arXiv subjects

D. Vretenar

Publications and source records attributed to D. Vretenar.

At least 19 recordsLinked to original sources

arXiv2026

Intrinsic generation of angular momenta and entanglement in fission

Nuclear time-dependent density functional theory is used to investigate spin generation and entanglement of fission fragments in spontaneous fission of $^{252}$Cf, incorporating both axial and non-axial deformations. Axially symmetric fission trajectories enforce strict constraints: counter rotation (twisting mode) along the fission axis and equiprobable bending/wriggling modes perpendicular to it. Non-axial modes broaden the distributions of fission fragment spin projection on the fission axis, and allow for axial (tilting) collective rotations, which are forbidden on axially symmetric trajectories. Mutual information analysis reveals that axial-symmetry breaking reduces spin-spin correlations along the fission axis of symmetric cases, while perpendicular correlations remain more resilient. The effect of triaxial degrees of freedom on the opening angle distribution between the spins of the fission fragments is analyzed.

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arXiv2026

Configuration-interaction time-dependent density functional theory for nuclear dynamics

A configuration-interaction time-dependent density functional theory (CI-TDDFT) for nuclear dynamics is developed. In this framework, the correlated nuclear many-body wave function is expanded in terms of time-dependent many-particle configurations built from a common set of orthonormal single-particle states. The equations of motion for both the expansion coefficients and the single-particle states are derived self-consistently using the Dirac-Frenkel time-dependent variational principle. This formulation extends conventional time-dependent density functional theory (TDDFT) by incorporating configuration mixing and beyond-mean-field correlations, while preserving energy and particle-number conservation. As an illustrative application, the method is implemented using the relativistic point-coupling functional PC-PK1 in the particle-hole channel and a monopole pairing interaction in the particle-particle channel, and is applied to the study of isoscalar giant monopole resonance in $^{58}$Ni and $^{60}$Ni. Numerical tests show that both the total energy and particle number are conserved, with relative deviations within $4\times 10^{-4}$ during the time evolution. Compared with conventional TDDFT, CI-TDDFT yields broader strength distributions for giant monopole resonances while keeping the main peak positions close to those from TDDFT. This broadening is associated with configuration mixing in the valence space and suggests a coupling of the monopole oscillation to additional collective degrees of freedom. These results demonstrate the potential of CI-TDDFT as a quantum, microscopic beyond-mean-field framework for nuclear dynamics.

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arXiv2026

De-excitation effects on entanglement in multi-nucleon transfer reactions

This study quantifies the impact of nuclear de-excitation on correlations in multi-nucleon transfer (MNT) reactions. To bridge the gap between initial collision dynamics and final experimental observables, we introduce a hybrid TDCDFT+GEMINI approach, integrating time-dependent covariant density functional theory (TDCDFT) with the statistical de-excitation model GEMINI++. Applied to the $^{40}$Ca + $^{208}$Pb reaction, our method demonstrates that the de-excitation is essential for reconciling theoretical cross sections with experimental data. Analysis of the cross-section Shannon entropy reveals that new reaction channels open abruptly at a specific energy threshold. By employing mutual information, we show that the de-excitation process significantly degrades the initial quantum entanglement between the projectile-like and the target-like fragments, revealing a key mechanism through which fundamental quantum correlations are lost.

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arXiv2025

Microscopic triaxial quadrupole-octupole collective Hamiltonian for low-energy nuclear excitations

We present a microscopic triaxial quadrupole-octupole collective Hamiltonian (TQOCH) that unifies collective rotations, quadrupole-octupole vibrations, and their couplings to model low-lying nuclear states of both parities. The TQOCH's dynamics are governed by collective parameters derived from multidimensionally constrained covariant density functional theory. The model's validity is demonstrated through calculations of $^{152}$Sm, including its deformation energy surfaces, excitation spectra, and transition probabilities. As a predictive tool, the TQOCH probes complex nuclear phenomena like shape coexistence and phase transitions, directly connecting their microscopic origins to spectroscopic observables.

