arXiv subjects

Takaharu Otsuka

Publications and source records attributed to Takaharu Otsuka.

At least 55 records · Page 3Linked to original sources

arXiv2012

Variational procedure for nuclear shell-model calculations and energy-variance extrapolation

We discuss a variational calculation for nuclear shell-model calculations and propose a new procedure for the energy-variance extrapolation (EVE) method using a sequence of the approximated wave functions obtained by the variational calculation. The wave functions are described as linear combinations of the parity, angular-momentum projected Slater determinants, the energy of which is minimized by the conjugate gradient method obeying the variational principle. The EVE generally works well using the wave functions, but we found some difficult cases where the EVE gives a poor estimation. We discuss the origin of the poor estimation concerning shape coexistence. We found that the appropriate reordering of the Slater determinants allows us to overcome this difficulty and to reduce the uncertainty of the extrapolation.

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arXiv2012

Tensor-force-driven Jahn-Teller effect and shape transitions in exotic Si isotopes

We show how the shape evolution of the neutron-rich exotic Si and S isotopes can be understood as a Jahn-Teller effect that comes in part from the tensor-driven evolution of single-particle energies. The detailed calculations we present are in excellent agreement with known experimental data, and we point out of new features that should be explored in new experiments. Potential energy surfaces are used to understand the shape evolutions. The sub-shell closed nucleus, $^{42}$Si, is shown to be a perfect example of a strongly oblate shape instead of a sphere through a robust Jahn-Teller mechanism. The distribution of spectroscopic factors measured by $^{48}$Ca(e,e'p) experiment is shown to be well described, providing a unique test on the tensor-driven shell evolution.

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arXiv2011

Beta Decays of Isotones with Neutron Magic Number of N=126 and R-process Nucleosynthesis

Beta decays of the isotones with N=126 are studied by shell model calculations taking into account both the Gamow-Teller (GT) and first-forbidden (FF) transitions. The FF transitions are found to be important to reduce the half-lives, by nearly twice to several times, from those by the GT contributions only. Possible implications of the short half-lives of the waiting point nuclei on the r-process nucleosynthesis during the supernova explosions are discussed. A slight shift of the third peak of the element abundances in the r-process toward higher mass region is found.

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arXiv2011

Renormalization persistency of tensor force in nuclei

In this work we analyze the tensor-force component of effective interactions appropriate for nuclear shell-model studies, with particular emphasis on the monopole term of the interactions. Standard nucleon-nucleon ($NN$) interactions such as AV8' and $χ$N$^3$LO are tailored to shell-model studies by employing $V_{low k}$ techniques to handle the short-range repulsion of the $NN$ interactions and by applying many-body perturbation theory to incorporate in-medium effects. We show, via numerical studies of effective interactions for the $sd$ and $pf$ shells, that the tensor-force contribution to the monopole term of the effective interaction is barely changed by these renormalization procedures, resulting in almost the same monopole term as the one of the bare $NN$ interactions. We propose to call this feature {\it Renormalization Persistency} of the tensor force, as it is a remarkable property of the renormalization and should have many interesting consequences in nuclear systems. For higher multipole terms, this feature is maintained to a somewhat smaller extent. We present general intuitive explanations for the renormalization persistency of the tensor force, as well as analyses of core-polarization terms in perturbation theory. The central force does not exhibit a similar renormalization persistency.

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arXiv2011

Microscopic Formulation of Interacting Boson Model for Rotational Nuclei

We propose a novel formulation of the Interacting Boson Model (IBM) for rotational nuclei with axially-symmetric strong deformation. The intrinsic structure represented by the potential energy surface (PES) of a given multi-nucleon system has a certain similarity to that of the corresponding multi-boson system. Based on this feature, one can derive an appropriate boson Hamiltonian as already reported. This prescription, however, has a major difficulty in rotational spectra of strongly deformed nuclei: the bosonic moment of inertia is significantly smaller than the corresponding nucleonic one. We present that this difficulty originates in the difference between the rotational response of a nucleon system and that of the corresponding boson system, and could arise even if the PESs of the two systems were identical. We further suggest that the problem can be cured by implementing $\hat{L} \cdot \hat{L}$ term into the IBM Hamiltonian, with coupling constant derived from the cranking approach of Skyrme mean-field models. The validity of the method is confirmed for rare-earth and actinoid nuclei, as their experimental rotational yrast bands are reproduced nicely.

