Temperature dependence of ultrafast phonon dynamics in graphite

Nonequilibrium optical phonons are generated in graphite following the excitation of electron-hole pairs with a femtosecond laser pulse. Their energy relaxation is probed by means of terahertz pulses. We find that the hot-phonon lifetime increases by a factor of 2 when the sample temperature decreases from 300 to 5K. These results suggest that the energy relaxation in graphite at room temperature and above is dominated by the anharmonic decay of hot A 1 ′ phonons at the K point into acoustic phonons with energies of about 10meV.