Observation of current-induced spin polarization in the topological insulator Bi_2Te_3 via circularly polarized photoconductive differential current

摘要

Current-induced spin polarization (CISP) in the topological insulator (TI) Bi_2Te_3 has been observed via circularly polarized photoconductive differential current (CPDC). It is found that the CPDC induced by the CISP is proportional to the square of the longitudinal electric field when the electric field is weak. The intensity of the CPDC decreases and finally reverses the sign when the thickness of Bi_2Te_3 films is increased from 3 to 20 QL, owing to the switching of the dominant contribution to the CPDC from the top surface states to the bottom surface states. The spin polarization degree (SPD) of the photoconduction and the critical electric field E_c at which the SPD becomes saturated are determined, and it is revealed that the SPD decreases while E_c increases with the increasing of the thickness of Bi_2Te_3 films. In addition, it is demonstrated that the CPDC excited by 1064 nm is more than one order larger than that excited by 1342 nm light, which is due to the larger optical absorption coefficient when excited by 1064 nm light. The temperature dependence of the CPDC and SPD of Bi_2Te_3 films is also investigated, and it is found that the SPD induced by CISP first increases and then decreases with decreasing temperature. This work proposes a method using CPDC to investigate the CISP of TIs.