arXiv subjects

J. M. Yao

Publications and source records attributed to J. M. Yao.

At least 91 records · Page 5Linked to original sources

arXiv2015

Relativistic description of nuclear matrix elements in neutrinoless double-$β$ decay

Neutrinoless double-$β$ ($0νββ$) decay is related to many fundamental concepts in nuclear and particle physics beyond the standard model. Currently there are many experiments searching for this weak process. An accurate knowledge of the nuclear matrix element for the $0νββ$ decay is essential for determining the effective neutrino mass once this process is eventually measured. We report the first full relativistic description of the $0νββ$ decay matrix element based on a state-of-the-art nuclear structure model. We adopt the full relativistic transition operators which are derived with the charge-changing nucleonic currents composed of the vector coupling, axial-vector coupling, pseudoscalar coupling, and weak-magnetism coupling terms. The wave functions for the initial and final nuclei are determined by the multireference covariant density functional theory (MR-CDFT) based on the point-coupling functional PC-PK1. The low-energy spectra and electric quadrupole transitions in ${}^{150}$Nd and its daughter nucleus ${}^{150}$Sm are well reproduced by the MR-CDFT calculations. The $0νββ$ decay matrix elements for both the $0_1^+\rightarrow 0_1^+$ and $0_1^+\rightarrow 0_2^+$ decays of ${}^{150}$Nd are evaluated. The effects of particle number projection, static and dynamic deformations, and the full relativistic structure of the transition operators on the matrix elements are studied in detail. The resulting $0νββ$ decay matrix element for the $0_1^+\rightarrow 0_1^+$ transition is $5.60$, which gives the most optimistic prediction for the next generation of experiments searching for the $0νββ$ decay in ${}^{150}$Nd.

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arXiv2014

Structure of hypernuclei in relativistic approaches

We review the relativistic mean-field approach to various aspects of hypernuclei. It was Brockmann and Weise who first applied this approach to hypernuclei. At that time, it had been already observed experimentally that the spin-orbit splittings in hypernuclei are significantly smaller than that in ordinary nuclei. The relativistic approach was suitable for a discussion of spin-orbit splittings in hypernuclei, as the spin-orbit interaction is naturally emerged with the relativistic framework. Since this seminal work, many developments have been done and the relativistic approach has been applied both to the ground state and to excited states of hypernuclei. We summarize those developments in the Chapter. We shall concentrate on $Λ$-hypernuclei and do not discuss $Σ$ hypernuclei, since the theoretical treatment is rather similar to each other.

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arXiv2014

Searching for 4$α$ linear-chain structure in excited states of $^{16}$O with a covariant density functional theory

A study of 4$α$ linear-chain structure in high-lying collective excitation states of $^{16}$O with a covariant density functional theory is presented. The low-spin states are obtained by configuration mixing of particle-number and angular-momentum projected quadrupole deformed mean-field states with generator coordinate method. The high-spin states are determined by cranking calculations. These two calculations are based on the same energy density functional PC-PK1. We have found a rotational band at low-spin with the dominated intrinsic configuration considered to be the one that 4$α$ clusters stay along a common axis. The strongly deformed rod shape also appears in the high-spin region with the angular momentum $13-18\hbar$; however whether the state is pure $4α$ linear chain or not is less obvious than that in the low-spin states.

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arXiv2014

Microscopic particle-rotor model for low-lying spectrum of Lambda hypernuclei

We propose a novel method for low-lying states of hypernuclei based on the particle-rotor model, in which hypernuclear states are constructed by coupling the hyperon to low-lying states of the core nucleus. In contrast to the conventional particle-rotor model, we employ a microscopic approach for the core states, that is, the generator coordinate method (GCM) with the particle number and angular momentum projections. We apply this microscopic particle-rotor model to $^9_Λ$Be employing a point-coupling version of the relativistic mean-field Lagrangian. A reasonable agreement with the experimental data for the low-spin spectrum is achieved using the $ΛN$ coupling strengths determined to reproduce the binding energy of the $Λ$ particle.

