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

TL;DR

This study uses time- and angle-resolved photoemission spectroscopy to investigate ultrafast electron dynamics at the Dirac point of Sb$_2$Te$_3$, revealing prolonged electron lifetimes above the Fermi energy due to low density of states.

Contribution

It provides new insights into Dirac fermion dynamics in Sb$_2$Te$_3$ with an unoccupied Dirac point, advancing understanding for spintronic applications.

Findings

Electrons above the Dirac point have longer lifetimes.

The Dirac point is completely above the Fermi energy.

Reduced density of states near the Dirac point affects electron dynamics.

Abstract

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 .