Coherent ultrafast spin-dynamics probed in three dimensional topological insulators

Abstract

Topological insulators are candidates to open up a novel route in spin based electronics. Different to traditional ferromagnetic materials, where the carrier spin-polarization and magnetization are based on the exchange interaction, the spin properties in topological insulators are based on the coupling of spin- and orbit interaction connected to its momentum. Specific ways to control the spin-polarization with light have been demonstrated: the energy momentum landscape of the Dirac cone provides spin-momentum locking of the charge current and its spin. We investigate a spin-related signal present only during the laser excitation studying real and imaginary part of the complex Kerr angle by disentangling spin and lattice contributions. This coherent signal is only present at the time of the pump-pulses’ light field and can be described in terms of a Raman coherence time. The Raman transition involves states at the bottom edge of the conduction band. We demonstrate a coherent femtosecond control of spin-polarization for electronic states at around the Dirac cone.

Perspective: Ultrafast magnetism and THz spintronics

“…143 An interesting pathway is to use this coherent polarization to trigger interactions with another part of the magnetic subsystem as, for example, the spin-polarized surface states in topological insulators, as seen in the different response for the components of the complex Kerr rotation from the Bi 2 Se 3 family, (Bi 0.57 Sb 0.43 ) 2 Te 3 shown in Figure 11(b). 144 It is believed that these processes are faster than the thermal demagnetization effect. Their investigation will shed light on the inverse Faraday effects and further ultrafast processes that happen faster than the scattering time of the electrons in a coherent state, ultimately leading to attosecond control 145 of magnetization.…”

Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

“…for the reflectivity dynamics and complex Kerr signal observed in recent state-of-the-art experiments on the same material [31].…”

“…As a consequence of the weak surface scattering these findings enabled, on the one hand, the observation of spin-polarized electrical currents originating from TSSs using circularly-polarized light in the time domain [6,30,31]. On the other hand, the use of linearly-polarized fs-infrared pulses enabled the suppression of the charge current on the surface [5,16], presumably giving rise to pure ultrafast spin currents evolving on a different time scale [16], so that the transient Dirac cone can be dynamically populated by the same number of excited elec- …”

Role of matrix elements in the time-resolved photoemission signal

“…The particular form of Z(τ) could stem from pump-induced modifications to the initial state wavefunction, warping of the Dirac cone, or even the nature of the photoemission final states. In the present context, one plausible contribution may derive from the commonly reported photoinduced A 1g optical phonons in TIs (see figure 3 and [19,49,51,53]). These out of plane modes, i.e.…”

“…High time-resolution TR-reflectivity has been widely employed for studying photoinduced phonons. In particular, A 1g phonons with a frequency of a few THz have been commonly observed in TIs after a near-IR perturbation [45][46][47][48][49][50][51]. To confirm the presence of lattice vibrations photoinduced by the pump, we have thus performed a high time-resolution TR-reflectivity investigation of BSTS, which is so far missing to the best of our knowledge.…”