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arXiv2025

Microscopic model for yields and total kinetic energy in nuclear fission

An extension of time-dependent density functional theory (TDDFT), the generalized time-dependent generator coordinate method (TDGCM), is applied to a study of induced nuclear fission dynamics. In the generalized TDGCM, the correlated nuclear wave function is represented as a coherent superposition of time-dependent DFT trajectories. In the first realistic application, a large basis of 25 TDDFT trajectories is employed to calculate the charge yields and total kinetic energy distribution for the fission of $^{240}$Pu. The results are compared with available data, and with those obtained using a standard TDDFT, that does not consider quantum fluctuations, and the adiabatic TDGCM+GOA (Gaussian overlap approximation). It is shown that fragment yields and kinetic energies can simultaneously be described in a consistent microscopic framework that includes fluctuations in the collective degrees of freedom and the one-body dissipation mechanism.

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arXiv2024

Large quadrupole deformation in $^{20}$Ne challenges rotor model and modern theory: urging for $α$ clusters in nuclei

The spectroscopic quadrupole moment of the first excited state, $Q_{_S}(2^{+}_{1})$, at 1.634 MeV in $^{20}$Ne was determined from sensitive reorientation-effect Coulomb-excitation measurements using a heavy target and safe energies well below the Coulomb barrier. Particle-$γ$ coincidence measurements were collected at iThemba LABS with a digital data-acquisition system using the {\sc AFRODITE} array coupled to an annular, doubled-sided silicon detector. A precise value of $Q_{_S}(2^{+}_{1})=-0.22(2)$ eb was determined at backward angles in agreement with the only safe-energy measurement prior to this work, $Q_{_S}(2^{+}_{1})=-0.23(8)$ eb. This result adopts 1$\hbarω$ shell-model calculations of the nuclear dipole polarizability of the 2$^+_1$ state that contributes to the effective quadrupole interaction and determination of $Q_{_S}(2^{+}_{1})$. It disagrees, however, with the ideal rotor model for axially-symmetric nuclei by almost $3σ$. Larger discrepancies are computed by modern state-of-the-art calculations performed in this and prior work, including {\it ab initio} shell model with chiral effective interactions and the multi-reference relativistic energy density functional ({\sc MR-EDF}) model. The intrinsic nucleon density of the 2$^+_1$ state in $^{20}$Ne calculated with the {\sc MR-EDF} model illustrates the presence of $α$ clustering, which explains the largest discrepancy with the rotor model found in the nuclear chart and motivates the explicit inclusion of $α$ clustering for full convergence of $E2$ collective properties.

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arXiv2024

Entanglement in multinucleon transfer reactions

Nuclear reactions present an interesting case for studies of the time-evolution of entanglement between complex quantum systems. In this work, the time-dependent nuclear density functional theory is employed to explore entanglement in multinucleon transfer reactions. As an illustrative example, for the reaction $^{40}$Ca $+$ $^{208}$Pb at $E_{\rm lab} = 249$ MeV, in the interval of impact parameters $4.65-7.40$ fm, and the relativistic density functional PC-PK1, we compute the von Neumann entropies, entanglement between fragments, nucleon-number fluctuations, and Shannon entropy for the nucleon-number observable. A simple linear correlation is established between the entanglement and nucleon-number fluctuation of the final fragments. The entanglement between the fragments can be related to the corresponding excitation energies and angular momenta. The relationship between the von Neumann entropy and the Shannon entropy for the nucleon-number observable is analyzed, as well as the time-evolution of the entanglement (nucleon-number fluctuation). The entanglement is also calculated for a range of incident energies and it is shown how, depending on the impact parameter, the entanglement increases with the collision energy.

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arXiv2024

Time-dependent density functional theory study of induced-fission dynamics of $^{226}$Th

A microscopic finite-temperature model based on time-dependent nuclear density functional theory (TDDFT), is employed to study the induced-fission process of $^{226}$Th. The saddle-to-scission dynamics of this process is explored, starting from various points on the deformation surface of Helmholtz free energy at a temperature that corresponds to the experimental excitation energy, and following self-consistent isentropic fission trajectories as they evolve toward scission. Dissipation effects and the formation of excited fragments are investigated and, in particular, the difference in the evolution of the local temperature along asymmetric and symmetric fission trajectories. The relative entropies and entanglement between fission fragments emerging at scission are analyzed.

