| Abstract | Recently, Duda et al. [J. C. Duda, P. E. Hopkins, Y. Shen, and M. C. Gupta, Phys. Rev. Lett. 110, 015902 (2013)] reported that the fullerene derivative [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) has the lowest thermal conductivity Lambda ever observed in a fully dense solid, Lambda approximate to 0.03 W m(-1) K-1. We have investigated a variety of phases and microstructures of PCBM and the closely related compound [6,6]-phenyl-C61-butyric acid n-butyl ester (PCBNB) and find that the thermal conductivities of PCBM and PCBNB films are mostly limited to the range 0.05 < Lambda < 0.06 W m(-1) K-1 with a few samples having slightly higher Lambda. The conductivities we observe are approximate to 70% larger than reported by Duda et al. but are still ``ultralow'' in the sense that the thermal conductivity is a factor of approximate to 3 below the conductivity predicted by the minimum thermal conductivity model using an estimate of the thermally excited modes per molecule. |
