Orientational discrete breathers in hydrogen-bonded chains.

We consider rotational motion of protons within a hydrogen-bonded zig-zag chain. Each proton is subjected to a Coulomb interaction from the three nearest heavy ions, as well as from the two neighboring protons. The hydrogen bonding is modeled with an additional double-minimum on-site potential. The system admits discrete breather solutions in the gap below the phonon band. The numerically exact procedure using an anticontinuum limit is exploited to obtain these solutions, which appear to be asymmetric due to the asymmetry of the interaction potential. Only single-well orbits are considered. A linear stability analysis is performed. The discrete breather solutions are shown to be linearly stable provided the nonresonance condition is satisfied, and they turn out to be unstable in the region of 2:3 parametric resonance. Phonon-breather solutions are found in the 1:2 resonance region. Two kinds of two-site breather solutions are investigated.