Speaker
Description
Understanding how lattice thermal conductivity changes under strain or pressure is increasingly important in materials science, yet difficult to predict because a material's behavior can range from monotonic increases or decreases to anomalous trends. Molecular crystals, including organic semiconductors, stand out for exhibiting unusually large pressure-induced increases relative to their (ultra-)low thermal conductivities. However, the atomistic mechanisms underlying these significant pressure enhancements have yet to be elucidated.
Our test system, naphthalene, crystallizes in a herringbone packing motif common in most π-conjugated organic semiconductors which is strongly affected by external pressure. Measurements up to 2 GPa from previous literature show an isotropic thermal conductivity that is up to four times higher than at ambient conditions. By combining highly accurate machine-learned potentials with the Wigner transport equation, we not only reproduce these findings but also elucidate how compression affects naphthalene's anisotropic thermal conductivity and how phonon tunneling becomes less relevant with increasing pressure. Finally, we trace the pressure-induced enhancement to frequency upshifts and the associated modifications in phonon scattering.