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.Comment: 6 pages, 4 figure

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.Comment: 7 pages (latex), 1 figure (ps file), an invited talk at the workshop "Symmetries and Spin - Praha 98", to be published in Czech.J.Phys., 199

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.Comment: 10 pages, 1 figure, contribution to Proceedings of 7th International Spring Seminar on Nuclear Physics, Maiori, Italy, May 27-31, 200

Individual low-energy E1 toroidal and compression states in light nuclei: deformation effect, spectroscopy and interpretation

The existence of individual low-energy E1 toroidal and compression states (TS and CS) in $^{24}$Mg was predicted recently in the framework of quasiparticle random-phase-approximation (QRPA) model with Skyrme forces. It was shown that the strong axial deformation of $^{24}$Mg is crucial to downshift the toroidal strength to the low-energy region and thus make the TS the lowest E1(K=1) dipole state. In this study, we explain this result by simple mean-field arguments. Comparing TS in two strongly axial nuclei, $^{24}$Mg and $^{20}$Ne, we show that the lowest TS is not not a universal phenomenon but rather a peculiarity of $^{24}$Mg. The spectroscopy of TS and CS is analyzed and some additional interpretation of these states is suggested.Comment: 6 pages, 8 figures, 2 tables. Submitted to proceedings of International Conference on Nuclear Structure and Related Topics - NSRT18 (Bulgaria, Bourgas, June 3-9, 2018). The paper was resubmitted to enlarge the reference lis

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.Comment: 22 pages, 13 figures, 2 table

Systematics of toroidal dipole modes in Ca, Ni, Zr, and Sn isotopes

We analyze the relation between isoscalar toroidal modes and so-called pygmy dipole resonance (PDR) which both appear in the same region of low-energy dipole excitations. To this end, we use a theoretical description within the fully self-consistent Skyrme quasiparticle random-phase approximation (QRPA). Test cases are spherical nuclei $^{40,48}$Ca, $^{58,72}$Ni, $^{90,100}$Zr, and $^{100,120,132}$Sn which cover four different elements and for each element at least two isotopes with different neutron excess, one small and another large. The structure of the modes is investigated in terms of strength functions, transition densities (TD) and transition currents (TC). For all considered nuclei, we see that, independently on whether PDR strength exists or not, the flow pattern in the lower part of the "PDR energy region" is basically the isoscalar vortical toroidal motion with a minor irrotational fraction. A one-to-one correspondence between calculated TD and TC can be established. The toroidal flow appears already in the uncoupled two-quasiparticle (2qp) excitations and becomes definitively strong for the QRPA modes. Altogether, we find that low-lying dipole strength often denoted as isoscalar PDR is actually an oversimplified imitation of the basically toroidal motion in nuclei with a sufficient neutron excess.Comment: 12 pages, 14 figure

Pairing and deformation effects in nuclear excitation spectra

We investigate effects of pairing and of quadrupole deformation on two sorts of nuclear excitations,$\gamma$-vibrational $K^{\pi}=2^+$ states and dipole resonances (isovector dipole, pygmy, compression, toroidal). The analysis is performed within the quasiparticle random-phase approximation (QRPA) based on the Skyrme energy functional using the Skyrme parametrization SLy6. Particular attention is paid to i) the role of the particle-particle (pp) channel in the residual interaction of QRPA, ii) comparison of volume pairing (VP) and surface pairing (SP), iii) peculiarities of deformation splitting in the various resonances. We find that the impact of the pp-channel on the considered excitations is negligible. This conclusion applies also to any other excitation except for the $K^{\pi}=0^+$ states. Furthermore, the difference between VP and SP is found small (with exception of peak height in the toroidal mode). In the low-energy isovector dipole (pygmy) and isoscalar toroidal modes, the branch $K^{\pi}=1^-$ is shown to dominate over $K^{\pi}=0^-$ one in the range of excitation energy $E <$ 8--10 MeV. The effect becomes impressive for the toroidal resonance whose low-energy part is concentrated in a high peak of almost pure $K^{\pi}=1^-$ nature. This peculiarity may be used as a fingerprint of the toroidal mode in future experiments. The interplay between pygmy, toroidal and compression resonances is discussed, the interpretation of the observed isoscalar giant dipole resonance is partly revised.Comment: 12 pages, 12 figure