Terahertz time-domain signatures of the inverse Edelstein effect in topological-insulator/ferromagnet heterostructures

Three-dimensional topological insulators possess topologically protected surface states with spin-momentum locking, which enable spin-charge-current interconversion (SCI) by the inverse Edelstein effect (IEE). However, it remains experimentally challenging to separate the surface-related IEE from the bulk-type inverse spin Hall effect (ISHE). Here we search for distinct time-domain signatures of the two SCI phenomena in an F/TI model stack of a ferromagnetic-metal layer F (Co and Fe) and a topological-insulator (TI) layer (Bi2Te₃, SnBi₂Te₄, and Bi1−xSbx with x = 0.15 and x = 0.3), where the focus is on Bi₂Te₃. A femtosecond laser pulse serves to induce a transient spin voltage μsF in F and, thus, drive an ultrafast spin current out of F. SCI results in a transverse charge current with a sheet density Ic that is detected by sampling of the emitted terahertz electric field. Analysis of the dynamics of Ic(t) vs time t relative to μsF(t) reveals two components with distinct timescales: (1) a quasi-instantaneous response and (2) a longer-lived response with a relaxation time of 270 fs, which is independent of the F material chosen. Component (1) is consistently ascribed to the ISHE. In contrast, we interpret component (2) as a signature of interfacial spin accumulation and the IEE at the F/Bi₂Te₃ interface, with a fraction of less than 10−2 of the incident spins participating. This assignment is fully consistent with respect to its dynamics and magnitude. We rate other possible signal contributions, such as spin trapping in intermediate states, as less likely. Our results show that the femtosecond dynamics of photocurrents allow us to differentiate the ISHE and the IEE and, more generally, provide important insights into the mechanisms of spin transport and SCI in F/TI stacks.