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arXiv2010

Novel Extrapolation Method in the Monte Carlo Shell Model

We propose an extrapolation method utilizing energy variance in the Monte Carlo shell model in order to estimate the energy eigenvalue and observables accurately. We derive a formula for the energy variance with deformed Slater determinants, which enables us to calculate the energy variance efficiently. The feasibility of the method is demonstrated for the full $pf$-shell calculation of $^{56}$Ni, and the applicability of the method to a system beyond current limit of exact diagonalization is shown for the $pf$+$g_{9/2}$-shell calculation of $^{64}$Ge.

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arXiv2010

Three-body forces and the limit of oxygen isotopes

The limit of neutron-rich nuclei, the neutron drip-line, evolves regularly from light to medium-mass nuclei except for a striking anomaly in the oxygen isotopes. This anomaly is not reproduced in shell-model calculations derived from microscopic two-nucleon forces. Here, we present the first microscopic explanation of the oxygen anomaly based on three-nucleon forces that have been established in few-body systems. This leads to repulsive contributions to the interactions among excess neutrons that change the location of the neutron drip-line from $^{28}$O to the experimentally observed $^{24}$O. Since the mechanism is robust and general, our findings impact the prediction of the most neutron-rich nuclei and the synthesis of heavy elements in neutron-rich environments.

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arXiv2010

Suppression of Charge Equilibration leading to the Synthesis of Exotic Nuclei

Charge equilibration between two colliding nuclei can take place in the early stage of heavy-ion collisions. A basic mechanism of charge equilibration is presented in terms of the extension of single-particle motion from one nucleus to the other, from which the upper energy-limit of the bombarding energy is introduced for significant charge equilibration at the early stage of the collision. The formula for this limit is presented, and is compared to various experimental data. It is examined also by comparison to three-dimensional time-dependent density functional calculations. The suppression of charge equilibration, which appears in collisions at the energies beyond the upper energy-limit, gives rise to remarkable effects on the synthesis of exotic nuclei with extreme proton-neutron asymmetry.

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arXiv2009

Novel features of nuclear forces and shell evolution in exotic nuclei

Novel simple properties of the monopole component of the effective nucleon-nucleon interaction are presented, leading to the so-called monopole-based universal interaction. Shell structures are shown to change as functions of $N$ and $Z$ consistently with experiments. Some key cases of this shell evolution are discussed, clarifying the effects of central and tensor forces. The validity of the present tensor force is examined in terms of the low-momentum interaction V$_{low k}$ and the Q$_{box}$ formalism.

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arXiv2008

Dynamical symmetry in spinor Bose-Einstein condensates

We demonstrate that dynamical symmetry plays a crucial role in determining the structure of the eigenspectra of spinor Bose-Einstein condensates (BECs). In particular, the eigenspectra of spin- 1 and spin-2 BECs in the single-mode approximation are shown to be completely determined by dynamical symmetries, where a spin-2 BEC corresponds to the U(5) limit of the interacting boson model in nuclear physics. The eigenspectrum of a spin-3 BEC is solved analytically for a specific class of coupling constants, while it is shown that dynamical symmetry alone is not enough to determine the spectrum for arbitrary coupling constants. We also study the low-lying eigenspectra of spin-1 and spin-2 BECs in the absence of external magnetic field, and find, in particular, that the quasidegenerate spectra emerge for antiferromagnetic and cyclic phases. This implies that these systems are highly susceptible to external perturbations and may undergo symmetry-breaking transitions to other states upon increasing the size of system.

cond-mat.other↗

arXiv2006

Neutrino Nucleus Reactions based on New Shell Model Hamiltonians

A new shell model Hamiltonian for p-shell nuclei which properly takes into account important roles of spin-isospin interactions is used to obtain cross sections of neutrino-^12C reactions induced by decay-at-rest (DAR) neutrinos as well as supernona neutrinos. Branching ratios to various decay channels are calculated by the Hauser-Feshbach theory. Neutrino-^4He reactions are also investigated by using recent shell model Hamiltonians. The reaction cross sections are found to be enhanced for both ^12C and ^4He compared with previous calculations. As an interesting consequence of this, a possible enhancement of the production yields of light elements, ^7Li and ^11B, during supernova explosions is pointed out.

