Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

Abstract

Using time-, spin-and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi2Te3 following optical excitation by fs-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that relax in about ∼2 ps. One state is located below ∼0.4 eV just above the bulk continuum, the other one at ∼0.8 eV inside a projected bulk band gap. At the onset of the excitation, both states exhibit a reversed spin texture with respect to that of the transient Dirac bands, in agreement with our one-step photoemission calculations. Our data reveal that the high-energy state undergoes spin relaxation within ∼0.5 ps, a process that triggers the subsequent spin dynamics of both the Dirac cone and the low-energy state, which behave as two dynamically-locked electron populations. We discuss the origin of this behavior by comparing the relaxation times observed for electrons with opposite spins to the ones obtained from a microscopic Boltzmann model of ultrafast band cooling introduced into the photoemission calculations. Our results demonstrate that the nonequilibrium surface dynamics is governed by electron-electron rather than electron-phonon scattering, with a characteristic time scale unambiguously determined by the complex spin texture of excited states above the Fermi level. Our findings reveal the critical importance of detecting momentum and energy-resolved spin textures with fs resolution to fully understand the sub-ps dynamics of transient electrons on the surface of topological insulators.

Surface State Dynamics of Topological Insulators Investigated by Femtosecond Time- and Angle-Resolved Photoemission Spectroscopy

“…Their electronic dynamics are shown in panels (b-c) of Figure 4. Several relaxation mechanisms have been proposed in the past, including electron-phonon interactions [22,31], spin-mediated channels [32], bulk diffusion [18], the photovoltage effect [23,33] and surface resonant states [34]. By comparing Figure 4b, the BB2, and Figure 4c, the TSS, on the same time scale we can clearly see that: (i) both relaxation dynamics are sensibly slower than BB1; (ii) there is a clear intensity build-up for p-doped samples (i.e., darker lines) ( Figure 4b); and (iii) the slowest relaxation, especially for the TSS, takes place when the DN is at the Fermi level (i.e., the Bi 0.8 Sb 1.2 Te 3 sample).…”

Ultrafast and Local Optoelectronic Transport in Topological Insulators

“…Polarized illumination can additionally induce a net spin polarization after the excitation via angular momentum selection rules. [ 74–76 ] The nonequilibrium population depolarizes and thermalizes via fast carrier–carrier interaction. The resulting ensemble can be described by a heated Fermi–Dirac distribution with electron temperature T e and shifted chemical potential μ +Δ μ (Figure 4b).…”

The Impact of Spin–Orbit Interaction on the Image States of High‐Z Materials

“…[ 1–5 ] Spin and momentum locking and protection by time‐reversal symmetry which are present in TI systems qualify these solid phases as prominent materials in the field of spintronics. [ 6–23 ] This way a quantitative understanding of the special spin topology is of great interest in the scientific community. Prototypical examples for such TI systems are Bi 2 Se 3 , Bi 2 Te 3 , or Sb 2 Te 3 but also slightly more complex compounds as for example Bi 2 Te 2 Se.…”