We report the terahertz (THz) emission from Bi1-xSbx/Co and Bi2SnTe4/Co bilayers including nanometer-thin ferromagnetic layer as spin-injectors and Bi1-xSbx and Bi2SnTe4 topological insulators (TI) from the Bi family grown by molecular beam epitaxy. Using THz emission spectroscopy, an efficient dynamical spin-to-charge conversion in the sub-picosecond timescale is demonstrated in these heterostructures with an output THz amplitude sizeable compared to reference metallic spintronic THz emitters. We investigate TI thickness dependence and azimuthal crystalline orientation dependence on the THz emission which are both in line with interfacially-mediated interconversion. We show that a strong reminiscent THz signal at the limit of small TI thickness is explained by a spin-charge interconversion occurring at the level of the first planes of their interface in contact with Co. This strongly suggests a spin-charge interconversion via inverse Rashba-Edelstein effect (IREE) onto spin-locked TI’s interface states.
Band gap opening and TI properties can be induced at room temperature in α-Sn (001) thin films by strain and quantum finite-size effects. Indeed, angle-resolved photoemission spectroscopy (ARPES) measurements by Ohtsubo et al. performed on thin α-Sn (001) films grown in-situ by molecular beam epitaxy revealed a Dirac cone (DC) linear dispersion with helical spin polarization around the Γ point of the surface Brillouin zone. We recently reported that a very efficient spin-to-charge conversion (SCC) can be achieved at room temperature by spin pumping into this α-Sn thin films, in clear relation with the inverse Edelstein effect (IEE) induced by the counterclockwise helical spin configuration of the DC identified by ARPES . We will present our work focused on the research of efficient interface for charge current to spin current. We will be discuss in details our study by ARPES of the thickness dependence, as well as the impact of a metallic or insulating capping layer on the α-Sn surface states. For all the thicknesses (20 to 50 atomic layers) a linear energy dispersion of the surface states has been observed, allowing the Fermi velocity, the Fermi level and the density of states to be estimated. Following ARPES experiments we performed magneto-transport experiments using the same samples. It is then possible by tracking the Shubnikov-de Haas oscillations to evidence the signature of the surface states, supporting that transport measurements can indeed reveal surface states.
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