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arXiv2014

A microscopic benchmark-study of triaxiality in low-lying states of 76Kr

We report on a seven-dimensional generator coordinate calculation in the two deformation parameters $β$ and $γ$ together with projection on three-dimensional angular momentum and two particle numbers for the low-lying states in $^{76}$Kr. These calculations are based on covariant density functional theory. Excellent agreement is found with the data for the spectrum and the electric multipole transition strengths. This answers the important question of dynamic correlations and triaxiality in a fully microscopic way. We find that triaxial configurations dominate both the ground state and the quasi $γ$-band. This yields a different picture from the simple interpretation in terms of "coexistence of a prolate ground state with an oblate low-lying excited state", which is based on the measured sign of spectroscopic quadrupole moments. This study also provides for the first time a benchmark for the collective Hamiltonian in five dimensions. Moreover, we point out that the staggering phase of the $γ$-band is not a safe signature for rigid triaxiality of the low-energy structure.

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arXiv2014

Global dynamical correlation energies in covariant density functional theory: cranking approximation

The global dynamical correlation energies for 575 even-even nuclei with proton numbers ranging from Z=8 to Z=108 calculated with the covariant density functional theory using the PC-PK1 parametrization are presented. The dynamical correlation energies include the rotational correction energies obtained with the cranking approximation and the quadrupole vibrational correction energies. The systematic behavior of the present correlation energies is in good agreement with that obtained from the projected generator coordinate method using the SLy4 Skyrme force although our values are systematically smaller. After including the dynamical correlation energies, the root-mean-square deviation predicted by the PC-PK1 for the 575 even-even nuclei masses is reduced from 2.58 MeV to 1.24 MeV.

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arXiv2014

Systematic study of the symmetry energy coefficient in finite nuclei

The symmetry energy coefficients in finite nuclei have been studied systematically with a covariant density functional theory (DFT) and compared with the values calculated using several available mass tables. Due to the contamination of shell effect, the nuclear symmetry energy coefficients extracted from the binding energies have large fluctuations around the nuclei with double magic numbers. The size of this contamination is shown to be smaller for the nuclei with larger isospin value. After subtracting the shell effect with the Strutinsky method, the obtained nuclear symmetry energy coefficients with different isospin values are shown to decrease smoothly with the mass number $A$ and are subsequently fitted to the relation $\dfrac{4a_{\rm sym}}{A}=\dfrac{b_v}{A}-\dfrac{b_s}{A^{4/3}}$. The resultant volume $b_v$ and surface $b_s$ coefficients from axially deformed covariant DFT calculations are $121.73$ and $197.98$ MeV respectively. The ratio $b_s/b_v=1.63$ is in good agreement with the value derived from the previous calculations with the non-relativistic Skyrme energy functionals. The coefficients $b_v$ and $b_s$ corresponding to several available mass tables are also extracted. It is shown that there is a strong linear correlation between the volume $b_v$ and surface $b_s$ coefficients and the ratios $b_s/b_v$ are in between $1.6-2.0$ for all the cases.

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arXiv2014

Low-energy structure and anti-bubble effect of dynamical correlations in 46Ar

The low-energy structure of 46Ar is of particular interest due to the possible coexistence of different shapes and the possible existence of proton bubble structure. In this work, we apply a beyond relativistic mean-field approach to study the low-energy structure of 46Ar. Correlations beyond the mean field are introduced by configuration mixing of both particle-number and angular-momentum projected axially deformed mean-field states. The low-lying spectroscopy and charge density in laboratory frame are calculated and an excellent agreement with available data is achieved. Even though an evident proton bubble structure is shown in the spherical state of 46Ar, it eventually disappears after taking into account the dynamical correlation effects. Moreover, our results indicate that the existence of proton bubble structure in argon isotopes is unlikely.

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arXiv2013

Effect of pairing correlations on nuclear low-energy structure: BCS and general Bogoliubov transformation

Low-lying nuclear states of Sm isotopes are studied in the framework of a collective Hamiltonian based on covariant energy density functional theory. Pairing correlation are treated by both BCS and Bogoliubov methods. It is found that the pairing correlations deduced from relativistic Hartree-Bogoliubov (RHB) calculations are generally stronger than those by relativistic mean-field plus BCS (RMF+BCS) with same pairing force. By simply renormalizing the pairing strength, the diagonal part of the pairing field is changed in such a way that the essential effects of the off-diagonal parts of the pairing field neglected in the RMF+BCS calculations can be recovered, and consequently the low-energy structure is in a good agreement with the predictions of the RHB model.

