We demonstrate that the stability of the semilocal vortex can be significantly improved by the presence of a dilatonic coupling of the form e qjÈj 2 2 F F with q > 0 where is the scale of symmetry breaking that gives rise to the vortex. For q ¼ 0 we obtain the usual embedded (semilocal) Nielsen-Olesen vortex. We find the stability region of the parameter ð m È m A Þ 2 (m È and m A are the masses of the scalar and gauge fields, respectively). We show that the stability region of is 0 < < max ðqÞ where max ðq ¼ 0Þ ¼ 1 (as expected) and max ðqÞ is an increasing function of q. This result may have significant implications for the stability of the electroweak vortex in the presence of a dilatonic coupling (dilatonic electroweak vortex).

We consider Lagrangians in 3+1 dimensions admitting topological defects where there is an additional coupling between the defect scalar field Φ and the gauge field kinetic term (eg B(|Φ| 2 )Fµν F µν ). Such a dilatonic coupling in the context of a static defect, induces a spatially dependent effective gauge charge and effective mass for the scalar field which leads to modified properties of the defect core. In particular, the scale of the core gets modified while the stability properties of the corresponding embedded defects are also affected. These modifications are illustrated for gauged (Nielsen-Olesen) vortices and for gauged ('t Hooft-Polyakov) monopoles. The corresponding dilatonic global defects are also studied in the presence of an external gauge field.