Optimizing phonon scattering by tuning surface-interdiffusion-driven intermixing to break the random-alloy limit of thermal conductivity

We investigate the evolution of the cross-plane thermal conductivity $\kappa$ of superlattices (SLs) as interfaces change from perfectly abrupt to totally intermixed, by using non-equilibrium molecular dynamics simulations in combination with the spectral heat current calculations. We highlight the role of surface-interdiffusion-driven intermixing by calculating the $\kappa$ of SLs with changing interface roughness, whose tuning allows for the $\kappa$ values much lower than the "alloy limit" and the abrupt interface limit in same cases. The interplay between alloy and interface scattering in different frequency ranges provides a physical basis to predict a minimum of thermal conductivity. More specifically, we also explore how the interface roughness affects the thermal conductivities for SLs materials with a broad span of atomic mass and bond strength. In particular, we find that (i) only when the "spacer" thickness of SLs increases up to a critical value the $\kappa$ of rough SLs can break the corresponding "alloy limit". (ii) Whether the $\kappa$ changes monotonically as interface roughness strongly depends on the period length and intrinsic behavior of phonon transport for SLs materials. Especially, for the SL with large period length, there exists an optimal interface roughness which can minimize the thermal conductivity. (iii) Surface-interdiffusion-driven intermixing is more effective in achieving the low $\kappa$ below the alloy limit for SL materials with large mass mismatch than with small one. (iv) It's possible for SLs materials with large lattice mismatch (i.e., bond strength) to design an ideally abrupt interface structure with $\kappa$ much below the "alloy limit". These results have a clear implications for optimization of thermal transport for heat management and for the development of thermoelectric materials.