Microscopic Analysis Of Low-Energy Spin And Orbital Magnetic Dipole Excitations In Deformed Nuclei

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 Dy-160,Dy-162,Dy-164 and Th-232. Accuracy of the method is justified by a good description of M1 spin-flip giant resonance. The calculations show that isotopes Dy-160,Dy-162,Dy-164 indeed have at 1.5-2.4 MeV (below OSR) (IK)-K-pi = 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 up arrow, 411 down arrow] and nn[521 up arrow, 521.] spin-flip configurations corresponding to pp[2d(3/2), 2d(5/2)] and nn[2 f(5/2), 2 f(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 interference: weakly destructive in SSR range and strongly constructive in OSR range. In Th-232, the M1 spin strength is very small. Two groups of I-pi = 1(+) states observed experimentally at 2.4-4 MeV in Dy-160,Dy-162,Dy-164 and at 2-4 MeV in Th-232 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 the spin-scissors one.