state in a model of two single-mode Bose–Einstein condensates coupled via Josephson
tunnelling. The energy gap is never zero when the tunnelling interaction is non-zero. The
gap exhibits no local minimum below a threshold coupling which separates a delocalized
phase from a self-trapping phase that occurs in the absence of the external potential. Above
this threshold point one minimum occurs close to the Josephson regime, and a set of minima …