Acoustic-optical phonon scattering observed in the thermal conductivity of polydiacetylene single crystals

The steady-state thermal conductivity of polydiacetylene single crystals has been measured in the range 50 mK-50K. Below 1K the temperature dependence of the thermal conductivity approaches T 3 as expected for single crystal whilst above 1K the conductivity is approximately proportional to T 1 2/. The data above 1K have been interpreted by assuming that the acoustic phonons responsible for heat transport are scattered by low-energy optical modes associated with the motion of the massive polymer sidegroups. The five lowest optical modes observed by far-infrared absorption and inelastic neutron scattering are sufficient to explain the observed temperature dependence. A good fit to the data was achieved with each mode having an acoustic-optical coupling matrix element of 5 meV; the associated splitting of the phonon dispersion curves is estimated to be about 0.6 meV.