Probing Spin-injection in the Time Domain in Topological Insulators via THz-TDS Spectroscopy

Spin-to-charge conversion (SCC) in femtosecond laser excited magnetic heterostructures may generate high efficiency and wide-bandwidth terahertz emission with a magnetically controllable polarization state¹. The origin of this THz emission has been assigned to the generation of a spin-polarized current and subsequent conversion of the spin current to a transverse charge current². Two main SCC mechanisms are generally involved: the Inverse Spin Hall Effect (ISHE) and the Inverse Rashba- Edelstein Effect (IREE). The discovery of metallic quantum states at the surface of 3D topological insulators (TIs) has opened exciting new functionalities owing to their time-reversal symmetry property and their spin-momentum locking (SML) properties. The resulting SCC combining strong spin-orbit coupling (SOC) and SML is expected to be enhanced compared to the spin Hall effect (SHE) of heavy metals. SCC has been demonstrated in a range of Bi-based TI compounds, including bismuth selenide Bi₂Se₃, bismuth telluride Bi₂Te₃, Bi₂(Se,Te)₃ or Bi_1-xSb_x (BiSb). To benefit fully from IREE, the charge currents should be confined in the surface states and any current flowing through the bulk states should be avoided.

In this talk, I will report particularly on our detailed investigation of the surface state SML properties of ultrathin (111)-oriented Bi_1-xSb_x epitaxial films. They exhibit a topological phase as recently confirmed by angular-resolved photo-emission spectroscopy (ARPES)³ and mainly in-plane spin texture as shown by SARPES. SCC mediated by the BiSb surface states is probed at the sub-picosecond timescale. Unprecedentedly large SCC is measured with efficiencies beyond the level of carefully optimized Co/Pt systems. I will present our more recent results dealing with the fabrication of sputtered BiSb materials giving rise to very efficient spin-charge conversion in the time domain at the level of up-to-date best bilayers spintronics emitters.

References
[1]. Seifert T. et al., “Efficient metallic spintronic emitters of ultrabroadband terahertz radiation”, Nature Photon 10, 483–488, 2016.
[2]. Dang T. H. et al., “Ultrafast spin-currents and charge conversion at 3d-5d interfaces probed by time-domain terahertz spectroscopy”, Appl. Phys. Rev. 7, 2020
[3]. L. Baringthon et al “Topological surface states in ultrathin Bi_1-xSb_x layers” Physical Review Materials 6, 074204, 2022
[4] E. Rongione et al., “Spin-Momentum Locking and Ultrafast Spin-Charge Conversion in Ultrathin Epitaxial Bi1 – xSbx Topological Insulator”, Adv. Sci. 2023, 2301124