thermal conductivity[@Wang2011] in nanocrystalline samples. Computational atomistic methods such as molecular dynamics[@Maiti1997; @Stevens2007; @Landry2009; @IhChoi2012; @Jones2013; @Yang2013; @Merabia2014; @Liang2014; @Schelling2002b] and atomistic Green’s functions[@Zhang2006; @Li2012b; @Tian2014; @Huang2010; @Hopkins2009] have been extensively applied to obtain the transmission coefficients at interfaces with realistic atomic structure. These calculations generally predict the coefficients to decrease with increasing phonon frequency due to reflections of short wavelength phonons by atomistic roughness, a trend that is supported by basic wave physics and indirectly by experiment.[@Wang2011; @Wilson2014b] However, a direct determination of the spectral transmission coefficients at an actual interface has not yet been reported.

Here, we report a metrology to extract the thermal phonon transmission coefficients at a solid interface. Our approach, based on combining experimental observations with ab-initio phonon transport modeling, exploits quasiballistic transport near the interface to significantly narrow the possible transmission coefficient profiles at a solid interface compared to the bounds obtained from traditional approaches. Applying our approach in conjunction with transmission electron microscopy (TEM), we are able to directly link atomic structure to the spectral content of heat crossing the interface. Our approach is a useful tool to elucidate the microscopic transport properties of thermal phonons at solid interfaces.

Overview of approach
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