Ultrafast surface carrier dynamics in the topological insulator Bi2Te3

TL;DR

This study investigates the ultrafast dynamics of surface and bulk carriers in the topological insulator Bi2Te3 after femtosecond laser excitation, revealing detailed temporal evolution and interband scattering processes.

Contribution

It provides real-time visualization of carrier dynamics in Bi2Te3, highlighting the role of bulk-surface interband scattering and the spatial extent of this process.

Findings

Surface state thermalization occurs within 0.5 ps due to bulk interband scattering.

Dirac cone electrons recover Fermi-Dirac distribution in a few ps.

Bulk-surface scattering is effective within 5 nm distance.

Abstract

We discuss the ultrafast evolution of the surface electronic structure of the topological insulator Bi$_2$Te$_3$ following a femtosecond laser excitation. Using time and angle resolved photoelectron spectroscopy, we provide a direct real-time visualisation of the transient carrier population of both the surface states and the bulk conduction band. We find that the thermalization of the surface states is initially determined by interband scattering from the bulk conduction band, lasting for about 0.5 ps; subsequently, few ps are necessary for the Dirac cone non-equilibrium electrons to recover a Fermi-Dirac distribution, while their relaxation extends over more than 10 ps. The surface sensitivity of our measurements makes it possible to estimate the range of the bulk-surface interband scattering channel, indicating that the process is effective over a distance of 5 nm or less. This establishes a correlation between the nanoscale thickness of the bulk charge reservoir and the evolution of the ultrafast carrier dynamics in the surface Dirac cone.