arXiv subjects

J. Kvasil

Publications and source records attributed to J. Kvasil.

53 records · Page 3Linked to original sources

arXiv2008

Skyrme-Random-Phase-Approximation description of E1 strength in 92-100Mo

The isovector dipole E1 strength in Mo isotopes with A=92,94,96,98,100 is analyzed within the self-consistent separable random-phase approximation (SRPA) model with Skyrme forces SkT6, SkM*, SLy6, and SkI3. The special attention is paid to the low-energy region near the particle thresholds (4-12 MeV), which is important for understanding of astrophysical processes. We show that, due to a compensation effect, the influence of nuclear deformation on E1 strength below 10-12 MeV is quite modest. At the same time, in agreement with previous predictions, the deformation increases the strength at higher energy. At 4-8 MeV the strength is mainly determined by the tail of E1 giant resonance. The four Skyrme forces differ in description of the whole giant resonance but give rather similar results below 12 MeV.

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arXiv2008

Skyrme-Rpa Description of Dipole Giant Resonance in Heavy and Superheavy Nuclei

The E1(T=1) isovector dipole giant resonance (GDR) in heavy and super-heavy deformed nuclei is analyzed over a sample of 18 rare-earth nuclei, 4 actinides and three chains of super-heavy elements (Z=102, 114 and 120). Basis of the description is self-consistent separable RPA (SRPA) using the Skyrme force SLy6. The self-consistent model well reproduces the experimental data (energies and widths) in the rare-earth and actinide region. The trend of the resonance peak energies follows the estimates from collective models, showing a bias to the volume mode for the rare-earths isotopes and a mix of volume and surface modes for actinides and super-heavy elements. The widths of the GDR are mainly determined by the Landau fragmentation which in turn is found to be strongly influenced by deformation. A deformation splitting of the GDR can contribute about one third to the width and about 1 MeV further broadening can be associated to mechanism beyond the mean-field description (escape, coupling with complex configurations).

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arXiv2007

Reflection symmetry instability at high spins in 162,164Yb

A shape evolution of 162,164Yb in yrast states is traced using the self-consistent Skyrme Hartree-Fock calculations. We found that nonaxial octupole deformations (in particular, Y_{31} term) become favorable at large rotational frequencies (> 0.4 MeV) in 162Yb, while in 164Yb a nonaxial quadrupole shape is dominant at fast rotation. The cranked Nilsson model and random phase approximation are used to understand the dynamics of octupole correlations in both nuclei. We demonstrate that the disappearance of one of the octupole vibrational modes in the rotating frame gives rise to the nonaxial octupole deformations in 162Yb, while the octupole modes are nonzero in 164Yb.

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arXiv2007

TDDFT with Skyrme Forces: Effect of Time-Odd Densities on Electric Giant Resonances

Time-odd densities and their effect on electric giant resonances are investigated within the self-consistent separable random-phase-approximation (SRPA) model for various Skyrme forces (SkT6, SkO, SkM*, SIII, SGII, SLy4, SLy6, SkI3). Time-odd densities restore Galilean invariance of the Skyrme functional, violated by the effective-mass and spin-orbital terms. In even-even nuclei these densities do not contribute to the ground state but can affect the dynamics. As a particular case, we explore the role of the current density in description of isovector E1 and isoscalar E2 giant resonances in a chain of Nd spherical and deformed isotopes with A=134-158. Relation of the current to the effective masses and relevant parameters of the Skyrme functional is analyzed. It is shown that current contribution to E1 and E2 resonances is generally essential and fully determined by the values and signs of the isovector and isoscalar effective-mass parameters of the force. The contribution is the same for all the isotope chain, i.e. for both standard and exotic nuclei.

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arXiv2007

Wobbling excitations at high spins in A~160

We found that in 156Dy and 162Yb the lowest odd spin gamma-vibrational states transform to the wobbling excitations after the backbending, associated with the transition from axially-symmetric to nonaxial shapes. The analysis of quadrupole electric transitions determines uniquely the sign of the gamma-deformation in both nuclei after the transition point.

