Microscopic description of Gamow-Teller transitions in middle pf--shell nuclei by a realistic shell model calculation

GT transitions in $N=28\sim 30$ nuclei are studied in terms of a large-scale realistic shell-model calculation, by using Towner's microscopic parameters. $B({\rm GT})$ values to low-lying final states are reproduced with a reasonable accuracy. Several gross properties with respect to the GT transitions are investigated with this set of the wavefunctions and the operator. While the calculated total GT$^-$ strengths show no apparent disagreement with the measured ones, the calculated total GT$^+$ strengths are somewhat larger than those obtained from charge-exchange experiments. Concerning the Ikeda sum-rule, the proportionality of $S_{\rm GT}$ to $(N-Z)$ persists to an excellent approximation, with a quenching factor of 0.68. For the relative GT$^-$ strengths among possible isospin components, the lowest isospin component gathers greater fraction than expected by the squared CG coefficients of the isospin coupling. It turns out that these relative strengths are insensitive to the size of model space. Systematics of the summed $B({\rm GT})$ values are discussed for each isospin component.