Emergence of nuclear clustering in electric-dipole excitations of $^6$Li

Abstract

Nuclear clustering plays an important role, especially in the dynamics of light nuclei. The importance of the emergence of the nuclear clustering was discussed in the recent measurement of the photoabsorption cross sections as it offered the possibility of the coexistence of various excitation modes which are closely related to the nuclear clustering. To understand the excitation mechanism, we study the electric-dipole ($E1$) responses of $^6$Li with a fully microscopic six-body calculation. The ground-state wave function is accurately described with a superposition of correlated Gaussian (CG) functions with the aid of the stochastic variational method. The final-state wave functions are also expressed by a number of the CG functions including important configurations to describe the six-body continuum states excited by the $E1$ field. We found that the out-of-phase transitions occur due to the oscillations of the valence nucleons against the $^4$He cluster at the low energies around 10 MeV indicating ``soft'' giant-dipole-resonance(GDR)-type excitations, which are very unique in the $^6$Li system but could be found in other nuclear systems. At the high energies beyond $\sim 30$ MeV typical GDR-type transitions occur. The $^3$He-$^3$H clustering plays an important role to the GDR phenomena in the intermediate energy regions around 20 MeV.