arXiv subjects

C. F. Jiao

Publications and source records attributed to C. F. Jiao.

11 recordsLinked to original sources

arXiv2026

Ab Initio Emergence and Collapse of Nuclear Collectivity near N = Z = 40

We present an \textit{ab initio} description of the emergence and collapse of enhanced quadrupole collectivity in nuclei near $N=Z=40$ using multi-shell valence-space Hamiltonians derived from chiral two- and three-nucleon forces with the in-medium similarity renormalization group. Projected generator-coordinate calculations with consistently evolved $E2$ operators capture the strong collectivity in $^{80}$Zr and its decrease toward $^{86}$Mo and $^{88}$Ru without empirical effective charges. While the $N,Z=40$ effective single-particle energy gaps remain open, the $1g_{9/2}$--$2d_{5/2}$ spacing is smallest near $^{76}$Sr and $^{80}$Zr, with its variation governed mainly by the proton-neutron monopole contribution. The associated occupancies are consistent with quadrupole correlations involving pseudo-SU(3) $pf$ holes and quasi-SU(3) particles. Excluding the occupation of the proton and neutron $2d_{5/2}$ from the variational space strongly suppresses the deformation and $B(E2;2_1^+\rightarrow0_1^+)$ in $^{80}$Zr. These results mark a steady step toward \textit{ab initio} computations of heavy open-shell nuclei.

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arXiv2026

First observation of multi-phonon $γ$-vibrations in an odd-odd nuclear system

The identification of the first multi-phonon $γ$-vibrational bands in an odd-odd neutron-rich nucleus of the nuclear chart is presented. These high spin structures of hard to access $^{104}_{41}$Nb$_{63}$, produced in fission, were studied by combining a spectrometer with isotopic resolution coupled to a $γ$-ray tracking array and independently high-fold $γ$ coincidence measurements. Triaxial Projected Shell Model calculations for the high-spin states are in good agreement with the measured observables for the yrast, one-phonon and two-phonon $γ$ bands. The possibility of an oblate shape of an isomeric state and coexistence of triaxial and oblate configurations are investigated from the decay of the 141 keV isomer. The present work illustrates the robustness of vibration excitations in the presence of odd valence proton and neutron as well as the possibly coexisting shapes beyond the $N=60$ transitional region.

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arXiv2025

Ab initio study in the island of inversion within the two-major-shell valence space

We present an \textit{ab initio} study of nuclear structure in the island of inversion around neutron number $N=20$, using multishell effective Hamiltonians derived from the valence-space in-medium similarity renormalization group approach combined with the quantum-number projected generator coordinate method. By progressively expanding the valence space from the \textit{sd} shell to the intermediate $sdf_{7/2}p_{3/2}$ space and, for the first time, to the full \textit{sdfp} shell, we investigate low-lying spectra, $E2$ transition strengths, deformation properties, and neutron occupancies in even-even Ne, Mg, and Si isotopes around $N=20$. Our results show that enlarging the valence space significantly improves the description of quadrupole collectivity, yielding better agreement with experimental data for key observables such as the lowered $2^+$ energies and the enhanced $B(E2;0^+_1\rightarrow2_1^+)$ values. The analysis reveals the critical role of cross-shell multi-particle multi-hole excitations in breaking the $N=20$ shell closure and establishing intruder-dominated ground states. It also demonstrates the ability of the VS-IMSRG+PGCM framework to capture both dynamical (short range) and static (long range) correlations across multiple major-oscillator shells.

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arXiv2025

Ab initio nuclear shape coexistence and emergence of island of inversion around $N=20$

We extend a nuclear ab initio framework based on chiral two- and three-nucleon interactions to investigate shape coexistence and the degradation of the $N=20$ magic number in both even-even and odd-even magnesium isotopes. The quantum-number projected generator coordinate method, combined with the in-medium similarity renormalization group (IMSRG), is employed to compute their low-lying states. This approach reasonably reproduces the coexistence of weakly and strongly deformed states at comparable energies, and allows us to track the emergence of the $N=20$ island of inversion through the continuous IMSRG evolution of the chiral Hamiltonian. Our results indicate that the ground state of $^{33}$Mg with spin-parity $3/2^-$ is predominantly a strongly deformed configuration with $K^π= 3/2^-$, while the lowest $7/2^-$ state is predicted to be a shape isomer, consisting of a mixture of weakly deformed configurations with different $K$ values. The results highlight the essential roles of both dynamical and static collective correlations in reproducing the ordering of nuclear states with distinct shapes.

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arXiv2025

Direct observation of $β$ and $γ$ decay from a high-spin long-lived isomer in $^{187}$Ta

$^{187}$Ta ($Z=73$, $N=114$) is located in the neutron-rich $A \approx 190$ region where a prolate-to-oblate shape transition via triaxial softness is predicted to take place. A preceding work on the $K^π = (25/2^-)$ isomer and a rotational band to which the isomer decays carried out by the same collaboration revealed that axial symmetry is slightly violated in this nucleus. This paper focuses on a higher-lying isomer, which was previously identified at 2933(14) keV by mass measurements with the Experimental Storage Ring at GSI. The isomer of interest has been populated by a multi-nucleon transfer reaction with a $^{136}$Xe primary beam incident on a natural tungsten target, using the KEK Isotope Separation System at RIKEN. New experimental findings obtained in the present paper include the internal and external $β$-decay branches from the high-spin isomer and a revised half-life of 136(24) s. The evaluated hindrances for $K$-forbidden transitions put constraints on the spin-parity assignment, which can be interpreted as being ascribed to a prolate shape with a five-quasiparticle configuration by model calculations.

