Deformed neutron halo nuclei and soft dipole excitations in the 40 < A < 90 mass region

We study deformed neutron halo nuclei in the mass region $40<A<90$ and their soft electric dipole $(E1)$ excitations based on the deformed relativistic Hartree-Bogoliubov theory in continuum. Three candidates, $^{43}\mathrm{Si}$, $^{69}\mathrm{Ti}$, and $^{75}\mathrm{Cr}$, are selected for detailed analysis. Unique features are identified in the decoupled densities of possible $s$- and $p$-wave deformed halo nuclei in this mass region, which are influenced by large high-$l$ configurations. It is shown that the dipole response is a highly sensitive observable to detect the halo component of the single-particle wave function in deformed halo nuclei, and it helps identify the configuration and the magnitude of deformation for halo nuclei in the $40<A<90$ mass region. Experimental confirmation of the dipole strength in the low-energy region is highly desirable to explore possible deformed halo candidates in the medium-heavy mass region.