Microscopic analysis of low-energy spin and orbital magnetic dipole excitations in deformed nuclei

Abstract

A low-energy magnetic dipole $(M1)$ spin-scissors resonance (SSR) located just below the ordinary orbital scissors resonance (OSR) was recently predicted in deformed nuclei within the Wigner Function Moments (WFM) approach. We analyze this prediction using fully self-consistent Skyrme Quasiparticle Random Phase Approximation (QRPA) method. Skyrme forces SkM*, SVbas and SG2 are implemented to explore SSR and OSR in ${}^{160,162,164}$Dy and ${}^{232}$Th. Accuracy of the method is justified by a good description of M1 spin-flip giant resonance. The calculations show that isotopes ${}^{160,162,164}$Dy indeed have at 1.5-2.4 MeV (below OSR) $I^{\pi }K=1^{+}1$ states with a large $M1$ spin strength ($K$ is the projection of the total nuclear moment to the symmetry z-axis). These states are almost fully exhausted by $pp[411\uparrow ,411\downarrow ]$ and $nn[521\uparrow ,521\downarrow ]$ spin-flip configurations corresponding to $pp[2d_{3/2},2d_{5/2}]$ and $nn[2f_{5/2},2f_{7/2}]$ structures in the spherical limit. So the predicted SSR is actually reduced to low-orbital (l=2,3) spin-flip states. Following our analysis and in contradiction with WFM spin-scissors picture, deformation is not the principle origin of the low-energy spin $M1$ states but only a factor affecting their features. The spin and orbital strengths are generally mixed and exhibit the interference: weak destructive in SSR range and strong constructive in OSR range. In ${}^{232}$Th, the $M1$ spin strength is found very small. Two groups of $I^{\pi }=1^{+}$ states observed experimentally at 2.4-4 MeV in ${}^{160,162,164}$Dy and at 2-4 MeV in ${}^{232}$Th are mainly explained by fragmentation of the orbital strength. Distributions of nuclear currents in QRPA states partly correspond to the isovector orbital-scissors flow but not to spin-scissors one.

15 pages, 13 figures

References in corpus (5)

• The Skyrme Interaction in finite nuclei and nuclear matter
• Skyrme-Rpa Description of Dipole Giant Resonance in Heavy and Superheavy Nuclei
• Skyrme-Rpa Description of Spin-Flip M1 Giant Resonance
• Pairing and deformation effects in nuclear excitation spectra
• Low-energy M1 excitations in ${}^{208}$Pb and the spin channel of the Skyrme energy-density functional

Cited by in corpus (6)

• Solution of universal nonrelativistic nuclear DFT equations in the Cartesian deformed harmonic-oscillator basis. (IX) HFODD (v3.06h): a new version of the program
• Non-iterative finite amplitude methods for E1 and M1 giant resonances
• QRPA calculations for M1 transitions with the noniterative finite amplitude method and the application to neutron radiative capture cross sections
• Microscopic analysis of dipole electric and magnetic strengths in ${}^{156}$Gd
• Low-energy M1 states in deformed nuclei: spin-scissors or spin-flip?
• Isovector giant monopole and quadrupole resonances in a Skyrme energy density functional approach with axial symmetry