Enhanced Spin-to-Charge Conversion Efficiency in Ultrathin Bi₂Se₃ Observed by Spintronic Terahertz Spectroscopy

Abstract

Owing to their remarkable spin−charge conversion (SCC) efficiency, topological insulators (TIs) are the most attractive candidates for spin−orbit torque generators. The simple method of enhancing SCC efficiency is to reduce the thickness of TI films to minimize the trivial bulk contribution. However, when the thickness reaches the ultrathin regime, the SCC efficiency decreases owing to intersurface hybridization. To overcome these contrary effects, we induced dehybridization of the ultrathin TI film by breaking the inversion symmetry between surfaces. For the TI film grown on an oxygen-deficient transition-metal oxide, the unbonded transition-metal d-orbitals affected only the bottom surface, resulting in asymmetric surface band structures. Spintronic terahertz emission spectroscopy, an emerging tool for investigating the SCC characteristics, revealed that the resulting SCC efficiency in symmetry-broken ultrathin Bi 2 Se 3 was enhanced by up to ∼2.4 times.

Topological Surface‐Dominated Spintronic THz Emission in Topologically Nontrivial Bi1−xSbx Films

“…Our experimental data were rather similar to those of epitaxial Bi 2 Se 3 films that exhibit surface‐dominated SCC. [ 18,19,29 ] In the ultrathin regime (often < 6 nm), the intersurface coupling of a topological insulator creates a gap at the Dirac point and reduces the spin polarization of the TSS, resulting in a critical decrease in the spin Hall angle. [ 19 ] The intersurface coupling of an 8 nm thick Bi 1− x Sb x is not sufficiently strong to create a gap at the $$\overline {{{\Gamma}}} $$ point, but the coupling elsewhere in the k ‐space can be significant owing to a large tunneling probability.…”

“…In this study, we investigated the spin‐to‐charge conversion (SCC) characteristics of epitaxial Bi 1− x Sb x films by spintronic terahertz (THz) spectroscopy, which is an emerging tool in the analysis of ultrafast SCC dynamics. [ 17 , 18 , 19 ] An ultrafast spin current was generated in the ferromagnet Co layer using an optical pump and converted to charge current in the Bi 1− x Sb x layer, resulting in THz radiation. Consequently, we observed a significantly stronger spintronic THz emission than those of Pt and Bi 2 Se 3 in the topologically nontrivial semimetal Bi 1− x Sb x films, indicating the application potential of Bi 1− x Sb x as a high‐performance spintronic device and THz emitter.…”

“…[ 18,19,29 ] In the ultrathin regime (often < 6 nm), the intersurface coupling of a topological insulator creates a gap at the Dirac point and reduces the spin polarization of the TSS, resulting in a critical decrease in the spin Hall angle. [ 19 ] The intersurface coupling of an 8 nm thick Bi 1− x Sb x is not sufficiently strong to create a gap at the $$\overline {{{\Gamma}}} $$ point, but the coupling elsewhere in the k ‐space can be significant owing to a large tunneling probability. [ 30–32 ] In contrast to the Bi 2 Se 3 , because the complicated surface state of Bi 1− x Sb x alloys generates many anisotropic electron and hole pockets, the spin polarization of the surface state away from the $$\overline {{{\Gamma}}} $$ point should be considered to investigate the SCC.…”

Ultrafast Spin‐Charge Conversion at SnBi2Te4/Co Topological Insulator Interfaces Probed by Terahertz Emission Spectroscopy

“…[22] THz emission TDS has proven to be a relevant experimental method [32][33][34] and is particularly important to probe the spin injection efficiency, the spin relaxation and related IREE mediated by interfacial states of TIs. [22,35] For experimental details about the THz-TDS setup, the readers may refer to Section S1, Supporting Information and ref. [27].…”

Ultrafast spin-charge conversion at SnBi$_2$Te$_4$/Co topological insulator interfaces probed by terahertz emission spectroscopy

“…These experiments rely on ultrafast spin injection and their conversion into a charge current at TI material interfaces as explored recently for Bi/Bi 2 Te 3 systems [22]. THz emission TDS has proven to be a relevant experimental method [32][33][34] and is particularly important to probe the spin injection efficiency, the spin relaxation and related IREE mediated by interfacial states of TIs [22,35]. For experimental details about the THz-TDS setup, the readers may refer to Suppl.…”