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Topological insulators (TIs) are a new quantum state of matter. Theirsurfaces and interfaces act as a topological boundary to generate masslessDirac fermions with spin-helical textures. Investigation of fermion dynamicsnear the Dirac point is crucial for the future development of spintronicdevices incorporating topological insulators. However, research so far has beenunsatisfactory because of a substantial overlap with the bulk valence band anda lack of a completely unoccupied Dirac point (DP). Here, we explore thesurface Dirac fermion dynamics in the TI Sb$_2$Te$_3$ by time- andangle-resolved photoemission spectroscopy (TrARPES). Sb$_2$Te$_3$ has a DPlocated completely above the Fermi energy ($E_F$) with an in-gap DP. Theexcited electrons in the upper Dirac cone stay longer than those below theDirac point to form an inverted population. This was attributed to a reduceddensity of states (DOS) near the DP .

Ultrafast electron dynamics at the Dirac node of the topological insulator Sb$_2$Te$_3$