Amplitude breathers in conjugated polymers

Nonlinear adiabatic dynamics of electron-phonon models of trans-(CH${)}_{\mathrm{x}}$, cis-(CH${)}_{\mathrm{x}}$, and (AB${)}_x$-type polymers are studied. Using both analytic low-amplitude expansion and numerical simulations, we find the existence of charge-neutral breathers—spatially localized, time-periodic nonlinear excitations. Simulations also show that a photoexcited electron-hole pair generates a breather in addition to a soliton-antisoliton pair. Semiclassical quantization shows that the nth breather is a bound state of n amplitude phonons. A semiclassical formalism of optical absorption is applied to trans-(CH${)}_{\mathrm{x}}$; the ground-state result accounts for a pronounced intragap absorption tail, while breathers can account for the observed photoinduced absorption at 1.35 eV.