearance of the wobbling motion in positive-γnuclei is clarified theoretically -- the rotational alignment of the πi_{13/2} quasiparticle(s) is the essential condition. This indicates that the wobbling motion is a collective motion that is sensitive to the single-particle alignment. Secondly, we prove that the observed unexpected rotational-frequency dependence of the wobbling frequency is an outcome of the rotational-frequency dependent dynamical moments of inertia.

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arXiv2002

Wobbling Motion in Atomic Nuclei with Positive-Gamma Shapes

The three moments of inertia associated with the wobbling mode built on the superdeformed states in 163Lu are investigated by means of the cranked shell model plus random phase approximation to the configuration with an aligned quasiparticle. The result indicates that it is crucial to take into account the direct contribution to the moments of inertia from the aligned quasiparticle so as to realize J_x > J_y in positive-gamma shapes. Quenching of the pairing gap cooperates with the alignment effect. The peculiarity of the recently observed 163Lu data is discussed by calculating not only the electromagnetic properties but also the excitation spectra.

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arXiv2000

Diabatic Mean-Field Description of Rotational Bands in Terms of the Selfconsistent Collective Coordinate Method

Diabatic description of rotational bands provides a clear-cut picture for understanding the back-bending phenomena, where the internal structure of the yrast band changes dramatically as a function of angular momentum. A microscopic framework to obtain the diabatic bands within the mean-field approximation is presented by making use of the selfconsistent collective coordinate method. Applying the framework, both the ground state rotational bands and the Stockholm bands are studied systematically for the rare-earth deformed nuclei. An overall agreement has been achieved between the calculated and observed rotational spectra. It is also shown that the inclusion of the double-stretched quadrupole-pairing interaction is crucial to obtain an overall agreement for the even-odd mass differences and the rotational spectra simultaneously.

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arXiv1996

Microscopic Structure of High-Spin Vibrational States in Superdeformed A=190 Nuclei

Microscopic RPA calculations based on the cranked shell model are performed to investigate the quadrupole and octupole correlations for excited superdeformed (SD) bands in even-even A=190 nuclei. The K=2 octupole vibrations are predicted to be the lowest excitation modes at zero rotational frequency. The Coriolis coupling at finite frequency produces different effects depending on the neutron and proton number of nucleus. The calculations also indicate that some collective excitations may produce J(2) moments of inertia almost identical to those of the yrast SD band. An interpretation of the observed excited bands invoking the octupole vibrations is proposed, which suggests those octupole vibrations may be prevalent in even-even SD A=190 nuclei.

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arXiv1996

Heuristic quantization of the cranking model

We propose a simple and systematic method to quantize the cranking model and to calculate the matrix elements of intrinsic operators entering the generalized intensity relations. An example of the application is given to show the Coriolis coupling effects for B(E3) values of the 3- octupole states in Gd isotopes.

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arXiv1996

A New Microscopic Approach to the Rotational Intensity Relations

The intensity relations for electromagnetic transition rates in the rotational coupling scheme have been a basic tool to understand the properties of nuclear collective rotations. In particular the correction terms to the leading-order relation give the response of the intrinsic motion to the Coriolis and centrifugal forces. We propose a simple and systematic method to calculate the matrix elements of intrinsic operators entering the generalized intensity relations that uses the microscopic cranking formalism. Examples are given to show the usefulness of the method.

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arXiv1995

Microscopic Structure of High-Spin Vibrational Excitations in Superdeformed 190,192,194Hg

Microscopic RPA calculations based on the cranked shell model are performed to investigate the quadrupole and octupole correlations for excited superdeformed bands in 190Hg, 192Hg, and 194Hg. The K=2 octupole vibrations are predicted to be the lowest excitation modes at zero rotational frequency. At finite frequency, however, the interplay between rotation and vibrations produces different effects depending on neutron number: The lowest octupole phonon is rotationally aligned in 190Hg, is crossed by the aligned two-quasiparticle bands in 192Hg, and retains the K=2 octupole vibrational character up to the highest frequency in 194Hg. The gamma vibrations are predicted to be higher in energy and less collective than the octupole vibrations. From a comparison with the experimental dynamic moments of inertia, a new interpretation of the observed excited bands invoking the K=2 octupole vibrations is proposed, which suggests those octupole vibrations may be prevalent in SD Hg nuclei.

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arXiv1994

Nuclear Wobbling Motion and Electromagnetic Transitions

The nuclear wobbling motion is studied from a microscopic viewpoint. It is shown that the expressions not only of the excitation energy but also of the electromagnetic transition rate in the microscopic RPA framework can be cast into the corresponding forms of the macroscopic rotor model. Criteria to identify the rotational band associated with the wobbling motion are given, based on which examples of realistic calculations are investigated and some theoretical predictions are presented.

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