Effect of low-frequency optical phonons on the thermal conductivity of 2H molybdenum disulfide

Phonon engineering is a novel and effective approach to tailor the thermal conductivity for the thermoelectric performance and heat dissipation. In general, the acoustic phonons rather than the optical phonons are dominant in heating carriers. Here we report an unprecedented large contribution, 47% overall, from the low-frequency optical phonons to the in-plane thermal conductivity in 2H molybdenum disulfide, revealed by low-wave-number high-pressure Raman technology assisted with first-principles calculations. The analysis of phonon dispersion curves and Gr\"uneisen parameters of individual phonon modes reveals that the large contribution originates in a joint effect of the large group velocity of low-frequency optical phonons and their strong anharmonic effects. The joint effect is continuously maintained when pressure increases, up to 20 GPa. Our work provides new insights into the optical phonon transport, paving the way for the phonon engineering and thermal management.