Momentum resolved spin dynamics of bulk and surface excited states in the topological insulator $\mathrm{Bi_{2}Se_{3}}$

TL;DR

This paper demonstrates that comprehensive understanding of optically excited electronic states in topological insulators like Bi2Se3 requires spin-resolved measurements over a wide range of parameters, revealing a surface-resonance state and spin-dependent relaxation dynamics.

Contribution

It introduces a method combining STAR-PES measurements with ab-initio calculations to identify surface-resonance states and models spin-dependent relaxation in topological insulators.

Findings

Identification of a surface-resonance state in Bi2Se3

Observation of spin-dependent relaxation of hot carriers

Validation of a spin dynamics model with two non-interacting systems

Abstract

The prospective of optically inducing a spin polarized current for spintronic devices has generated a vast interest in the out-of-equilibrium electronic and spin structure of topological insulators (TIs). In this Letter we prove that only by measuring the spin intensity signal over several order of magnitude in spin, time and angle resolved photoemission spectroscopy (STAR-PES) experiments is it possible to comprehensively describe the optically excited electronic states in TIs materials. The experiments performed on $\mathrm{Bi_{2}Se_{3}}$ reveal the existence of a Surface-Resonance-State in the 2nd bulk band gap interpreted on the basis of fully relativistic ab-initio spin resolved photoemission calculations. Remarkably, the spin dependent relaxation of the hot carriers is well reproduced by a spin dynamics model considering two non-interacting electronic systems, derived from the excited surface and bulk states, with different electronic temperatures.