arXiv subjects

Yasutaka Taniguchi

Publications and source records attributed to Yasutaka Taniguchi.

At least 19 recordsLinked to original sources

arXiv2026

Oblate-prolate shape mixing and E0 transition in 28Si

Background: oblate-prolate shape coexistence in $^{28}$Si has been discussed for decades, but the degree of shape mixing between these configurations remains poorly constrained. Purpose: We constrain the oblate-prolate mixing amplitudes in $^{28}$Si using available experimental information and discuss the inter-band E0 transition strength. Methods: Oblate and prolate $0^+$ and $2^+$ configurations are obtained by antisymmetrized molecular dynamics combined with the generator coordinate method. Using these configurations as the basis states, we constrain the mixing amplitudes by simultaneously reproducing the measured charge radius, the quadrupole moment of the $2_1^+$ state, and the in-band and inter-band $B(\mathrm{E2})$ values. The strength of the density-dependent term in the Gogny interaction is also varied within a reasonable range. Results: In the ground state, the oblate component is dominant, and the prolate component in the ground state is limited to less than about $20\%$. For the $2_1^+$ state, the allowed prolate component is smaller than that in the ground state. The present analysis does not tightly constrain the corresponding E0 transition strength, but an upper limit of $ρ^2(\mathrm{E0};0_3^+\rightarrow0_1^+) \lesssim 0.206$ is obtained. Conclusions: The low-lying $0^+$ states of $^{28}$Si may exhibit substantial oblate-prolate mixing. A measurement of the inter-band E0 transition strength would provide a quantitative determination of the mixing amplitude.

nucl-th↗

arXiv2025

Deformation and core$+n$ decoupling in the spectrum of $^{17}$C

The coexistence of various structures, such as diverse shapes and cluster structures, is a fundamental property of atomic nuclei. In neutron-rich nuclei, a core$+n$ structure can compete with nuclear deformation due to the small neutron separation energy. A neutron-rich carbon isotope, $^{17}$C, exemplifies the appearance of the deformation and the core+$n$ decoupling in its spectrum, which is desirable for a deeper understanding of the coexistence phenomena in neutron-rich nuclei. We aim to describe and understand this coexistence phenomenon in the low-lying levels of $^{17}$C in a unified manner considering explicitly the degrees of freedom of both the quadrupole deformation and the relative motion between a $^{16}$C core and a valence neutron. We adopt the generator coordinate method (GCM) with the antisymmetrized molecular dynamics (AMD) to describe various configurations. We superpose various basis wave functions generated by the energy variation by imposing two types of constraints: one incorporating the degree of the quadrupole deformation and the other taking care of the relative motion between a $^{16}$C core and a valence neutron. We find that the experimental energy level is well reproduced by the present method, including both deformed and $^{16}$C+$n$ configurations. The ground $3/2^{+}$ and second excited $5/2^{+}$ states exhibit a triaxially deformed shape, while the main component of the first excited $1/2^{+}$ state is a $^{16}$C($0^{+}$) core plus an $s$-wave neutron configuration. The tail of the valence neutron is significantly improved by including the $^{16}$C+$n$ basis functions explicitly. The explicit inclusion of both the quadrupole deformation and the relative motion between a core and a valence neutron is essential to describe the coexistence phenomena observed in neutron-rich nuclei in the AMD+GCM framework.

nucl-th↗

arXiv2024

Investigation of the determination of nuclear deformation using high-energy heavy-ion scattering

