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In spite of their importance for understanding phonon transport phenomena inthin films and polycrystalline solids, the effects of boundary roughnessscattering on phonon specularity and coherence are poorly understood becausethere is no general method for predicting their dependence on phonon momentum,frequency, branch and boundary morphology. Using the recently formulatedatomistic S-matrix method, we develop a theory of boundary roughness scatteringto determine the mode-resolved phonon coherence and specularity parameters fromthe scattering amplitudes. To illustrate the theory, we apply it to phononscattering in realistic nonsymmetric graphene grain boundary (GB) modelsderived from atomic structure predictions. The method is validated by comparingits predictions with frequency-resolved results from lattice dynamics-basedcalculations. We prove that incoherent scattering is almost perfectlydiffusive. We show that phonon scattering at the graphene GB is not diffusealthough coherence and specularity are significantly reduced forlong-wavelength flexural acoustic phonons. Our approach can be generalized toother atomistic boundary models.

Structure-specific, mode-resolved phonon coherence and specularity at graphene grain boundaries