Quasiparticle-vibration Coupling Research Articles

Toward a Unified Description of Isoscalar Giant Monopole Resonances in a Self-Consistent Quasiparticle-Vibration Coupling Approach.

The nuclear incompressibility is a key parameter of the nuclear equation of state that can be extracted from the measurements of the so-called "breathing mode" of finite nuclei. The most serious discrepancy so far is between values extracted from Pb and Sn, that has provoked the longstanding question "Why is tin so soft?". To solve this puzzle, a fully self-consistent quasiparticle random-phase approximation plus quasiparticle-vibration coupling approach based on Skyrme-Hartree-Fock-Bogoliubov is developed. We show that the many-body correlations introduced by quasiparticle-vibration coupling, which shift the isoscalar giant monopole resonance energy in Sn isotopes by about 0.4MeV more than the energy in ^{208}Pb, play a crucial role in providing a unified description of the isoscalar giant monopole resonance in Sn and Pb isotopes. The best description of the experimental strength functions is given by SV-K226 and KDE0, which are characterized by incompressibility values K_{∞}=226 MeV and 229MeV, respectively, at mean field level.

Relativistic approach to the nuclear breathing mode

Microscopic theory of the nuclear response based on the relativistic meson-nucleon Lagrangian is applied to the description of the isoscalar giant monopole resonance (ISGMR) in a variety of nuclear systems. It is shown that the parameter-free inclusion of beyond-mean-field correlations of the quasiparticle-vibration coupling (qPVC) type in the leading approximation allows for a simultaneous realistic description of the ISGMR in nuclei of lead, tin, zirconium, and nickel mass regions, which is difficult on the mean-field level. The calculations employ the finite-range effective meson-nucleon interaction, which, in combination with the qPVC, has consistently demonstrated the ability to reliably describe many other nuclear structure phenomena. Systematic calculations of the isoscalar monopole response for nickel isotopes help reveal the central role of the coupling between the ISGMR and the low-energy quadrupole states in the placement of the ISGMR centroids.

Microscopic response theory for strongly coupled superfluid fermionic systems

A consistent microscopic theory for the response of strongly-coupled superfluid fermionic systems is formulated. After defining the response as a two-point two-fermion correlation function in the basis of the Bogolyubov's quasiparticles, the equation of motion (EOM) method is applied using the most general fermionic Hamiltonian with a bare two-body interaction, also transformed to the quasiparticle space. As a superfluid extension of the case of the normal phase, the resulting EOM is of the Bethe-Salpeter-Dyson form with the static and dynamical interaction kernels, where the former determines the short-range correlations and the latter is responsible for the long-range ones. Both kernels as well as the entire EOM have the double dimension as compared to that of the normal phase. Non-perturbative approximations via the cluster decomposition of the dynamical kernel are discussed, with the major focus on a continuous derivation of the quasiparticle-phonon coupling variant of the latter kernel, where the phonons (vibrations) are composite correlated two-quasiparticle states unifying both the normal and pairing modes. The developed theory is adopted for nuclear structure applications, such as the nuclear response in various channels. In particular, the finite-amplitude method generalized beyond the quasiparticle random phase approximation, taking into account the quasiparticle-vibration coupling, is formulated for prospective calculations in non-spherical nuclei.

Beyond-mean-field calculations of allowed and first-forbidden β− decays of r-process waiting-point nuclei

β-decay rates of neutron-rich nuclei, in particular those located at neutron shell closures, play a central role in simulations of the heavy-element nucleosynthesis and resulting abundance distributions. We present β-decay half-lives of even-even N = 82 and N = 126 r-process waiting-point nuclei calculated in the approach based on relativistic quasiparticle random phase approximation with quasiparticle-vibration coupling. The calculations include both allowed and first-forbidden transitions. In the N = 82 chain, the quasiparticlevibration coupling has an important impact close to stability, as it increases the contribution of Gamow-Teller modes and improves the agreement with the available data. In the N = 126 chain, we find the decay to proceed dominantly via first-forbidden transitions, even when the coupling to vibrations is included.

