Toroidal nature of the low-energyE1mode

Abstract

The nature of E1 low-energy strength (LES), often denoted as a "pygmy dipole resonance", is analyzed within the random-phase approximation (RPA) in 208 Pb using Skyrme forces in a fully self-consistent manner. A first overview is given by the strength functions for the dipole, compressional, and toroidal operators. More detailed insight is gained by averaged transition densities and currents where the latter provide a very illustrative flow pattern. The analysis reveals clear isoscalar toroidal flow in the low-energy bin 6.0-8.8 MeV of the LES and a mixed isoscalar/isovector toroidal/compression flow in the higher bin 8.8-10.5 MeV. Thus the modes covered by LES embrace both vortical and irrotational motion. The simple collective picture of the LES as a "pygmy" mode (oscillations of the neutron excess against the nuclear core) is not confirmed.

Decay of quadrupole-octupole1−states inCa40andCe140Derya1, Tsoneva2, Aumann3 et al. 2016Phys. Rev. C12120View full textAdd to dashboardCite

“…Furthermore, experiments with complementary probes like α-particles at intermediate energy, indicate an isospin splitting of the low-lying 1 − states [21]. Similar to the experimental findings, theoretical models show that at lower energies the E1 strength is predominantly of isoscalar character which gradually becomes more isovector with increasing excitation energy toward the IVGDR [18,19,[22][23][24][25][26].…”

“…These include the low-energy tail of the IVGDR, PDR [18,19,27,28], multi-phonon excitations [19,29], toroidal modes [23] and α-cluster vibrations [30,31]. Furthermore, in recent studies it has been pointed out that the interaction between quasiparticles and phonons is important for a correct theoretical description of the low-lying E1 strength because it can influence its fragmentation and mixing with the core polarization and the IVGDR [18,19,29,[32][33][34][35].…”

Cluster and toroidal aspects of isoscalar dipole excitations in C12

“…Since the ISM and ISD operators corresponding to compressive modes can directly excite inter-cluster motions, they are good probes for cluster states as discussed by Yamada et al [1] and Chiba et al [2]. Indeed, in such nuclei as 16 O and 24 Mg, the LE-ISM and ISD strengths are described by cluster states, which appear separating from the collective modes of the GRs.…”

“…The energy of the TD mode is expected to be lower than the ISGDR energy because it conserves the nuclear density. In these years, microscopic calculations with quasiparticle phonon model (QPM) and random phase approximation (RPA) have been achieved, and toroidal natures of the LED excitations in various nuclei have been investigated [3,[12][13][14][15][16][17].…”

The Nuclear Symmetry Energy and Other Isovector Observables from the Point of View of Nuclear Structure

“…A toroidal component has been found in the calculations of Refs. [20,21]. Despite these well-taken warnings, the values of the slope parameter and neutron skin (for 208 Pb) that have been extracted in [15], namely L = 64.8 ± 15.7 MeV;…”