Background: Nuclear deformation provides a crucial characteristic of nuclear structure. Conventionally, the quadrupole deformation length of a nucleus, $δ_{2}$, has often been determined based on a macroscopic model through a deformed nuclear potential with the deformation length $δ^{\rm (pot)}_{2}$, which is determined to reproduce the nuclear scattering data. This approach assumes $δ_{2}=δ^{\rm (pot)}_{2}$ although there is no theoretical foundation. Purpose: We clarify the relationship between $δ_{2}$ and $δ^{\rm (pot)}_{2}$ for high-energy heavy-ion scattering systematically to evaluate the validity of the conventional approach to determine the nuclear deformation. Method: The deformation lengths for the $^{12}$C inelastic scattering by $^{12}$C, $^{16}$O, $^{40}$Ca, and $^{208}$Pb targets at $E/A$ = 50--400 MeV are examined. First, we perform microscopic coupled-channel (CC) calculations to relate $δ_{2}$ of the deformed density into the inelastic scattering cross section. Second, we use the deformed potential model to determine $δ^{\rm (pot)}_{2}$ so as to reproduce the microscopic CC result. We then compare $δ^{\rm (pot)}_{2}$ with $δ_{2}$. Results: We find that $δ^{\rm (pot)}_{2}$ is about 20--40 \% smaller than presumed $δ_{2}$, showing strong energy and target dependence. Further analysis, which considers higher-order deformation effects beyond the derivative model, reveals that $δ^{\rm (pot)}_{2}$ is still about 15--35 \% smaller than $δ_{2}$. Conclusion: Our results suggest that one needs to be careful when the deformed potential model for the high-energy heavy-ion scattering is used to extract the nuclear deformation. The conventional approach may underestimate the deformation length $δ_2$ systematically.

nucl-th↗

arXiv2024

An implementation of nuclear many-body wave functions by the superposition of localized Gaussians

We introduce a new framework for the low-energy nuclear structure calculations, which describes the single-particle wave function as a superposition of localized Gaussians. It is a hybrid of the Hartree-Fock and antisymmetrized molecular dynamics models. In the numerical calculations of oxygen, calcium isotopes and 100Sn, the framework shows its potential by significantly improving upon AMD and yielding the results consistent with or even better than Hartree-Fock(-Bogoliubov) calculations based on harmonic oscillator expansions. In addition to the basic equations, general form of the matrix elements is also given.

nucl-th↗

arXiv2024

Shape of 12C

We have examined the hypothesis by Bijker and Iachello who asserted that 12C has an internal structure with three alpha particles arranged in a triangular shape, leading to the formation of the ground rotational band consisting of 0+, 2+, 3-, 4$^\pm$ and 5- states. Following this idea, we reconstructed the intrinsic shape of 12C using experimental electron scattering data with minimal theoretical assumption. Our sole assumption was that the observed 0+, 2+, 3-, and 4+ states share a common internal structure, forming a rotational spectrum. The reconstructed intrinsic density showed a beautiful triangular shape with three peaks implying alpha cluster formation in the ground band.

nucl-th↗

arXiv2024

Impact of the molecular resonances on the 12C+12C fusion reaction rate

The properties of the low-energy 12C+12C molecular resonances, which potentially enhance the fusion reaction rate at low temperatures, have been investigated by a full-microscopic nuclear model employing various nuclear energy density functionals. We show that some density functionals plausibly describe the observed high-spin 12C+12C molecular resonances and predict many 0+ and 2+ resonances at low energies, which enhance the reaction rate. We also discuss how the uncertainty in the nuclear energy density functionals propagates to that of the reaction rate.

nucl-th↗

arXiv2021

4$α$ linear-chain state produced by $^{9}$Be+$^{9}$Be collision

Extreme nuclear deformations provide great insight into the geometric formation of quantum many-body systems. In this work, the $4α+2n$ linear chain is assessed in $^{18}$O. We predict excitation energies, moment-of-inertia, $α$-, and $^{9}$Be-decay widths by using the antisymmetrized molecular dynamics. We show that the $K^π=3^-$ linear-chain states may be verified by the head-on $^{9}{\rm Be}+{}^{9}{\rm Be}$ collision experiments.

nucl-th↗

arXiv2021

$\mathbf{{}^{12}{C} + {}^{12}{C}}$ Fusion $\boldsymbol{S^*}$-factor from a Full-microscopic Nuclear Model

The ${}^{12}\mathrm{C} + {}^{12}\mathrm{C}$ fusion reaction plays a vital role in the explosive phenomena of the universe. The resonances in the Gamow window rule its reaction rate and products. Hence, the determination of the resonance parameters by nuclear models is indispensable as the direct measurement is not feasible. Here, for the first time, we report the resonances in the ${}^{12}\mathrm{C} + {}^{12}\mathrm{C}$ fusion reaction described by a full-microscopic nuclear model. The model plausibly reproduces the measured low-energy astrophysical $S$-factors and predicts the resonances in the Gamow window. Contradictory to the hindrance model, we conclude that there is no low-energy suppression of the $S$-factor.