Many-body theory for quasiparticle states in superfluid fermionic systems

We present a formalism for the fermionic quasiparticle propagator in a superfluid fermionic system. Starting from a general many-body hamiltonian confined by the two-body instantaneous interaction, the equation of motion for the fermionic propagator is obtained in the Dyson form. Before making any approximation, the interaction kernel is found to be decomposed into the static and dynamical (time-dependent) contributions, while the latter translates to the energy-dependent and the former maps to the energy-independent terms in the energy domain. The three-fermion correlation function being the heart of the dynamical part of the kernel is factorized into the two-fermion and one-fermion ones. With the relaxed particle number constraint, the normal propagator is coupled to the abnormal one via both the static and dynamical kernels, that is formalized by introducing the generalized quasiparticle propagator of the Gor'kov type. The dynamical kernel in the factorized form is associated with the quasiparticle-vibration coupling (QVC) with the vibrations unifying both the normal and pairing phonons. The QVC vertices are related to the variations of the Hamiltonian of the Bogoliubov quasiparticles, which can be obtained by the finite amplitude method.

Gamow\u2013Teller strength distributions of {^{116} text {Sb}} and {^{122} text {Sb}} using the {(^{3} text {He}, t)} charge-exchange reaction

The Gamow–Teller strength distributions of {^{116} text {Sb}} and {^{122} text {Sb}} were measured with the {^{116,122} text {Sn} (^{3} text {He},t) ^{116,122} text {Sb}} charge-exchange reactions at {140 text {MeV/u}}. The measurements were carried out at the Research Center for Nuclear Physics (RCNP) at Osaka University in Osaka, Japan using the Grand Raiden spectrometer. The data were analysed by Multipole-Decomposition Analysis (MDA). The Gamow–Teller strengths summed up to {28 text {MeV}} are {(38 pm 7) %} and {(48 pm 6) %} of the Ikeda sum rule for {^{116} text {Sb}} and {^{122} text {Sb}}, respectively, if the quasi-free scattering (QFS) contribution is not subtracted. These percentages are {(29 pm 7) %} and {(35 pm 5) %}, respectively, if the QFS contribution is maximally subtracted. These results were compared to those from previous measurements of the same isotopes, to recent measurements of {^{150} text {Pm}}, and to a Quasi-particle Random-Phase Approximation (QRPA) calculation with Quasi-Particle Vibration Coupling (QPVC). The data suggest that the true QFS contribution is small for {^{116} text {Sb}}, but are inconclusive about whether the QFS contribution is small or significant for {^{122} text {Sb}}. Therefore, these data may provide an interesting test for the general quenching phenomenon of the Gamow–Teller Resonance (GTR). However, more research to reveal the nature of the QFS contribution is still needed on both the experimental and the theoretical side.

Interplay of quasiparticle-vibration coupling and pairing correlations on β-decay half-lives

The nuclear β-decay half-lives of Ni and Sn isotopes, around the closed shell nuclei 78Ni and 132Sn, are investigated by computing the distribution of the Gamow–Teller strength using the Quasiparticle Random Phase Approximation (QRPA) with quasiparticle-vibration coupling (QPVC), based on ground-state properties obtained by Hartree–Fock–Bogoliubov (HFB) calculations. We employ the effective interaction SkM⁎ and a zero-range effective pairing force. The half-lives are strongly reduced by including the QPVC. We study in detail the effects of isovector (IV) and isoscalar (IS) pairing. Increasing the IV strength tends to increase the lifetime for nuclei in the proximity of, but lighter than, the closed-shell ones in QRPA calculations, while the effect is significantly reduced by taking into account the QPVC. On the contrary, the IS pairing mainly plays a role for nuclei after the shell closure. Increasing its strength decreases the half-lives, and the effect at QRPA and QRPA+QPVC level is comparable. The effect of IS pairing is particularly pronounced in the case of the Sn isotopes, where it turns out to be instrumental to obtain good agreement with experimental data.

