Tuning of thermoelectric properties using coherent phonon

Recently, improvement of thermoelectric properties, decrease in thermal conductivity and increase in power factor, of single-walled carbon nanotubes (SWNTs) due to fullerene encapsulation at room temperature has been observed [Nat. Mater. 295, 828 (2017)]. We reveal how the fullerene encapsulation affects phonon properties of SWNTs. The fullerene encapsulation causes expansion in radial direction and wavy deformation in outer shell, SWNT. The structural deformation lead to variations of phonon dispersion: softening of longitudinal modes, zone-folding, and hybridization between vibrations of SWNT and encapsulated fullerenes. The modulation of phonon dispersion decreases both group velocity and relaxation time. The contributions of change of group velocity and relaxation time to the decrease in thermal conductivity are comparable regardless phonon modes and frequency. Our detailed analysis paves the new way for thermal management using coherent phonons.