nucl-th↗

arXiv2021

Unexpectedly enhanced $α$-particle preformation in $^{48}$Ti probed by the $(p,pα)$ reaction

The formation of $α$ particle on nuclear surface has been a fundamental problem since the early age of nuclear physics. It strongly affects the $α$ decay lifetime of heavy and superheavy elements, level scheme of light nuclei, and the synthesis of the elements in stars. However, the $α$-particle formation in medium-mass nuclei has been poorly known despite its importance. Here, based on the $^{48}{\rm Ti}(p,pα)^{44}{\rm Ca}$ reaction analysis, we report that the $α$-particle formation in a medium-mass nucleus $^{48}{\rm Ti}$ is much stronger than that expected from a mean-field approximation, and the estimated average distance between $α$ particle and the residue is as large as 4.5 fm. This new result poses a challenge of describing four nucleon correlations by microscopic nuclear models.

nucl-th↗

arXiv2019

Quantitative description of the $^{20}$Ne($p$,$pα$)$^{16}$O cross section as a means of probing the surface $α$ amplitude

The proton-induced $α$ knockout reaction has been utilized for decades to investigate the $α$ cluster states of nuclei, of the ground state in particular. However, even in recent years, it is reported that the deduced $α$ spectroscopic factors from $α$ knockout experiments and reaction analyses with a phenomenological $α$ cluster wave function diverge depending on the kinematical condition of the reaction. In the present study we examine the theoretical description of the $^{20}$Ne($p$,$pα$)$^{16}$O cross section based on the antisymmetrized molecular dynamics and the distorted wave impulse approximation by comparing with existing experimental data. We also investigate the correspondence between the $α$ cluster wave function and the $α$ knockout cross section. The existing $^{20}$Ne($p$,$pα$)$^{16}$O data at 101.5 MeV is well reproduced by the present framework. Due to the peripherality of the reaction, the surface region of the cluster wave function is selectively reflected to the knockout cross section. A quantitatively reliable $α$ cluster wave function, $p$-$α$ cross section, and distorting potentials between scattering particles, $α$-$^{16}$O in particular, are crucial for the quantitative description of the ($p$,$pα$) cross section. Due to the peripherality of the reaction, the ($p$,$pα$) cross section is a good probe for the surface $α$ amplitude.

nucl-th↗

arXiv2019

$\bf ^{12}{C} + {}^{16}{O}$ molecular resonances at deep sub-barrier energy

The existence of $^{12}{\rm C} + {}^{16}{\rm O}$ molecular resonances at sub-barrier energy has been a significant problem in nuclear astrophysics because they strongly affect the $^{12}{\rm C} + {}^{16}{\rm O}$ fusion reaction rate in type Ia supernovae and heavy stars. However, experimental surveys have been limited to 4~MeV and cannot access the deep sub-barrier energy due to a very small fusion cross section. Here we predict a couple of resonances with $J^π=0^+$, $2^+$, and $4^+$ in the deep sub-barrier energy based on the antisymmetrized molecular dynamics calculation that reproduces the known resonances and low-lying spectrum of $^{28}{\rm Si}$.

nucl-th↗

arXiv2019

Investigation of spatial manifestation of $α$ clusters in $^{16}$O via $α$-transfer reactions

