Measurement of intrinsic Dirac fermion cooling on the surface of a topological insulator Bi$_2$Se$_3$ using time- and angle-resolved photoemission spectroscopy

TL;DR

This study investigates the ultrafast relaxation dynamics of surface and bulk electrons in the topological insulator Bi$_2$Se$_3$ using time- and angle-resolved photoemission spectroscopy, revealing temperature-dependent phonon-assisted coupling and Dirac fermion cooling behavior.

Contribution

It provides the first detailed analysis of Dirac fermion cooling mechanisms and their dependence on temperature and carrier density in Bi$_2$Se$_3$.

Findings

Strong phonon-assisted surface-bulk coupling at high temperature

Suppression of inelastic scattering at low temperature

Power law dependence of surface temperature decay on carrier density

Abstract

We perform time- and angle-resolved photoemission spectroscopy of a prototypical topological insulator Bi$_2$Se$_3$ to study the ultrafast dynamics of surface and bulk electrons after photo-excitation. By analyzing the evolution of surface states and bulk band spectra, we obtain their electronic temperature and chemical potential relaxation dynamics separately. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature. In this low temperature regime, the unique cooling of Dirac fermions in TI by acoustic phonons is manifested through a power law dependence of the surface temperature decay rate on carrier density.