Emergent soft monopole modes in weakly bound deformed nuclei

Abstract

Based on the Hartree-Fock-Bogoliubov solutions in large deformed coordinate spaces, the finite amplitude method for the quasiparticle random-phase approximation (FAM-QRPA) has been implemented, providing a suitable approach to probing collective excitations of weakly bound nuclei embedded in the continuum. The monopole excitation modes in magnesium isotopes up to the neutron drip line have been studied with the FAM-QRPA framework on both the coordinate-space and harmonic oscillator basis methods. Enhanced soft monopole strengths and collectivity as a result of weak-binding effects have been unambiguously demonstrated. Nuclei close to the particle drip lines are weakly bound superfluid quantum systems and can exhibit exotic threshold phenomena [1], sharing interdisciplinary interests with weakly bound systems such as multiquark states, Rydberg atoms, and quantum droplets [1][2][3]. Since the discovery of nuclear halos with radioactive beams [4], there have been numerous theo retical developments aiming at weakly bound nuclei and their dilute surfaces [5], Extensive Hartree-Fock-Bogoliubov (HFB) studies have provided successful descriptions of continuum couplings and halo features in ground states of weakly bound nuclei [6][7][8][9][10][11][12]. On the other hand, excitations in weakly bound nuclei opened vast possibilities to probing novel collective modes, as well as continuum effects and components of the effective interaction that are suppressed in ground states [13][14][15], To address these issues, along with the operation of forthcoming facilities such as the Facility for Rare Isotope Beams (FRIB) at Michigan State University, an accurate and self-consistent treatment of continuum together with pairing correlations, deformations, and large spatial extensions is essential.Among the excited states in weakly bound nuclei, the emergent soft excitation modes (or pygmy resonances) which correspond to the collective motion between neutron halo/skins and cores are particularly intriguing. These modes can impact astrophysical neutron capture rates and r-process nucleosynthesis. However, the collectivity of observed pygmy resonances, as a crucial verification of coherence, is still under debate [13,14,16], This Rapid Communication is devoted to the low-energy monopole excitations in weakly bound nuclei caused by the soft incompressibility of halos, as the dilute nuclear matter has a decreased incompressibility compared to saturated densities [17], The low-energy monopole modes indeed have been predicted, e.g., in the neutron-rich nickel isotopes (observed very recently in 68Ni [ 18]), as a rather non collective excitation [19]; however, it may hardly be expected in another random-phase approximation (RPA) calculation with a proper treatment of continuum [20]. Besides, the collectivity could be enhanced due to weak-binding effects [15], Therefore, the emergence of collective soft monopole modes, as well as the role of continuum contributions with the fully self-consistent continuum quasiparticle randomphase approximati...

Finite amplitude method applied to the giant dipole resonance in heavy rare-earth nuclei

“…Especially for the low-lying excitations of loosely bound nuclei, the HFB method has an advantage over the BCS method for the treatment of pairing correlations, especially for nuclei close to the neutron drip line [64,65]. For this purpose, FAM-QRPA embedded into the coordinate-space HFB solver could be a good choice of method [30].…”

“…[29], FAM-QRPA was incorporated into the axially symmetric Skyrme-HFB solver, based on the harmonic oscillator basis. To date, FAM has been applied also to the axially symmetric coordinate-space HFB solver [30] and to the relativistic mean-field framework [31,32]. Various applications of the FAM include descriptions of giant and pygmy dipole excitations [33,34], efficient computation of the QRPA matrix elements [35], and evaluation of beta-decay rates, including the proton-neutron pairing correlations [36,37].…”

Self-consistent collective coordinate for reaction path and inertial mass

“…To evaluate the matrix elements of A ± B in Eq. (13), we adopt the finite amplitude method (FAM) [30][31][32][36][37][38][39][40][41], especially the matrix FAM (m-FAM) prescription [32]. The FAM requires only the calculations of the single-particle Hamiltonian constructed with independent bra and ket states [30], providing us an efficient tool to solve the RPA problem.…”

Multipole modes of excitation in triaxially deformed superfluid nuclei

“…[25,26,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] for applications of the FAM). We start from the HFB code using the two-basis method of the 3D Cartesian coordinate representation.…”