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J. Meng

Publications and source records attributed to J. Meng.

At least 37 records · Page 2Linked to original sources
arXiv2024

Generalized time-dependent generator coordinate method for induced fission dynamics

The generalized time-dependent generator coordinate method (TD-GCM) is extended to include pairing correlations. The correlated GCM nuclear wave function is expressed in terms of time-dependent generator states and weight functions. The particle-hole channel of the effective interaction is determined by a Hamiltonian derived from an energy density functional, while pairing is treated dynamically in the standard BCS approximation with time-dependent pairing tensor and single-particle occupation probabilities. With the inclusion of pairing correlations, various time-dependent phenomena in open-shell nuclei can be described more realistically. The model is applied to the description of saddle-to-scission dynamics of induced fission. The generalized TDGCM charge yields and total kinetic energy distribution for the fission of 240Pu, are compared to those obtained using the standard time-dependent density functional theory (TD-DFT) approach, and with available data.

nucl-th↗
arXiv2023

Isospin-dependence of the charge-changing cross-section shaped by the charged-particle evaporation process

We present the charge-changing cross sections (CCCS) of $^{11-15}$C, $^{13-17}$N, and $^{15,17-18}$O at around 300 MeV/nucleon on a carbon target, which extends to $p$-shell isotopes with $N < Z$ for the first time. The Glauber model, which considers only the proton distribution of projectile nuclei, underestimates the cross sections by more than 10\%. We show that this discrepancy can be resolved by considering the contribution from the charged-particle evaporation process (CPEP) following projectile neutron removal. Using nucleon densities from the deformed relativistic Hartree-Bogoliubov theory in continuum, we investigate the isospin-dependent CPEP contribution to the CCCS for a wide range of neutron-to-proton separation energy asymmetry. Our calculations, which include the CPEP contribution, agree well with existing systematic data and reveal an ``evaporation peak" at the isospin symmetric region where the neutron-to-proton separation energy is close to zero. These results suggest that analysis beyond the Glauber model is crucial for accurately determining nuclear charge radii from CCCSs.

nucl-ex↗
arXiv2023

Effects of rotation and valence nucleons in molecular-like $α$-chain nuclei

Effects of rotation and valence nucleons in molecular-like linear $α$-chain nuclei are analyzed using a three-dimensional lattice cranking model based on covariant density functional theory. The structure of $^{16}$C and $^{16}$Ne is investigated as a function of rotational frequency. The valence nucleons, with respect to the 3$α$ linear chain core of $^{12}$C, at low frequency occupy the $π$ molecular orbital. With increasing rotational frequency these nucleons transition from the $π$ orbital to the $σ$ molecular orbital, thus stabilizing the 3$α$ linear chain structure. It is predicted that the valence protons in $^{16}$Ne change occupation from the $π$ to the $σ$ molecular orbital at $\hbarω\approx 1.3$ MeV, a lower rotational frequency compared to $\hbarω\approx 1.7$ MeV for the valence neutrons in $^{16}$C. The same effects of valence protons are found in $^{20}$Mg, compared to the four valence neutrons in $^{20}$O. The model is also used to examine the effect of alignment of valence nucleons on the relative positions and size of the three $α$-clusters in $^{16}$C and $^{16}$Ne.

nucl-th↗
arXiv2023

Impact of isovector pairing fluctuation on neutrinoless double-beta decay in multi-reference covariant density functional theory

We extend the multi-reference covariant density functional theory (MR-CDFT) by including fluctuations in quadrupole deformations and average isovector pairing gaps simultaneously for the nuclear matrix elements (NMEs) of neutrinoless double-beta $(0νββ)$ decay in the candidate nuclei $^{76}$Ge, $^{82}$Se, $^{100}$Mo, $^{130}$Te, and $^{136}$Xe assuming the exchange of either light or heavy neutrinos. The results indicate a linear correlation between the predicted NMEs and the isovector pairing strengths, as well as the excitation energies of $2^{+}_1$ and $4^{+}_1$ states. By adjusting the pairing strengths based on the excitation energies of the $2^{+}_1$ states, we calculate the NMEs for $0νββ$ decay, which are reduced by approximately $12\%$ to $62\%$ compared to the results obtained in the previous studies by Song et al. [Phys. Rev. C95, 024305 (2017)]. Additionally, upon introducing the average isovector pairing gap as an additional generator coordinate in the calculation, the NMEs increase by a factor ranging from $56\%$ to $218\%$.

nucl-th↗
arXiv2023

Generalized time-dependent generator coordinate method for small and large amplitude collective motion

