The Evolution Of Pygmy Dipole Resonance In Axial-deformed Nuclei

Random phase approximation(RPA)method and quasiparticle random phase approximation(QRPA)method based on the nuclear covariant density functional theory are important tools to investigate the low-lying excitation properties of nuclei both stable and far from the stability line.However,in the axial-deformed symmetry case,the configuration space of the conventional(Q)RPA through diagonalization is huge and demands enormous computing resources.In order to circum-vent the numerical difficulties,we fulfill a feasible approach,i.e.,the finite amplitude method(FAM).In FAM,the variation of hamiltonian is calculated directly instead of evaluating the matrix ele-ments of residual interaction explicitly,which requires just a mild numerical cost and makes the systematic investigation of pygmy dipole resonance(PDR)in axial-deformed nuclei possible.The emergence of PDR in neutron-rich nuclei is an interesting phenomena and is intensely discussed in recent years.In this work,we have studied the collectivity and isospin character of this distinct resonance structure,as well as the evolution of PDR with the neutron skin thickness.The existence of pygmy inertial excess neutrons(PIEN)is confirmed.Meanwhile,the influence of deformation is investigated,especially the PDR built on the shape isomer of 68Ni:which can be used to explain the experimental conflict of such low-lying E1 excitation between different measurements.