Henri Jaffres
Terahertz (THz) spintronic emitters (STE) are promising candidates for covering the THz gap, offering a gapless broadband emission up to 30 THz [1]. They generally consist in nanometer-thin multilayers made of a ferromagnetic (FM) layer and a heavy metal (HM) from the 3d-5d family offering the ability of spin- charge conversion (SCC) and THz emission scaling with the spin Hall angle. Recently, efforts have been made to increase the STE output power by layer thickness engineering and interface optimization to reduce THz absorption in metallic layers and spin current absorption in thick FMs [2, 3]. Alternative path now turns to bilayers of FM and topological insulator (TI). TIs present conductive topological surface states (TSS) which allow interfacial SCC via the inverse Rashba-Edelstein effect (IREE) [5-7]. In these systems, enhanced SCC is expected owing to i) the large Fermi velocity of related TSS together with ii) the insulating behavior of the TI bulk, strongly reducing THz absorption.
In this study, we demonstrate large THz emission in Bi-based TIs using the TSS of Bi1-xSbx and Bi2SnTe4 TIs. In a second part, we will emphasize more on the different features of THz emission from dynamical spin- injection into Bi1-xSbx (thickness ranging from 2.5 to 50 nm) compared to Co/Pt STE based on ISHE phenomena. Emission performances of Bi1-xSbx are shown to be about the same order of magnitude as Co/Pt ISHE SCC state-of-the-art STE [6-7]. Moreover, the relative thickness and Sb content independence of the THz signal obtained on this family of materials is in favor of an interfacial SCC carried by IREE at the short extended interfacial states. Besides, the azimuthal crystalline dependence of the THz magnetic contribution reveals an isotropic emission, expected from the linear response theory of our IREE calculations for SCC developed in our multiband sp3 tight-binding (TB) approach. In conclusion, TIs here illustrated by Bi1-xSbx and Bi2SnTe4 would be suitable candidates for strong STE output power and THz emission spectroscopy allows to explore TSS mediated SCC mechanism.
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] Hawecker J. et al., “Spin Injection Efficiency at Metallic Interfaces Probed by THz Emission Spectroscopy”, Adv. Optical Mater., 2100412, (2021).
[4] Wang X. et al., “Ultrafast Spin-to-Charge Conversion at the Surface of Topological Insulator Thin Films”, Adv. Mater. 30, 1802356 (2018).
[5] E. Rongione et al., “Ultrafast Spin-Charge Conversion at SnBi2Te4/Co Topological Insulator Interfaces Probed by Terahertz Emission Spectroscopy”, Adv. Optical Mater. 2022, 10, 2102061 (2022)
[6] E. Rongione et al., “Spin-Momentum Locking and Ultrafast Spin-ChargeConversion in Ultrathin Epitaxial Bi1 − xSbx Topological Insulator’’, Adv. Sci. 2023, 2301124 (2023)
[7] H. Park et al., “Topological Surface-Dominated Spintronic THz Emission in Topologically Nontrivial Bi1−xSbx Films.’’, Adv. Sci. 2022, 9, 2200948 (2023)