An implementation of the generalized time-dependent generator coordinated method (TD-GCM) is developed, that can be applied to the dynamics of small- and large-amplitude collective motion of atomic nuclei. Both the generator states and weight functions of the GCM correlated wave function depend on time. The initial generator states are obtained as solutions of deformation-constrained self-consistent mean-field equations, and are evolved in time by the standard mean-field equations of nuclear density functional theory (TD-DFT). The TD-DFT trajectories are used as a generally non-orthogonal and overcomplete basis in which the TD-GCM wave function is expanded. The weights, expressed in terms of a collective wave function, obey a TD-GCM (integral) equation. In this explorative paper, the generalized TD-GCM is applied to the excitation energies and spreading width of giant resonances, and to the dynamics of induced fission. The necessity of including pairing correlations in the basis of TD-DFT trajectories is demonstrated in the latter example.

nucl-th↗
arXiv2023

Fully-gapped superconductivity and topological aspects of the noncentrosymmetric TaReSi superconductor

We report a study of the noncentrosymmetric TaReSi superconductor by means of muon-spin rotation and relaxation ($μ$SR) technique, complemented by electronic band-structure calculations. Its superconductivity, with $T_c$ = 5.5 K and upper critical field $μ_0H_\mathrm{c2}(0)$ $\sim$ 3.4 T, was characterized via electrical-resistivity- and magnetic-susceptibility measurements. The temperature-dependent superfluid density, obtained from transverse-field $μ$SR, suggests a fully-gapped superconducting state in TaReSi, with an energy gap $Δ_0$ = 0.79 meV and a magnetic penetration depth $λ_0$ = 562 nm. The absence of a spontaneous magnetization below $T_c$, as confirmed by zero-field $μ$SR, indicates a preserved time-reversal symmetry in the superconducting state. The density of states near the Fermi level is dominated by the Ta- and Re-5$d$ orbitals, which account for the relatively large band splitting due to the antisymmetric spin-orbit coupling. In its normal state, TaReSi behaves as a three-dimensional Kramers nodal-line semimetal, characterized by an hourglass-shaped dispersion protected by glide reflection. By combining non\-triv\-i\-al electronic bands with intrinsic superconductivity, TaReSi is a promising material for investigating the topological aspects of noncentrosymmetric superconductors.

cond-mat.supr-con↗
arXiv2023

Fully-gapped superconductivity with preserved time-reversal symmetry in NiBi$_3$ single crystals

We report a study of NiBi$_3$ single crystals by means of electrical-resistivity-, magnetization-, and muon-spin rotation and relaxation ($μ$SR) measurements. As a single crystal, NiBi$_3$ adopts a needle-like shape and exhibits bulk superconductivity with $T_c \approx 4.1$ K. By applying magnetic fields parallel and perpendicular to the $b$-axis of NiBi$_3$, we establish that its lower- and upper critical fields, as well as the magnetic penetration depths show slightly different values, suggesting a weakly anisotropic superconductivity. In both cases, the zero-temperature upper critical fields are much smaller than the Pauli-limit value, indicating that the superconducting state is constrained by the orbital pair breaking. The temperature evolution of the superfluid density, obtained from transverse-field $μ$SR, reveals a fully-gapped superconductivity in NiBi$_3$, with a shared superconducting gap $Δ_0$ = 2.1 $k_\mathrm{B}$$T_c$ and magnetic penetration depths $λ_0$ = 223 and 210 nm for $H \parallel b$- and $H \perp b$, respectively. The lack of spontaneous fields below $T_c$ indicates that time-reversal symmetry is preserved in NiBi$_3$. The absence of a fast muon-spin relaxation and/or precession in the zero-field $μ$SR spectra definitely rules out any type of magnetic ordering in NiBi$_3$ single crystals. Overall, our investigation suggests that NiBi$_3$ behaves as a conventional $s$-type superconductor.

cond-mat.supr-con↗
arXiv2023

Nuclear chiral rotation induced by superfluidity

The microscopic understanding on the influence of the pairing correlations or the superfluidity on the nuclear chiral rotation has been a longstanding and challenging problem. Based on the three-dimensional cranking covariant density functional theory, a shell-model-like approach with exact particle number conservation is implemented to take into account the pairing correlations and applied for the chiral doublet bands in Nd135. The data available are well reproduced. It is found that the superfluidity can reduce the critical frequency and make the chiral rotation easier. The mechanism is that the particle/hole alignments along the short/long axis are reduced by the pairing correlations, resulting in the enhanced preference of the collective rotation along the intermediate axis, and inducing the early appearance of the chiral rotation.

nucl-th↗
arXiv2023

Coexisting single-particle excitations and octupole correlations in transitional nucleus $\mathbf{^{217}Ra}$

