Effects of ground-state correlations on magnetic dipole excitations in $^{40}$Ca
The effects of ground-state correlations on the magnetic dipole excitations in $^{40}$Ca are studied using an extended random phase approximation (ERPA) derived from the time-dependent density-matrix theory. Comparison is made with other extended RPA approaches, the renormalized RPA, the self-consistent RPA and the extended second RPA which also include the effects of ground-state correlations. It is pointed out that direct excitations from two particle - two hole space which are properly treated in ERPA cause strong magnetic dipole transitions in $^{40}$Ca.