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Single-particle and collective motion in unbound deformed $ ^{39}\text{Mg} $

Background: Deformed neutron-rich magnesium isotopes constitute a fascinating territory where the interplay between collective rotation and single-particle motion is strongly affected by the neutron continuum. The unbound $fp$-shell nucleus $ ^{39}\text{Mg} $ is an ideal candidate to study this interplay. Purpose: In this work, we predict the properties of low-lying resonant states of $ ^{39}\text{Mg} $, using a suite of realistic theoretical approaches rooted in the open quantum system framework. Method: To describe the spectrum and decay modes of $ ^{39}\text{Mg} $ we use the conventional Shell Model, Gamow Shell Model, Resonating Group Method, Density Matrix Renormalization Group method, and the non-adiabatic Particle-Plus-Rotor model formulated in the Berggren basis. Results: The unbound ground state of $ ^{39}\text{Mg} $ is predicted to be either a $ J^π = {7/2}^- $ state or a $ {3/2}^- $ state. A narrow $ J^π = {7/2}^- $ ground-state candidate exhibits a resonant structure reminiscent of that of its one-neutron halo neighbor $ ^{37}\text{Mg} $, which is dominated by the $ f_{7/2} $ partial wave at short distances and a $ p_{3/2} $ component at large distances. A $ J^π={3/2}^- $ ground-state candidate is favored by the large deformation of the system. It can be associated with the $ {1/2}^- [321] $ Nilsson orbital dominated by the $\ell=1$ wave; hence its predicted width is large. The excited $J^π = {1/2}^-$ and $5/2^-$ states are expected to be broad resonances, while the $ J^π = {9/2}^- $ and $ {11/2}^- $ members of the ground-state rotational band are predicted to have very small neutron decay widths. Conclusion: We demonstrate that the subtle interplay between deformation, shell structure, and continuum coupling can result in a variety of excitations in an unbound nucleus just outside the neutron drip line.