Kink-kink solutions in BPS impurity models

Abstract

We show that BPS-impurity theories may support BPS kink-kink solutions i.e., an energetically degenerated family of solutions describing two kinks at any mutual distance. This requires a singular impurity. As an example we consider the sine-Gordon and $\phi^6$ models coupled with such a BPS impurity. Interestingly, obtained solutions are identical to double sine-Gordon kinks and Christ-Lee kinks respectively. We also study the spectral flow on the moduli space. All the modes have an odd number of nodes to cancel the singularity of the impurity.