Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ultrafast electron dynamics at the Dirac node of the topological
  insulator Sb$_2$Te$_3$   Topological insulators (TIs) are a new quantum state of matter. Their
surfaces and interfaces act as a topological boundary to generate massless
Dirac fermions with spin-helical textures. Investigation of fermion dynamics
near the Dirac point is crucial for the future development of spintronic
devices incorporating topological insulators. However, research so far has been
unsatisfactory because of a substantial overlap with the bulk valence band and
a lack of a completely unoccupied Dirac point (DP). Here, we explore the
surface Dirac fermion dynamics in the TI Sb$_2$Te$_3$ by time- and
angle-resolved photoemission spectroscopy (TrARPES). Sb$_2$Te$_3$ has a DP
located completely above the Fermi energy ($E_F$) with an in-gap DP. The
excited electrons in the upper Dirac cone stay longer than those below the
Dirac point to form an inverted population. This was attributed to a reduced
density of states (DOS) near the DP .

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The categories of this paper are: hep-ex
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The categories of this paper are: hep-ph
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The categories of this paper are: physics
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The categories of this paper are: econ
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The categories of this paper are: gr-qc
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The categories of this paper are: mtrl-th
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The categories of this paper are: quant-ph
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The categories of this paper are: supr-con
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The categories of this paper are: comp-gas
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The categories of this paper are: bayes-an
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The categories of this paper are: adap-org
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The categories of this paper are: solv-int
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The categories of this paper are: q-fin
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The categories of this paper are: cmp-lg
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The categories of this paper are: chem-ph
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The categories of this paper are: math-ph