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arXiv2006

Giant dipole resonance in deformed nuclei: dependence on Skyrme forces

The giant dipole resonance (GDR) in deformed nuclei is analyzed using the self-consistent separable random-phase-approximation (SRPA) with Skyrme forces SkT6, SkM$^*$, SLy6 and SkI3. The deformed nuclei $^{150}$Nd and $^{238}$U are used as representative rare-earth and actinide samples. Dependencies of the dipole strength distributions on some basic characteristics of the Skyrme functional and nuclear matter properties (isoscalar and isovector effective masses, time-odd contributions) are discussed. Particular attention is paid to the fragmentation structure of the GDR strength which are shown to depend sensitively to spin-orbit intruder states with large angular momentum.

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arXiv2006

Self-Consistent Separable Rpa for Skyrme Forces: Giant Resonances in Axial Nuclei

We formulate the self-consistent separable random-phase-approximation (SRPA) method and specify it for Skyrme forces with pairing for the case of axially symmetric deformed nuclei. The factorization of the residual interaction allows to avoid diagonalization of high-rank RPA matrices, which dramatically reduces the computational expense. This advantage is crucial for the systems with a huge configuration space, first of all for deformed nuclei. SRPA takes self-consistently into account the contributions of both time-even and time-odd Skyrme terms as well as of the Coulomb force and pairing. The method is implemented to description of isovector E1 and isoscalar E2 giant resonances in a representative set of deformed nuclei: $^{154}$Sm, $^{238}$U, and $^{254}$No. Four different Skyrme parameterizations (SkT6, SkM*, SLy6, and SkI3) are employed to explore dependence of the strength distributions on some basic characteristics of the Skyrme functional and nuclear matter. In particular, we discuss the role of isoscalar and isovector effective masses and their relation to time-odd contributions. High sensitivity of the right flank of E1 resonance to different Skyrme forces and the related artificial structure effects are analyzed.

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arXiv2005

Self-Consistent Separable Rpa Approach for Skyrme Forces: Axial Nuclei

The self-consistent separable RPA (random phase approximation) method is formulated for Skyrme forces with pairing. The method is based on a general self-consistent procedure for factorization of the two-body interaction. It is relevant for various density- and current-dependent functionals. The contributions of the time-even and time-odd Skyrme terms as well as of the Coulomb and pairing terms to the residual interaction are taken self-consistently into account. Most of the expression have a transparent analytical form, which makes the method convenient for the treatment and analysis. The separable character of the residual interaction allows to avoid diagonalization of high-rank RPA matrices and thus to minimize the calculation effort. The previous studies have demonstrated high numerical accuracy and efficiency of the method for spherical nuclei. In this contribution, the method is specified for axial nuclei. We provide systematic and detailed presentation of formalism and discuss different aspects of the model.

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arXiv2005

Microscopic analysis of shape-phase transitions in even-even N~90 rotating nuclei

We study in cranked Nilsson plus random phase approximation shape transitions in fast rotating nuclei undergoing backbending, more specifically 156Dy and 162Yb. We found that a backbending in 156Dy is correlated with the disappearance of the collective, positive signature gamma-vibrational mode in the rotating frame, and, a shape transition (from axial to nonaxial) is accompanied with a large acquiring of the gamma-deformation. We show that such a shape transition can be considered as a phase transition of the first order. In 162Yb the quasiparticle alignment dominates in the backbending and the shape transition (from axial to nonaxial) is accompanied with a smooth transition from zero to nonzero values of the gamma-deformation. We extend the classical Landau theory for rotating nuclei and show that the backbending in 162Yb is identified with the second order phase transition. A description of spectral and decay properties of the yrast states and low-lying excitations demonstrates a good agreement between our results and experimental data.

