We study the soft electric dipole ($E1$) response of deformed halo nuclei $^{ 31}$Ne and $^{ 37}$Mg using a deformed Woods-Saxon potential, with the potential depth adjusted to reproduce empirical separation energy of last neutron orbit, i.e., 150 keV for $^{ 31}$Ne and 220 keV for $^{ 37}$Mg. The configuration dependence of the $E1$ strength near the neutron threshold is pointed out. The halo configurations $[321]3/2$ at $\beta_2=0.5$ and $[330]1/2$ at $\beta_2=0.24$ in $^{ 31}$Ne contain large amplitudes of halo $p$-shell orbits, which significantly enhance the threshold strength by several times compared to the non-halo configuration $[202]5/2$ at $\beta_2=0.32$. In $^{ 37}$Mg, the last neutron configuration is assigned as $[321]1/2$ at a large deformation of $\beta_2=0.46$, which involves a halo $p$-shell configuration that significantly enhances the soft dipole strength. This enhancement is about 60\% larger than that of the $[321]3/2$ configuration in $^{ 31}$Ne because of large $p$-shell probability in $^{ 37}$Mg. Experimental confirmation of the soft dipole strength is highly desired to determine the deformation and the configuration of the last neutron orbits both in $^{ 31}$Ne and $^{ 37}$Mg.
Categories: nucl-th