Extremely space- and time-limited phonon propagation from electron-lattice scattering induced by Sb/Bi codoping in Ge0.86Sb0.08Bi0.06Te single crystal

Abstract

Phonon dispersions and linewidths of layered thermoelectric Sb/Bi codoped ${\mathrm{Ge}}_{0.86}{\mathrm{Sb}}_{0.08}{\mathrm{Bi}}_{0.06}\mathrm{Te}$ covering 300 to 630 K in the $R3m$ phase are mapped by inelastic neutron scattering measurement. The acoustic phonons depart greatly from the harmonic frequencies with downturns in dispersion revealing soft phonon energies at larger wave vectors for phonons propagating in the crystallographic [110] direction. Two components that scatter phonons are identified. One is smaller in magnitude having a linear wave vector $q$ dependency that links to the three-phonon scattering. The dominant component for phonon softening has a wave vector square ${q}^{2}$ dependency with a Fermi-Dirac thermal reduction rate linked to the electron-phonon scattering. The scattering of phonons is so strong that they only propagate over a few unit cells in length, with lifetimes as short as \ensuremath{\sim}1 ps. Our results reveal the origin of the extremely low thermal conductivity of ${\mathrm{Ge}}_{0.86}{\mathrm{Sb}}_{0.08}{\mathrm{Bi}}_{0.06}\mathrm{Te}$ in the $R3m$ phase.