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arXiv2005

Practicable factorized TDLDA for arbitrary density- and current-dependent functionals

We propose a practicable method for describing linear dynamics of different finite Fermi systems. The method is based on a general self-consistent procedure for factorization of the two-body residual interaction. It is relevant for diverse density- and current-dependent functionals and, in fact, represents the self-consistent separable random-phase approximation (RPA), hence the name SRPA. SRPA allows to avoid diagonalization of high-rank RPA matrices and thus dwarfs the calculation expense. Besides, SRPA expressions have a transparent analytical form and so the method is very convenient for the analysis and treatment of the obtained results. SRPA demonstrates high numerical accuracy. It is very general and can be applied to diverse systems. Two very different cases, the Kohn-Sham functional for atomic clusters and Skyrme functional for atomic nuclei, are considered in detail as particular examples. SRPA treats both time-even and time-odd dynamical variables and, in this connection, we discuss the origin and properties of time-odd currents and densities in initial functionals. Finally, SRPA is compared with other self-consistent approaches for the excited states, including the coupled-cluster method.

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arXiv2004

Backbending and $Gamma$-Vibrations

We propose that the backbending phenomenon can be explained as a result of the disappearance of collective $gamma$-vibrational mode in the rotating frame. Using a cranking+random phase approximation approach for the modified Nilsson potential + monopole pairing forces, we show that this mechanism is responsible for the backbending in $^{156}$Dy, $^{158}$Er and obtain a good agreement between theoretical and experimental results.

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arXiv2004

Microscopic study of collective excitations in rotating nuclei

We have carried out a unified microscopic study of electric monopole, quadrupole and magnetic dipole excitations in fast rotating nuclei undergoing backbending, with special attention at the magnetic excitations. We found, among other results, that the strength of the orbital magnetic dipole excitations (scissors mode) gets enhanced by more than a factor four at high rotational frequency, above the backbending region. We provide a physical explanation for such an enhancement.

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arXiv2002

Separable RPA for self-consistent nuclear models

Self-consistent factorization of two-body residual interaction is proposed for arbitrary density- and current-dependent energy functionals. Following this procedure, a separable RPA (SRPA) method is constructed. SRPA considerably simplifies the calculations and demonstrates quick convergence to exact results. The method is tested for SkI3 and SkM* forces.

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arXiv2001

Self-Consistent Separable RPA For Density- and Current-Dependent Forces

Self-consistent factorization of two-body residual interaction is proposed for arbitrary density- and current-dependent energy functionals. Following this procedure, a separable RPA (SRPA) method is constructed. SRPA dramatically simplifies the calculations and demonstrates quick convergence to exact results. The method is tested for SkM* forces.

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arXiv2001

Signature inversion in axially deformed $^{160,162}$Tm

The microscopic analysis of experimental data in $^{160,162}$Tm is presented within the two-quasiparticle-phonon model. The model includes the interaction between odd quasiparticles and their coupling with core vibrations. The coupling explains naturally the attenuation of the Coriolis interaction in rotating odd-odd nuclei. It is shown that the competition between the Coriolis and neutron-proton interactions is responsible for the signature inversion phenomenon.

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arXiv2000

Octupole deformations in actinides at high spins within the cranking Skyrme-Hartree-Fock approach

The cranked Skyrme III effective Hamiltonian is applied for the analysis of the rotational dependence of the quadrupole and octupole moments in Ra, Th, and U isotopes. A comparison of the intrinsic electric dipole moments calculated in the model with available experimental and theoretical values is presented. It is found that the non-axial octupole deformation $Y_{32}$ becomes favorable at high spins for the actinide nuclei.

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arXiv1999

Electromagnetic Modes in Deformed Nuclei

A strength function method is adopted to describe a coupling between electric and magnetic modes of different multipolarity. The collective vibrations are analysed for a separable residual interaction in the framework of the random-phase approximation. The coupling between $M2$ and $E1$ giant resonances is considered as an illustrative example.

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