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We perform time- and angle-resolved photoemission spectroscopy of aprototypical topological insulator Bi$_2$Se$_3$ to study the ultrafast dynamicsof surface and bulk electrons after photo-excitation. By analyzing theevolution of surface states and bulk band spectra, we obtain their electronictemperature and chemical potential relaxation dynamics separately. Thesedynamics reveal strong phonon-assisted surface-bulk coupling at high latticetemperature and total suppression of inelastic scattering between the surfaceand the bulk at low lattice temperature. In this low temperature regime, theunique cooling of Dirac fermions in TI by acoustic phonons is manifestedthrough a power law dependence of the surface temperature decay rate on carrierdensity.

physical review letters

Measurement of intrinsic Dirac fermion cooling on the surface of a  topological insulator Bi$_2$Se$_3$ using time- and angle-resolved  photoemission spectroscopy