Systematics of the total strength and the contribution of orbital current for the M1 excitations in the even-even sd-shell nuclei

It is shown that the total M1 excitation strengths of the even-even sd-shell nuclei calculated in the full (sd)sup(n) basis are about 50% of the jj-coupling single-particle limit. Compared with the calculated values, the observed M1 strengths are reduced by typically 20%. It is found that the orbital current contribution is almost independent of the mass number, in contrast to the spin current. A large orbital-to-spin ratio for 20Ne is explained from the viewpoint of supermultiplet SU(4) symmetry. The total strength of Gamow-Teller excitation is also calculated. (author)