Low-temperature thermal conductivity of bulk and film-like rubrene single crystals

Thermal conductivity of rubrene single crystals is measured for both bulk and film-like crystals down to 0.5 K in order to estimate the density of crystalline defects quantitatively from their phonon mean-free paths. The temperature profile of the rubrene crystals exhibit a pronounced peak at similar to 10 K in the thermal conductivity due to very long mean-free paths of their phonons, which indicates extremely low-level defect density in the region of 10(15)-10(16) cm(-3) depending on different growth methods. The crystals grown from the gas phase tend to have less defects than those grown from solution. The method is applied even for micrometer-thick crystals used for field-effect transistors developed for a new membrane device for thermal-conductivity measurement of film-like samples.