Vortex Solutions in Two-Higgs-Doublet Systems
  We analyze the existence of string-like defects in a two-Higgs-doublet system having $SU(2) \times U(1)_Y \times U(1)_{Y^{\prime}}$ as gauge group. We are able to show that, when certain relations among the parameters hold, these configurations satisfy a set of first order differential equations (Bogomol'nyi equations) and their energy is proportional to their topological charge.} 
G. Bimonte and G. Lozano