The level structure of the transitional nucleus $\mathrm{^{217}Ra}$ has been extended with the addition of around 20 new transitions. The discrepancies between the placements of several transitions reported in the earlier studies are resolved. The newly-established negative-parity sequence at low excitation energies hints at the expected parity-doublet structures in this nucleus. The properties of the observed simplex bands are compared with that of similar bands in neighboring nuclei. Since the presence of parity-doublet structures reflect octupole correlations, theoretical calculations using reflection-asymmetric triaxial particle rotor model (RAT-PRM) have been performed. A comparison of the observed features of the simplex bands with the predictions of the RAT-PRM calculations suggests that $\mathrm{^{217}Ra}$ exhibits an intermediate the behavior between the extremes of spherical and octupole-deformed nuclei. The termination of the simplex bands at intermediate energies and the structures lying above reflect the dominance of the single-particle excitations at higher excitation energies.

nucl-ex↗
arXiv2023

$0νββ$-decay nuclear matrix elements in self-consistent Skyrme quasiparticle random phase approximation: uncertainty from pairing interaction

The uncertainty in the nuclear matrix elements (NMEs) of $0νββ$ decay for $^{76}$Ge, $^{82}$Se, $^{128}$Te, $^{130}$Te, and $^{136}$Xe in the self-consistent quasiparticle random phase approximation (QRPA) method is investigated by using eighteen Skyrme interactions supplemented with either a volume- or surface-type of pairing interactions. The NMEs for the isotopes concerned (except $^{136}$Xe) are less sensitive to the particle-hole ($ph$) interactions, while strongly dependent on the employed isovector particle-particle ($pp$) pairing interactions even though the pairing strengths are optimized to the same pairing gap. The results indicate that a precise determination of the isovector $pp$ pairing interaction in the Skyrme energy density functional is of importance to reduce the uncertainty in the NMEs within the QRPA framework.

nucl-th↗
arXiv2023

Very early warning of a moderate-to-strong El Niño in 2023

The El Niño Southern Oscillation (ENSO) is the strongest driver of year-to-year variations of the global climate and can lead to extreme weather conditions and disasters in various regions around the world. Here, we review two different approaches for the early forecast of El Niño that we have developed recently: the climate network-based approach allows forecasting the onset of an El Niño event about 1 year ahead, while the complexity-based approach allows additionally to estimate the magnitude of an upcoming El Niño event in the calendar year before. For 2023, both approaches predict the onset of an El Niño event, with a combined onset probability of about 89%. The complexity-based approach predicts a moderate-to-strong El Niño with a magnitude of $1.49\pm0.37$°C. Since El Niño events temporarily increase the global temperature, we expect that the coming El Niño will increase the global temperature by about +0.2°C, likely making 2024 the hottest year since the beginning of instrumental observations. It is possible that as a consequence of this El Niño, the +1.5°C target (compared to pre-industrial levels) will be temporarily breached already in 2024.

physics.ao-ph↗
arXiv2022

Possible neutron halo in triaxial nucleus 42Al

A microscopic self-consistent triaxial relativistic Hartree-Bogoliubov theory in continuum (TRHBc), which simultaneously takes into account the triaxiality and pairing correlations as well as continuum effects, is established and applied to explore the novel halo phenomenon in aluminum isotopes. The experimental proton drip line and the available data of neutron separation energies and charge radii are reproduced well without any free parameters. The neutron-richest odd-odd aluminum isotope observed so far, 42Al, is predicted to be triaxially deformed with beta=0.35 and gamma=42. Its one-neutron separation energy is predicted to be 0.68 MeV, in agreement with the AME2020, and the neutron rms radius is 3.94 fm, remarkably larger than the empirical value. The density distribution of the valance neutron, which extends much farther in space than the core, suggests a possible neutron halo in 42Al. The dominant components responsible for the spatial extension of the halo are revealed by the single-neutron orbitals around the Fermi energy. A novel phenomenon, the exchange of the intermediate and short axes between the triaxial core with beta=0.38 and gamma=50, and the triaxial halo with beta=0.79 and gamma=-23, is found. Future experiments to explore the halo phenomenon and the novel shape decoupling in 42Al are highly demanded.

nucl-th↗
arXiv2022

Stability and interpolation properties for Stokes-like virtual element spaces

We prove stability bounds for Stokes-like virtual element spaces in two and three dimensions. Such bounds are also instrumental in deriving optimal interpolation estimates. Furthermore, we develop some numerical tests in order to investigate the behaviour of the stability constants also from the practical side.