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arXiv2024

Ternary quasifission in collisions of actinide nuclei

The microscopic framework of time-dependent covariant density functional theory is applied to a systematic study of ternary quasifission in collisions of pairs of $^{238}$U nuclei. It is shown that the inclusion of octupole degree of freedom in the case of head-to-head collisions, extends the energy window in which ternary quasifission occurs, and greatly enhances the number of nucleons contained in a middle fragment. Dynamical pairing correlations, included here in the time-dependent BCS approximation, prevent the occurrence of ternary quasifission in head-to-head collisions, and have an effect on the location of the energy window in which a middle fragment is formed in tail-to-tail collisions. In the latter case, as well as for tail-to-side collisions, the formation of very heavy neutron-rich systems in certain energy intervals is predicted.

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arXiv2024

Multinucleon transfer with time-dependent covariant density functional theory

The microscopic framework of time-dependent covariant density functional theory is applied to study multinucleon transfer reactions, with transfer probabilities calculated using the particle number projection method. It is found that similar total cross sections are obtained with two different relativistic density functionals, PC-PK1 and DD-ME2, as well as with the Skyrme functional SLy5 in a previous study, for multinucleon transfer in the reactions: $^{40}{\rm Ca}+{}^{124}{\rm Sn}$ at $E_{\rm lab} = 170$ MeV, $^{40}{\rm Ca}+{}^{208}{\rm Pb}$ at $E_{\rm lab} = 249$ MeV, and $^{58}{\rm Ni}+{}^{208}{\rm Pb}$ at $E_{\rm lab} = 328.4$ MeV. We report the first microscopic calculation of total cross sections for the reactions: $^{40}{\rm Ar}+{}^{208}{\rm Pb}$ at $E_{\rm lab} = 256$ MeV and $^{206}{\rm Pb}+{}^{118}{\rm Sn}$ at $E_{\rm lab} = 1200$ MeV. Compared to the results obtained with the GRAZING model, the cross sections predicted by the time-dependent covariant density functional theory are in much better agreement with data, and demonstrate the potential of microscopic models based on relativistic density functionals for the description of reaction dynamics.

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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.

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arXiv2023

Coupling shape and pairing vibrations in a collective Hamiltonian based on nuclear energy density functionals (II): low-energy excitation spectra of triaxial nuclei

The triaxial quadrupole collective Hamiltonian, based on relativistic energy density functionals, is extended to include a pairing collective coordinate. In addition to triaxial shape vibrations and rotations, the model describes pairing vibrations and the coupling between triaxial shape and pairing degrees of freedom. The parameters of the collective Hamiltonian are determined by a covariant energy density functional, with constraints on the intrinsic triaxial shape and pairing deformations. The effect of coupling between triaxial shape and pairing degrees of freedom is analyzed in a study of low-lying spectra and transition rates of $^{128}$Xe. When compared to results obtained with the standard triaxial quadrupole collective Hamiltonian, the inclusion of dynamical pairing compresses the low-lying spectra and improves interband transitions, in better agreement with data. The effect of zero-point energy (ZPE) correction on low-lying excited spectra is also discussed.

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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.

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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.

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arXiv2023

Microscopic description of $α$, $2α$, and cluster decays of $^{216-220}$Rn and $^{220-224}$Ra

Alpha and cluster decays are analyzed for heavy nuclei located above $^{208}$Pb on the chart of nuclides: $^{216-220}$Rn and $^{220-224}$Ra, that are also candidates for observing the $2 α$ decay mode. A microscopic theoretical approach based on relativistic Energy Density Functionals (EDF), is used to compute axially-symmetric deformation energy surfaces as functions of quadrupole, octupole and hexadecupole collective coordinates. Dynamical least-action paths for specific decay modes are calculated on the corresponding potential energy surfaces. The effective collective inertia is determined using the perturbative cranking approximation, and zero-point and rotational energy corrections are included in the model. The predicted half-lives for $α$-decay are within one order of magnitude of the experimental values. In the case of single $α$ emission, the nuclei considered in the present study exhibit least-action paths that differ significantly up to the scission point. The differences in alpha-decay lifetimes are not only driven by Q values, but also by variances of the least-action paths prior to scission. In contrast, the $2 α$ decay mode presents very similar paths from equilibrium to scission, and the differences in lifetimes are mainly driven by the corresponding Q values. The predicted $^{14}$C cluster decay half-lives are within three orders of magnitudes of the empirical values, and point to a much more complex pattern compared to the alpha-decay mode.

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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.

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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.

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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.

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