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arXiv2023

Optimization of generator coordinate method with machine-learning techniques for nuclear spectra and neutrinoless double-beta decay: ridge regression for nuclei with axial deformation

The generator coordinate method (GCM) is an important tool of choice for modeling large-amplitude collective motion in atomic nuclei. The computational complexity of the GCM increases rapidly with the number of collective coordinates. It imposes a strong restriction on the applicability of the method. In this work, we propose a subspace-reduction algorithm that employs optimal statistical ML models as surrogates for exact quantum-number projection calculations for norm and Hamiltonian kernels. The model space of the original GCM is reduced to a subspace relevant for nuclear low energy spectra and the NME of ground state to ground state $0νββ$ decay based on the orthogonality condition (OC) and the energy-transition-orthogonality procedure (ENTROP), respectively. For simplicity, the polynomial ridge regression (RR) algorithm is used to learn the norm and Hamiltonian kernels of axially deformed configurations. The efficiency and accuracy of this algorithm are illustrated for 76Ge and 76Se by comparing results obtained using the optimal RR models to direct GCM calculations. The low-lying energy spectra of $^{76}$Ge and $^{76}$Se, as well as the $0νββ$-decay NME between their ground states, are computed. The results show that the performance of the GCM+OC/ENTROP+RR is more robust than that of the GCM+RR alone, and the former can reproduce the results of the original GCM calculation accurately with a significantly reduced computational cost.

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arXiv2019

Neutrinoless double-$β$ decay of $^{124}$Sn, $^{130}$Te, and $^{136}$Xe in the Hamiltonian-based generator-coordinate method

We present a generator-coordinate method for realistic shell-model Hamiltonians that closely approximates the full shell model calculations of the matrix elements for the neutrinoless double-$β$ decay of $^{124}$Sn, $^{130}$Te, and $^{136}$Xe. We treat not only quadrupole deformations but also the proton-neutron pairing amplitudes as generator coordinates. We validate this method by calculating and comparing spectroscopic quantities with the exact shell model results and experimental data. Our Hamiltonian-based generator-coordinate method produces $0νββ$ matrix elements much closer to the shell model ones, compared to the existing energy-density-functional-based generator-coordinate approaches. The remaining overestimation of $0νββ$ nuclear matrix element suggests that additional correlations may be needed to be taken into account for $^{124}$Sn, $^{130}$Te, and $^{136}$Xe when calculating with the Hamiltonian-based generator-coordinate method. The validation of this method may open the possibility of calculating $0νββ$ matrix element of $^{150}$Nd in a large shell-model space.

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arXiv2018

Generator-coordinate reference states for spectra and $0νββ$ decay in the in-medium similarity renormalization group

We use a reference state based on symmetry-restored states from deformed mean-field or generator-coordinate-method (GCM) calculations in conjunction with the in-medium similarity-renormalization group (IMSRG) to compute spectra and matrix elements for neutrinoless double-beta ($0νββ$) decay. Because the decay involves ground states from two nuclei, we use evolved operators from the IMSRG in one nucleus in a subsequent GCM calculation in the other. We benchmark the resulting IMSRG+GCM method against complete shell-model diagonalization for both the energies of low-lying states in $^{48}$Ca and $^{48}$Ti and the $0νββ$ matrix element for the decay of $^{48}$Ca, all in a single valence shell. Our approach produces better spectra than either the IMSRG with a spherical-mean-field reference or GCM calculations with unevolved operators. For the $0νββ$ matrix element the improvement is slight, but we expect more significant effects in full ab-initio calculations.

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arXiv2017

Neutrinoless double-beta decay matrix elements in large shell-model spaces with the generator-coordinate method

We use the generator-coordinate method with realistic shell-model interactions to closely approximate full shell-model calculations of the matrix elements for the neutrinoless double-beta decay of $^{48}$Ca, $^{76}$Ge, and $^{82}$Se. We work in one major shell for the first isotope, in the $f_{5/2}pg_{9/2}$ space for the second and third, and finally in two major shells for all three. Our coordinates include not only the usual axial deformation parameter $β$, but also the triaxiality angle $γ$ and neutron-proton pairing amplitudes. In the smaller model spaces our matrix elements agree well with those of full shell-model diagonalization, suggesting that our Hamiltonian-based GCM captures most of the important valence-space correlations. In two major shells, where exact diagonalization is not currently possible, our matrix elements are only slightly different from those in a single shell.

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arXiv2016

Searching for high-$K$ isomers in the proton-rich $A\sim80$ mass region

Configuration-constrained potential-energy-surface calculations have been performed to investigate the $K$ isomerism in the proton-rich $A\sim80$ mass region. An abundance of high-$K$ states are predicted. These high-$K$ states arise from two and four-quasi-particle excitations, with $K^π=8^{+}$ and $K^π=16^{+}$, respectively. Their excitation energies are comparatively low, making them good candidates for long-lived isomers. Since most nuclei under studies are prolate spheroids in their ground states, the oblate shapes of the predicted high-$K$ states may indicate a combination of $K$ isomerism and shape isomerism.

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arXiv2015

Shape coexistence and evolution in neutron-deficient krypton isotopes

Total Routhian Surface (TRS) calculations have been performed to investigate shape coexistence and evolution in neutron-deficient krypton isotopes ${}^{72,74,76}$Kr. The ground-state shape is found to change from oblate in ${}^{72}$Kr to prolate in ${}^{74,76}$Kr, in agreement with experimental data. Quadrupole deformations of the ground states and coexisting $0^{+}_{2}$ states as well as excitation energies of the latter are also well reproduced. While the general agreement between calculated moments of inertia and those deduced from observed spectra confirms the prolate nature of the low-lying yrast states of all three isotopes (except the ground state of ${}^{72}$Kr), the deviation at low spins suggests significant shape mixing. The role of triaxiality in describing shape coexistence and evolution in these nuclei is finally discussed.

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