Transition from vibrational to rotational character in low-lying states of hypernuclei

In order to clarify the nature of hypernuclear low-lying states, we carry out a comprehensive study of the structure of 00000.Λ145-155Sm hypernuclei, which exhibit a transition from vibrational to rotational character as the neutron number increases. To this end, we employ a microscopic particle-core coupling scheme based on a covariant density functional theory. We find that the positive-parity ground-state band in the hypernuclei shares a similar structure to that of the corresponding core nucleus. That is, regardless of whether the core nucleus is spherical or deformed, each hypernuclear state is dominated by the single configuration of the Λ particle in the s1/2 state (Λs1/2) coupled to one core state of the ground band. In contrast, the low-lying negative-parity states mainly consist of Λp1/2 and Λp3/2 configurations coupled to plural nuclear core states. We show that, while the mixing amplitude between these configurations is negligibly small in spherical and weakly deformed nuclei, it strongly increases as the core nucleus undergoes a transition to a well deformed shape, which is consistent with the Nilsson wave functions. We demonstrate that the structure of these negative-parity states with spin I can be well understood based on a naive LS coupling scheme, with total orbital angular momentum L=[I - 1] and spin angular momentum S=1/2.