Stability analysis of π-kinks in a 0-π Josephson junction

We consider a spatially nonautonomous discrete sine-Gordon equation with constant forcing and its continuum limit(s) to model a 0-π Josephson junction with an applied bias current. The continuum limits correspond to the strong coupling limit of the discrete system. The nonautonomous character is due to the presence of a discontinuity point, namely, a jump of π in the sine-Gordon phase. The continuum model admits static solitary waves which are called π-kinks and are attached to the discontinuity point. For small forcing, there are three types of π-kinks. We show that one of the kinks is stable and the others are unstable. There is a critical value of the forcing beyond which all static π-kinks fail to exist. Up to this value, the (in)stability of the π-kinks can be established analytically in the strong coupling limits. Applying a forcing above the critical value causes the nucleation of 2π-kinks and -antikinks. Besides a π-kink, the unforced system also admits a static 3π-kink. This state is unstable in the continuum models. By combining analytical and numerical methods in the discrete model, it is shown that the stable π-kink remains stable and that the unstable π-kinks cannot be stabilized by decreasing the coupling. The 3π-kink does become stable in the discrete model when the coupling is sufficiently weak.