Quasiparticle random-phase approximation with quasiparticle-vibration coupling: Application to the Gamow-Teller response of the superfluid nucleusSn120

We propose a self-consistent quasi-particle random phase approximation (QRPA) plus quasi-particle-vibration coupling (QPVC) model with Skyrme interactions to describe the width and the line shape of giant resonances in open-shell nuclei, in which the effect of superfluidity should be taken into account in both the ground state and the excited states. We apply the new model to the Gamow-Teller resonance in the superfluid nucleus $^{120}$Sn, including both the isoscalar spin-triplet and the isovector spin-singlet pairing interactions. The strength distribution in $^{120}$Sn is well reproduced and the underlying microscopic mechanisms, related to QPVC and also to isoscalar pairing, are analyzed in detail.

Impact of collective vibrations on quasiparticle states of open-shell odd-mass nuclei and possible interference with the tensor force

The energy splittings between the proton $1h_{11/2}$ and $1g_{7/2}$ states in the Sb (Z=51) isotopes and the neutron $1i_{13/2}$ and $1h_{9/2}$ states in the ${N=83}$ isotones are analyzed within relativistic quasiparticle-vibration coupling (QVC) model. Although a common viewpoint is that an effective tensor interaction is needed to reproduce these splittings, we show, for the first time, that quasiparticle-vibration coupling also improves their description considerably. The remaining differences between the experiment and the QVC calculations point to a complex scenario in which both tensor interaction and quasiparticle-vibration coupling are needed to reproduce accurately the observed energy splittings. This scenario, however, requires a considerably weaker tensor interaction as compared with the previous estimates.

Quasiparticle-vibration coupling in a relativistic framework: Shell structure ofZ=120isotopes

For the first time, the shell structure of open-shell nuclei is described in a fully self-consistent extension of the covariant energy density functional theory. The approach implies quasiparticle-vibration coupling for superfluid systems. A one-body Dyson equation formulated in the doubled quasiparticle space of Dirac spinors is solved for nucleonic propagators in tin isotopes which represent the reference case: The obtained energies of the single-quasiparticle levels and their spectroscopic amplitudes are in agreement with data. The model is applied to describe the shell evolution in a chain of superheavy isotopes ${}^{292,296,300,304}$120 and finds a rather stable proton spherical shell closure at $Z=120$. An interplay of the pairing correlations and the quasiparticle-phonon coupling gives rise to a smooth evolution of the neutron shell gap between $N=172$ and $N=184$ neutron numbers. Vibrational corrections to the alpha-decay energies reach several hundred keV and can be either positive or negative, thus also smearing out the shell effects.

Signature inversion driven by wobbling motion in negative-gamma nuclei.

Effects of the wobbling motion on the signature splitting in triaxial odd-{ital A} nuclei are studied analytically and numerically by means of the quasiparticle-vibration coupling model that is an extension of the cranking model. The signature inversion in negative-gamma three-quasiparticle bands is shown to be driven by the rotational {ital K} mixing in the wobbling mode.

Negative parity states ofGe71

Energy spectra and spectroscopic factors of negative-parity states in $^{71}\mathrm{Ge}$ are calculated within the framework of the quasiparticle-vibration coupling approach in the unified nuclear model. The calculated results are in very good agreement with the experimental data.NUCLEAR STRUCTURE $^{71}\mathrm{Ge}$; calculated levels, $J$, $\ensuremath{\pi}$, and spectroscopic factors. Compared with experiments.

Nuclear properties of the low-lying states inMo99

Energy levels and spectroscopic factors for the low-lying levels in $^{99}\mathrm{Mo}$ are calculated within the framework of the quasiparticle-vibration coupling approach in the unified nuclear model. The calculated results are in very good agreement with the experimental data. Nuclear moments for the ground and first excited states are also predicted.NUCLEAR STRUCTURE $^{99}\mathrm{Mo}$; calculated levels, $J$, $\ensuremath{\pi}$, and spectroscopic factors; $\ensuremath{\mu}$ and $q$ for the ground and first excited states. Compared with experiments.