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arXiv2013

Simultaneous quadrupole and octupole shape phase transitions in Thorium

The evolution of quadrupole and octupole shapes in Th isotopes is studied in the framework of nuclear Density Functional Theory. Constrained energy maps and observables calculated with microscopic collective Hamiltonians indicate the occurrence of a simultaneous quantum shape phase transition between spherical and quadrupole-deformed prolate shapes, and between non-octupole and octupole-deformed shapes, as functions of the neutron number. The nucleus $^{224}$Th is closest to the critical point of a double phase transition. A microscopic mechanism of this phenomenon is discussed in terms of the evolution of single-nucleon orbitals with deformation.

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arXiv2013

Does a proton "bubble" structure exist in the low-lying states of 34Si?

The possible existence of a "bubble" structure in the proton density of $^{34}$Si has recently attracted a lot of research interest. To examine the existence of the "bubble" structure in low-lying states, we establish a relativistic version of configuration mixing of both particle number and angular momentum projected quadrupole deformed mean-field states and apply this state-of-the-art beyond relativistic mean-field method to study the density distribution of the low-lying states in $^{34}$Si. An excellent agreement with the data of low-spin spectrum and electric multipole transition strengths is achieved without introducing any parameters. We find that the central depression in the proton density is quenched by dynamic quadrupole shape fluctuation, but not as significantly as what has been found in a beyond non-relativistic mean-field study. Our results suggest that the existence of proton "bubble" structure in the low-lying excited $0^+_2$ and $2^+_1$ states is very unlikely.

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arXiv2013

Beyond relativistic mean-field studies of low-lying states in neutron-deficient krypton isotopes

Neutron-deficient krypton isotopes are of particular interest due to the coexistence of oblate and prolate shapes in low-lying states and the transition of ground-state from one dominate shape to another as a function of neutron number. A detailed interpretation of these phenomena in neutron-deficient Kr isotopes requires the use of a method going beyond a mean-field approach that permits to determine spectra and transition probabilities. The aim of this work is to provide a systematic calculation of low-lying state in the even-even 68-86Kr isotopes and to understand the shape coexistence phenomenon and the onset of large collectivity around N=40 from beyond relativistic mean-field studies. The starting point of our method is a set of relativistic mean-field+BCS wave functions generated with a constraint on triaxial deformations (beta, gamma). The excitation energies and electric multipole transition strengths of low-lying states are calculated by solving a five-dimensional collective Hamiltonian (5DCH) with parameters determined by the mean-field wave functions. To examine the role of triaxiality, a configuration mixing of both particle number (PN) and angular momentum (AM) projected axially deformed states is also carried out within the exact generator coordinate method (GCM) based on the same energy density functional. The energy surfaces, the excitation energies of 0^+_2, 2^+_1, 2^+_2 states, as well as the E0 and E2 transition strengths are compared with the results of similar 5DCH calculations but with parameters determined by the non-relativistic mean-field wave functions, as well as with the available data...

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arXiv2012

Collective excitations of $Λ$ hypernuclei

We discuss low-lying collective excitations of $Λ$ hypernuclei using the self-consistent mean-field approaches. We first discuss the deformation properties of $Λ$ hypernuclei in the $sd$-shell region. Based on the relativistic mean-field (RMF) approach, we show that the oblate deformation for $^{28}$Si nucleus may disappear when a $Λ$ particle is added to this nucleus. We then discuss the rotational excitations of $^{25}_Λ$Mg nucleus using the three-dimensional potential energy surface in the deformation plane obtained with the Skyrme-Hartree-Fock method. The deformation of $^{25}_Λ$Mg nucleus is predicted to be slightly reduced due to an addition of $Λ$ particle. We demonstrate that this leads to a reduction of electromagnetic transition probability, $B(E2)$, in the ground state rotational band. We also present an application of random phase approximation (RPA) to hypernuclei, and show that a new dipole mode, which we call a soft dipole $Λ$ mode, appears in hypernuclei, which can be interpreted as an oscillation of $Λ$ particle against the core nucleus.