Recently, we have determined surface distributions of $α$ clusters in the ground state of $^{20}\mathrm{Ne}$ from $α$-transfer cross sections, without investigating the properties of its excited states. In this paper we extend our comprehension of $α$-cluster structures in excited states of nuclei through reaction studies. In particular we focus on $^{16}\mathrm{O}$, for which attention has been paid to advances of structure theory and assignment regarding $4^+$-resonance states. We study the surface manifestation of the $α$-cluster states in both the ground and excited states of $^{16}\mathrm{O}$ from the analysis of the $α$-transfer reaction $^{12}\mathrm{C}(^6\mathrm{Li},d)^{16}\mathrm{O}$. The $α$-transfer reaction is described by the distorted-wave Born approximation. We test two microscopic wave functions as an input of reaction calculations. Then a phenomenological potential model is introduced to clarify the correspondence between cluster-wave functions and transfer-cross sections. Surface peaks of the $α$-wave function of $^{16}\mathrm{O}(0^+)$ are sensitively probed by transfer-cross sections at forward angles, while it remains unclear how we trace the surface behavior of $^{16}\mathrm{O}(4^+)$ from the cross sections. We are able to specify that the $α$-cluster structure in the $0_1^+$ and $0_2^+$ states prominently manifests itself at the radii $\sim 4$ and $\sim 4.5$~fm, respectively. It is remarkable that the $4_1^+$ state has the $^{12}\mathrm{C}+α$-cluster component with the surface peak at the radius $\sim 4$ or outer, whereas the $^{12}\mathrm{C}+α$-cluster component in the $4_2^+$ state is found not to be dominant. The $4_2^+$ state is difficult to be interpreted by a simple potential model assuming the $^{12}\mathrm{C}+α$ configuration only.

nucl-th↗

arXiv2019

$α$ and triton clustering in $^{35}$Cl

Coupling of cluster and deformed structures are important for dynamics of nuclear structure. Threshold energy has been discussed to explain cluster structures coupling to deformed states but relation between threshold energy and excitation energy has open problems. Negative-parity superdeformed (SD) states were observed by a $γ$-spectroscopy experiment in $^{35}$Cl but its detailed structure is unclear. By analyzing coupling of cluster structures in deformed states and high-lying cluster states in $^{35}$Cl, cluster structures coupling to deformed states and excitation energy of high-lying cluster states are investigated. The antisymmetrized molecular dynamics (AMD) and the generator coordinate method (GCM) are used. An AMD wave function is a Slater determinant of Gaussian wave packets. By energy variational calculations with constraints on deformation and clustering, wave functions of deformed structures and $α$- and $t$-cluster structures are obtained. Adopting those wave functions as GCM basis, wave functions of ground and excited states are calculated. Various deformed bands are obtained and predicted. A $K^π= \frac{1}{2}^-$ deformed band, which corresponds to the observed SD band, dominates deformed structure and compact $α$- and $t$-cluster structure components. Particle-hole configurations of the dominant components with deformed and cluster structures are similar. In high-lying states, almost pure $α$- and $t$-cluster states are obtained in negative-parity states, and excitation energies of the $t$-cluster states are higher than those of $α$-cluster states. In conclusions, particle-hole configurations of cluster structure with small intercluster distance are important for coupling to low-energy deformed states. Threshold energies reflect to excitation energies of high-lying almost pure cluster states.

nucl-th↗

arXiv2016

Alignment of wave functions for angular momentum projection

Angular momentum projection is used to obtain eigen states of angular momentum from general wave functions. Multi-configuration mixing calculation with angular momentum projection is an important microscopic method in nuclear physics. For accurate multi-configuration mixing calculation with angular momentum projection, concentrated distribution of $z$ components $K$ of angular momentum in the body-fixed frame ($K$-distribution) is favored. Orientation of wave functions strongly affects $K$-distribution. Minimization of variance of $\hat{J}_z$ is proposed as an alignment method to obtain wave functions that have concentrated $K$-distribution. Benchmark calculations are performed for $α$-$^{24}$Mg cluster structure, triaxially superdeformed states in $^{40}$Ar, and Hartree-Fock states of some nuclei. The proposed alignment method is useful and works well for various wave functions to obtain concentrated $K$-distribution.

nucl-th↗

arXiv2016

Probing surface distributions of $α$ clusters in $^{20}$Ne via $α$-transfer reaction

