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We numerically investigate thermal transport at solid-solid interfaces withgraded intermediate layers whose masses vary exponentially from one side to theother. Using Non-Equilibrium Green's Function and Non-Equilibrium MolecularDynamics simulations, we show that an exponentially mass-graded junction with afinite thickness can result in 68\% of enhancement in thermal conductancelarger compared to a single bridging layer (29\%) and a linear mass-gradedjunction (64\%) of similar thickness. We examine how the thermal conductance atsuch interfaces is influenced by geometric qualities and strength ofanharmonicity. For geometric properties, we tested the effects from number oflayers and the junction thickness. In the absence of anharmonicity, increasingthe number of layers results in better elastic phonon transmission at eachindividual boundary, countered by the decrease of available conductingchannels. Consequently, in the harmonic regime, conductance initially increaseswith number of layers due to better bridging, but quickly saturates. Thepresence of slight anharmonic effects (at ultra-low temperature T = 2 K) turnsthe saturation into a monotonically increasing trend. Anharmonic effects canfacilitate interfacial thermal transport through the thermalization of phonons.At high temperature, however, the role of anharmonicity as a facilitator ofinterfacial thermal transport reverses. Strong anharmonicity introducessignificant intrinsic resistance, overruling the enhancement in thermalconduction at the boundaries. Our analysis shows that in our modelLennard-Jones system, the influence of a mass-graded junction on thermalconductance is dominated by the phonon thermalization through anharmoniceffects, while elastic phonon transmission plays a secondary role.

Phonon-mediated thermal transport at exponentially mass-graded interfaces: A computational study