Tunneling of a quantum breather in a one-dimensional chain

We investigate a chain of particles (bonds) with harmonic interbond and anharmonic intrabond interactions. In the classical limit we consider a breather solution that is strongly localized (essentially a single-site excitation). For the quantum case we study tunneling of this excitation to epsilon. neighboring site. In that case we neglect the anharmonicity except for the two sites between which the tunneling occurs. Within this model the breather tunneling reduces to the tunneling in a dimer coupled to two adjacent harmonic chains. Application of Feynman's path instanton technique yields the tunneling splitting Delta E.For the isolated dimer we reproduce the exponential factor for the splitting Delta E-(0), obtained earlier by a perturbative approach. Assuming the frequency omega of the breather to be much larger than the inverse instanton width we use an adiabatic approximation to derive Delta E for the dimer coupled to the harmonic chains. We find that Delta E can be obtained from Delta E-(0) just by scaling the Planck constant. We argue that independent of the density of states tunneling can never be suppressed, if omega is large enough.