Minimum thermal conductivity in the context of diffuson-mediated thermal transport

The concept of minimum thermal conductivity, κ_min, is important for materials screening and device design, particularly for thermoelectric and thermal barrier technologies. Unlike current phonon-based models of κ_min [Phys. Rev. B 46, 6131 (1992); Surf. Coat. Technol. 163, 67-74 (2003)], this study recognizes that atomic vibrations in complex materials at high temperature may be better described by diffuson quasi-particles [Philos. Mag. B 79, 1715-1731 (1999)]. Using the experimentally determined vibrational density of states, we derive the diffuson thermal conductivity, κ_diff, and show that it is defined by the average vibrational frequency, ω_avg. Furthermore, ω_avg is found to be highly correlated with the Debye temperature, allowing κ_diff to be estimated from readily accessible speed of sound measurements. Using κ_diff as an estimate of κ_min gives values that are 37% lower than the Cahill model and 18% lower than the Clarke model, which may reconcile some experimental findings of thermal conductivity below κ_min. Additionally, κ_diff is proposed to be a good metric for identifying materials with extraordinary physics leading to ultralow thermal conductivity, such as phonon focusing.