Proton-neutron pairing vibrations in N = Z nuclei: Precursory soft mode of isoscalar pairing condensation

L = 0 proton-neutron (pn) pair-addition and pair-removal strengths in Ca-40 and Ni-56 are investigated by means of the pn particle-particle random-phase approximation employing a Skyrme energy-density functional. It is found that the collectivity of the lowest J(pi) = 1(+) state in the adjacent odd-odd nuclei becomes stronger as the strength of the isoscalar (T = 0) pairing interaction increases. The results suggest the emergence of the T = 0 pn-pairing vibrational mode as a possible critical phenomenon toward the T = 0 pairing condensation.