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arXiv2012

Systematics of low-lying states of even-even nuclei in the neutron-deficient lead region from a beyond-mean-field calculation

Nuclei located in the neutron-deficient Pb region have a complex structure, rapidly evolving as a function of neutron and proton numbers. The most famous example is $^{186}$Pb where the three lowest levels are $0^+$ states, the two excited $0^+$ ones being located at low excitation energy around 600 keV. Coexisting structures with different properties are found in the neighboring nuclei. Many experiments have been performed over the last few years in which in-band and out-of-band $γ$ transition probabilities have been measured. A detailed interpretation of experimental data requires the use of a method going beyond a mean-field approach that permits to determine spectra and transition probabilities. Such methods have already been applied to selected isotopes in this mass region. Our aim is to provide a systematic investigation of this mass region in order to determine how well experimental data can be understood using a state-of-the-art method for nuclear structure. The starting point of our method is a set of mean-field wave functions generated with a constraint on the axial quadrupole moment and using a Skyrme energy density functional. Correlations beyond the mean field are introduced by projecting mean-field wave functions on angular-momentum and particle number and by mixing the symmetry restored wave functions as a function of the axial quadrupole moment. A detailed comparison with the available data is performed for energies, charge radii, spectroscopic quadrupole moments, $E0$ and $E2$ transition probabilities for the isotopic chains of neutron deficient Hg, Pb, Po and Rn. The connection between our results and the underlying mean field is also analyzed. Qualitative agreement with the data is obtained although our results indicate that the actual energy density functionals have to be improved further to achieve a quantitative agreement.

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arXiv2012

Enhanced collectivity in neutron-deficient Sn isotopes in energy functional based collective Hamiltonian

The low-lying collective states in Sn isotopes are studied by a five-dimensional collective Hamiltonian with parameters determined from the triaxial relativistic mean-field calculations using the PC-PK1 energy density functional. The systematics for both the excitation energies of $2^+_1$ states and $B(E2;0^+_1\to 2^+_1)$ values are reproduced rather well, in particular, the enhanced E2 transitions in the neutron-deficient Sn isotopes with N<66. We show that the gradual degeneracy of neutron levels 1g7/2 and 2d5/2 around the Fermi surface leads to the increase of level density and consequently the enhanced paring correlations from N=66 to 58. It provokes a large quadrupole shape fluctuation around the spherical shape, and leads to an enhanced collectivity in the isotopes around N=58.

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arXiv2012

Energy Density Functional analysis of shape evolution in N=28 isotones

The structure of low-energy collective states in proton-deficient N=28 isotones is analyzed using structure models based on the relativistic energy density functional DD-PC1. The relativistic Hartree-Bogoliubov model for triaxial nuclei is used to calculate binding energy maps in the $β$-$γ$ plane. The evolution of neutron and proton single-particle levels with quadrupole deformation, and the occurrence of gaps around the Fermi surface, provide a simple microscopic interpretation of the onset of deformation and shape coexistence. Starting from self-consistent constrained energy surfaces calculated with the functional DD-PC1, a collective Hamiltonian for quadrupole vibrations and rotations is employed in the analysis of excitation spectra and transition rates of $^{46}$Ar, $^{44}$S, and $^{42}$Si. The results are compared to available data, and previous studies based either on the mean-field approach or large-scale shell-model calculations. The present study is particularly focused on $^{44}$S, for which data have recently been reported that indicate pronounced shape coexistence.

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arXiv2012

An efficient method for computing the Thouless-Valatin inertia parameters

Starting from the adiabatic time-dependent Hartree-Fock approximation (ATDHF), we propose an efficient method to calculate the Thouless-Valatin moments of inertia for the nuclear system. The method is based on the rapid convergence of the expansion of the inertia matrix. The accuracy of the proposed method is verified in the rotational case by comparing the results with the exact Thouless-Valatin moments of inertia calculated using the self-consistent cranking model. The proposed method is computationally much more efficient than the full ATDHF calculation, yet it retains a high accuracy of the order of 1%.

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arXiv2012

Rapid structural change in low-lying states of neutron-rich Sr and Zr isotopes

The rapid structural change in low-lying collective excitation states of neutron-rich Sr and Zr isotopes is tudied by solving a five-dimensional collective Hamiltonian with parameters determined by both relativistic mean-field and non-relativistic Skyrme-Hartree-Fock calculations using the PC-PK1 and SLy4 forces respectively. Pair correlations are treated in BCS method with either a separable pairing force or a density-dependent zero-range force. The isotope shifts, excitation energies, electric monopole and quadrupole transition strengths are calculated and compared with corresponding experimental data. The calculated results with both the PC-PK1 and SLy4 forces exhibit a picture of spherical-oblate-prolate shape transition in neutron-rich Sr and Zr isotopes. Compared with the experimental data, the PC-PK1 (or SLy4) force predicts a more moderate (or dramatic) change in most of the collective properties around N=60. The underlying microscopic mechanism responsible for the rapid transition is discussed.

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