Spin- and deformation-dependent orbital M1 strength in rare-earth nuclei

Abstract

The M1 excitations in three isotope chains, 142,146,148,150Nd, 144,148,150,152,154Sm, and 156,158Gd, are studied within the quasi-particle random-phase approximation using a mean field given by a deformed Woods-Saxon potential and including quadrupole-quadrupole and spin-spin residual interactions. A residual rotation-vibration coupling ensures the exclusion of the spurious state. The behaviour of the M1 strength distribution is studied systematically in the whole range of experimentally explored excitation energies and compared with the large variety of experimental information, which includes both low-lying orbital and higher-lying spin-flip 1+ excitations. We have found a quadratic dependence of the M1 strength summed up to 4 MeV on the nuclear deformation reproducing at the same time the main features of the low-lying M1 spectra. The double-peaked structure of the spin M1 strength distribution found experimentally in several nuclei is qualitatively reproduced and interpreted as isoscalar and isovector peaks.