Figure 4: Model of the spin injection-induced modification of the surface spin photocurrent.

(a) A simplified energy diagram by lining up the vacuum levels of GaAs and Bi₂Te₃ without considering any charge transfer process between the two materials. (b) A schematic diagram showing the preferred generation of spin-up holes in the Dirac cone and the resulting spin-polarized photocurrent on the top and bottom surface, arising from the CPGE with the direct optical transitions in Bi₂Te₃ under the σ⁻ polarized excitation. (c) Illustration of selective annihilation of the spin-down holes on the bottom surface with the spin-up electrons injected from the GaAs substrate under the σ⁻ polarized excitation, which enhances the bottom-surface spin photocurrent and competes with the spin photocurrent on the top surface generated by the CPGE. Effect of the spin injection on the top-surface spin current is expected to be negligible due to an extremely short spin diffusion length in the bulk Bi₂Te₃, and is therefore not shown here.