  A shape phase transition between stable octupole deformation and octupole
vibrations in Th nuclei is analyzed in a microscopic framework based on nuclear
density functional theory. The relativistic functional DD-PC1 is used to
calculate axially-symmetric quadrupole-octupole constrained energy surfaces.
Observables related to order parameters are computed using an interacting-boson
Hamiltonian, with parameters determined by mapping the microscopic energy
surfaces to the expectation value of the Hamiltonian in the boson condensate.
The systematics of constrained energy surfaces and low-energy excitation
spectra point to the occurrence of a phase transition between octupole-deformed
shapes and shapes characterized by octupole-soft potentials.