Direct evidence of the $α$-cluster manifestation in bound states has not been obtained yet, although a number of experimental studies were carried out to extract the information of the clustering. In particular in conventional analyses of $α$-transfer reactions, there exist a few significant problems on reaction models, which are insufficient to qualitatively discuss the cluster structure. We aim to verify the development of the $α$-cluster structure from observables. As the first application, we plan to extract the spatial information of the cluster structure of the $^{20}$Ne nucleus in its ground state through the cross section of the $α$-transfer reaction $^{16}$O($^6$Li,~$d$)$^{20}$Ne. For the analysis of the transfer reaction, we work with the coupled-channel Born approximation (CCBA) approach, in which the breakup effect of $^6$Li is explicitly taken into account by means of the continuum-discretized coupled-channel method based on the three-body $α+ d + {}^{16}$O model. The two methods are adopted to calculate the overlap function between $^{20}$Ne and $α+ {}^{16}$O; one is the microscopic cluster model (MCM) with the generator coordinate method, and the other is the phenomenological two-body potential model (PM). We show that the CCBA calculation with the MCM wave function gives a significant improvement of the theoretical result on the angular distribution of the transfer cross section, which is consistent with the experimental data. Employing the PM, it is discussed which region of the cluster wave function is probed on the transfer cross section. It is found that the surface region of the cluster wave function is sensitive to the cross section. The present work is situated as the first step in obtaining important information to systematically investigate the cluster structure.

nucl-th↗

arXiv2015

Nonlinear electronic excitations in crystalline solids using meta-generalized gradient approximation and hybrid functional in time-dependent density functional theory

We develop numerical methods to calculate electron dynamics in crystalline solids in real-time time-dependent density functional theory employing exchange-correlation potentials which reproduce band gap energies of dielectrics; a meta generalized gradient approximation (meta-GGA) proposed by Tran and Blaha [Phys. Rev. Lett. 102, 226401 (2009)] (TBm-BJ) and a hybrid functional proposed by Heyd, Scuseria, and Ernzerhof [J. Chem. Phys. 118, 8207 (2003)] (HSE). In time evolution calculations employing the TB-mBJ potential, we have found it necessary to adopt a predictor-corrector step for stable time-evolution. Since energy functional is not known for the TB-mBJ potential, we propose a method to evaluate electronic excitation energy without referring to the energy functional. Calculations using the HSE hybrid functional is computationally expensive due to the nonlocal Fock-like term. We develop a computational method for the operation of the Fock-like term in Fourier space, for which we employ massively parallel computers equipped with graphic processing units. To demonstrate significances of utilizing potentials providing correct band gap energies, we compare electronic excitations induced by femtosecond laser pulses using the TB-mBJ, HSE, and a simple local density approximation (LDA). At low laser intensities, electronic excitations are found to be sensitive to the band gap energy: results using TB-mBJ and HSE are close to each other, while the excitation of the LDA calculation is more intensive than the others. At high laser intensities close to a damage threshold, we have found that electronic excitation energies are similar among the three cases.

cond-mat.mtrl-sci↗

arXiv2014

$^{16}$O + $^{16}$O molecular structures of positive- and negative-parity superdeformed bands in $^{34}$S

The structures of excited states in $^{34}$S are investigated using the antisymmetrized molecular dynamics and generator coordinate method (GCM). The GCM basis wave functions are calculated via energy variation with a constraint on the quadrupole deformation parameter $β$. By applying the GCM after parity and angular momentum projections, the coexistence of two positive- and one negative-parity superdeformed (SD) bands are predicted, and low-lying states and other deformed bands are obtained. The SD bands have structures of $^{16}$O + $^{16}$O + two valence neutrons in molecular orbitals around the two $^{16}$O cores in a cluster picture. The configurations of the two valence neutrons are $δ^2$ and $π^2$ for the positive-parity SD bands and $π^1δ^1$ for the negative-parity SD band. The structural changes of the yrast states are also discussed.

nucl-th↗

arXiv2014

Approximation of reduced width amplitude and application to cluster decay width

We propose a simple method to approximately evaluate reduced width amplitude (RWA) of a two-body spinless cluster channel using the norm overlap with the Brink-Bloch cluster wave function at the channel radius. The applicability of the present approximation is tested for the $^{16}$O+$α$ channel in $^{20}$Ne as well as the $α$+$α$ channel in $^8$Be. The approximation is found to be reasonable to evaluate the RWA for states near the threshold energy and it is useful to estimate the $α$-decay width of resonance states. The approximation is also applied to $^9$Li, and the partial decay width of the $^6$He($0^+_1$)+$t$ channel is discussed.

nucl-th↗