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arXiv2004

Anomalous Properties of Quadrupole Collective States in $^{136}$Te and beyond

The ground and low-lying states of neutron-rich exotic Te and Sn isotopes are studied in terms of the nuclear shell model by the same Hamiltonian used for the spherical-deformed shape phase transition of Ba isotopes, without any adjustment. An anomalously small value is obtained for $B(E2;0^+_1\to 2^+_1)$ in $^{136}$Te, consistently with a recent experiment. The levels of $^{136}$Te up to yrast $12^+$ are shown to be in agreement with observed ones. It is pointed out that $^{136}$Te can be an exceptionally suitable case for studying mixed-symmetry 1$^+$, 2$^+$ and 3$^+$ states, and predictions are made for energies, M1 and E2 properties. Systematic trends of structure of heavier and more exotic Sn and Te isotopes beyond $^{136}$Te are studied by Monte Carlo Shell Model, presenting an unusual and very slow evolution of collectivity/deformation.

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arXiv2004

Onset of intruder ground state in exotic Na isotopes and evolution of the N=20 shell gap

The onset of intruder ground states in Na isotopes is investigated by comparing experimental data and shell-model calculations. This onset is one of the consequences of the disappearance of the N=20 magic structure, and the Na isotopes are shown to play a special role in clarifying the change of this magic structure. Both the electromagnetic moments and the energy levels clearly indicate an onset of ground state intruder configurations at neutron number N=19 already, which arises only with a narrow N=20 shell gap in Na isotopes resulting from the spin-isospin dependence of the nucleon-nucleon interaction (as compared to a wider gap in stable nuclei like 40Ca). It is shown why the previous report based on the mass led to a wrong conclusion.

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arXiv2004

New effective interaction for $pf$-shell nuclei and its implications for the stability of the $N$=$Z$=28 closed core

The effective interaction GXPF1 for shell-model calculations in the full $pf$ shell is tested in detail from various viewpoints such as binding energies, electro-magnetic moments and transitions, and excitation spectra. The semi-magic structure is successfully described for $N$ or Z=28 nuclei, $^{53}$Mn, $^{54}$Fe, $^{55}$Co and $^{56,57,58,59}$Ni, suggesting the existence of significant core-excitations in low-lying non-yrast states as well as in high-spin yrast states. The results of $N=Z$ odd-odd nuclei, $^{54}$Co and $^{58}$Cu, also confirm the reliability of GXPF1 interaction in the isospin dependent properties. Studies of shape coexistence suggest an advantage of Monte Carlo Shell Model over conventional calculations in cases where full-space calculations still remain too large to be practical.

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arXiv2001

Magic numbers in exotic nuclei and spin-isospin properties of {\it NN} interaction

The magic numbers in exotic nuclei are discussed, and their novel origin is shown to be the spin-isospin dependent part of the nucleon-nucleon interaction in nuclei. The importance and robustness of this mechanism is shown in terms of meson exchange, G-matrix and QCD theories. In neutron-rich exotic nuclei, magic numbers such as N = 8, 20, etc. can disappear, while N = 6, 16, etc. arise, affecting the structure of lightest exotic nuclei to nucleosynthesis of heavy elements.

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arXiv1999

Excitation spectrum and effective mass of the even-fraction quantum Hall liquid

To probe the nature of the even-fraction quantum Hall system, we have investigated the low-lying excitation spectrum by means of exact diagonalization for finite systems. We have found (i) a striking one-to-one correspondence (i.e., a shell structure) between the spectrum and those for free (composite) fermions, (ii) a surprisingly straight scaling plot for the excitation energy that gives a zero gap (metal) in the thermodynamic limit, (iii) the effective mass evaluated from the scaling becoming heavier for filling factor = 1/2, 1/4, 1/6, but (iv) some deviations from the single-mode or the Hartree-Fock composite fermion approximation as well.

cond-mat.mes-hall↗

arXiv1996

Shape transitions in proton-neutron systems

We explore the possibility of shape transitions in proton-neutron systems driven by deformation differences between proton and neutron fluids. Within the framework of the proton-neutron interacting boson model, we show that such dynamic shape transitions cannot occur in well deformed nuclei but are a possibility in transitional nuclei. Likely candidates in the Os-Pt isotopes are discussed and predictions of the model are compared with the existing data.

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arXiv1996

Nuclear Shell Model by the Quantum Monte Carlo Diagonalization Method

The feasibility of shell-model calculations is radically extended by the Quantum Monte Carlo Diagonalization method with various essential improvements. The major improvements are made in the sampling for the generation of shell-model basis vectors, and in the restoration of symmetries such as angular momentum and isospin. Consequently the level structure of low-lying states can be studied with realistic interactions. After testing this method on $^{24}$Mg, we present first results for energy levels and $E2$ properties of $^{64}$Ge, indicating its large and $γ$-soft deformation.

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