  Large-amplitude collective dynamics of shape phase transition in the
low-lying states of 30-36Mg is investigated by solving the five-dimensional
(5D) quadrupole collective Schroedinger equation. The collective masses and
potentials of the 5D collective Hamiltonian are microscopically derived with
use of the constrained Hartree-Fock-Bogoliubov plus local quasiparticle RPA
method. Good agreement with the recent experimental data is obtained for the
excited 0+ states as well as the ground bands. For 30Mg, the shape coexistence
picture that the deformed excited 0+ state coexists with the spherical ground
state approximately holds. On the other hand, large-amplitude quadrupole-shape
fluctuations dominate in both the ground and the excited 0+ states in 32Mg, so
that the interpretation of 'coexisting spherical excited 0+ state' based on the
naive inversion picture of the spherical and deformed configurations does not
hold.
