Auxiliary Material for EPAPS: Measurement of intrinsic Dirac fermion cooling on the surface of a topological insulator Bi2Se3 using time- and angle-resolved photoemission spectroscopy

Two types of single crystal Bi2Se3 samples are prepared which allow us to span the surface doping range investigated in the main text. For the high doping level samples where ED = 0.28 eV below EF , stoichiometric mixture of Bi and Se are used. And for the low doping level samples where ED = 0.06 eV, the initial mixture of Bi:Se = 2:4.06 are used. ED in between these two levels are tuned through the surface doping effect of Bi2Se3 [Fig. S1] [1]. Bulk carrier density can be estimated from the Fermi energy and effective mass m∗ = 0.13me [2]. For the high doping sample, it is 3×1019cm−3 and for the low doping sample 3×1018cm−3. ARPES spectra [Fig. S1 (c)] show rigid shift of ED over time, consistent with the surface doping effect. A typical scan of TrARPES spectra is performed in 20 minutes for one doping level, during which the band-bending effect is negligible [1, 3].