math.NA↗
arXiv2022

Fission dynamics, dissipation and clustering at finite temperature

The saddle-to-scission dynamics of the induced fission process is explored using a microscopic finite-temperature model based on time-dependent nuclear density functional theory (TDDFT), that allows to follow the evolution of local temperature along fission trajectories. Starting from a temperature that corresponds to the experimental excitation energy of the compound system, the model propagates the nucleons along isentropic paths toward scission. For the four illustrative cases of induced fission of $^{240}$Pu, $^{234}$U, $^{244}$Cm, and $^{250}$Cf, characteristic fission trajectories are considered, and the partition of the total energy into various kinetic and potential energy contributions at scission is analyzed, with special emphasis on the energy dissipated along the fission path and the prescission kinetic energy. The model is also applied to the dynamics of neck formation and rupture, characterized by the formation of few-nucleon clusters in the low-density region between the nascent fragments.

nucl-th↗
arXiv2022

Beyond-mean-field approaches for nuclear neutrinoless double beta decay in the standard mechanism

Nuclear weak decays provide important probes to fundamental symmetries in nature. A precise description of these processes in atomic nuclei requires comprehensive knowledge on both the strong and weak interactions in the nuclear medium and on the dynamics of quantum many-body systems. In particular, an observation of the hypothetical double beta decay without emission of neutrinos ($0νββ$) would unambiguously demonstrate the Majorana nature of neutrinos and the existence of the lepton-number-violation process. It would also provide unique information on the ordering and absolute scale of neutrino masses. The next-generation tonne-scale experiments with sensitivity up to $10^{28}$ years after a few years of running will probably provide a definite answer to these fundamental questions based on our current knowledge on the nuclear matrix element (NME), the precise determination of which is a challenge to nuclear theory. Beyond-mean-field approaches have been frequently adapted for the study of nuclear structure and decay throughout the nuclear chart for several decades. In this review, we summarize the status of beyond-mean-field calculations of the NMEs of $0νββ$ decay assuming the standard mechanism of an exchange of light Majorana neutrinos. The challenges and prospects in the extension and application of beyond-mean-field approaches for $0νββ$ decay are discussed.

nucl-th↗
arXiv2022

Dynamical synthesis of 4He in the scission phase of nuclear fission

In the exothermic process of fission decay, an atomic nucleus splits into two or more independent fragments. Several aspects of nuclear fission are not properly understood, in particular the formation of the neck between the nascent fragments, and the subsequent mechanism of scission into two or more independent fragments. Using an implementation of time-dependent density functional theory, based on a relativistic energy density functional and including pairing correlations, we analyze the final phase of the process of induced fission of $^{240}$Pu, and show that the time-scale of neck formation coincides with the assembly of two $α$-like clusters (less than 1 zs = 10$^{-21}$ s). Because of its much larger binding energy, the dynamical synthesis of 4He in the neck predominates over other light clusters, e.g., $^3$H and $^6$He. At the instant of scission the neck ruptures exactly between the two $α$-like clusters, which separate because of the Coulomb repulsion and are eventually absorbed by the two emerging fragments. The newly proposed mechanism of light charged clusters formation at scission provides a natural explanation of ternary fission.

nucl-th↗
arXiv2022

Microscopic analysis of induced nuclear fission dynamics

The dynamics of low-energy induced fission is explored using a consistent microscopic framework that combines the time-dependent generator coordinate method (TDGCM) and time-dependent nuclear density functional theory (TDDFT). While the former presents a fully quantum mechanical approach that describes the entire fission process as an adiabatic evolution of collective degrees of freedom, the latter models the dissipative dynamics of the final stage of fission by propagating the nucleons independently toward scission and beyond. By combining the two methods, based on the same nuclear energy density functional and pairing interaction, we perform an illustrative calculation of the charge distribution of yields and total kinetic energy for induced fission of $^{240}$Pu. For the saddle-to-scission phase a set of initial points for the TDDFT evolution is selected along an iso-energy curve beyond the outer fission barrier on the deformation energy surface, and the TDGCM is used to calculate the probability that the collective wave function reaches these points at different times. Fission observables are computed with both methods and compared with available data. The relative merits of including quantum fluctuations (TDGCM) and the one-body dissipation mechanism (TDDFT) are discussed.

nucl-th↗
arXiv2022

Configuration interaction projected density functional theory: effects of four-quasiparticle configurations and time-odd interactions

The effects of four-quasiparticle configurations and time-odd interactions are investigated in the framework of configuration interaction projected density functional theory by taking the yrast states of 60Fe as examples. Based on the universal PC-PK1 density functional, the energies of the yrast states with spin up to 20\hbar and the available B(E2) transition probabilities are well reproduced. The yrast states are predicted to be of four-quasiparticle structure above spin I = 16\hbar. The inclusion of the time-odd interactions increases the kinetic moments of inertia and delays the appearance of the first band crossing, and, thus, improves the description of the data.

nucl-th↗