Probing Dirac Fermion Dynamics in Topological Insulator Bi$_2$Se$_3$ Films with Scanning Tunneling Microscope

TL;DR

This study uses scanning tunneling microscopy to explore the femtosecond dynamics of Dirac fermions in ultrathin Bi$_2$Se$_3$ films, revealing how quantum confinement and scattering processes influence electron behavior.

Contribution

It provides direct measurements of phase relaxation length and inelastic scattering lifetime in topological insulator films, highlighting the energy dependence and effects of film thickness.

Findings

Inelastic lifetime $ au$ scales as $(E-E_F)^{-2}$

Scattering properties depend on film thickness

Quantum confinement affects electron dynamics

Abstract

Scanning tunneling microscopy and spectroscopy have been used to investigate the femtosecond dynamics of Dirac fermions in the topological insulator Bi$_2$Se$_3$ ultrathin films. At two-dimensional limit, bulk electrons becomes quantized and the quantization can be controlled by film thickness at single quintuple layer level. By studying the spatial decay of standing waves (quasiparticle interference patterns) off steps, we measure directly the energy and film thickness dependence of phase relaxation length $l_{\phi}$ and inelastic scattering lifetime $\tau$ of topological surface-state electrons. We find that $\tau$ exhibits a remarkable $(E-E_F)^{-2}$ energy dependence and increases with film thickness. We show that the features revealed are typical for electron-electron scattering between surface and bulk states.