Scattering of phonons by high-concentration isotopic impurities in ultrathin graphite

As the isotopic concentration of ultrathin graphite is varied from natural abundance to nearly pure 13C, the thermal conductivity displays a slight dependence on the isotope concentration at temperatures near its maximum ∼150K. The strength of phonon-isotope scattering in the high-isotope impurity-concentration regime is found to be well below that given by a commonly used incoherent and independent isotope impurity scattering model. This finding is in agreement with some recent theoretical predictions that coherent multiple scattering of phonons is important in the measured thermal conductivity of low-dimensional materials in the high-isotope impurity-concentration regime.