Room Temperature Spin to Charge Conversion in Amorphous Topological Insulating Gd-Alloyed BixSe1-x/CoFeB Bilayers

Abstract: Disordered topological insulator (TI) films have gained intense interest by benefiting from both the TIs exotic transport properties and the advantage of mass production by sputtering. Here, we report on the clear evidence of spin-charge conversion (SCC) in amorphous Gd-alloyed BixSe1-x (BSG)/CoFeB bilayers fabricated by sputtering, which could be related to the amorphous TI surface states. Two methods have been employed to study SCC in BSG/CoFeB(5 nm) bilayers with different BSG thicknesses. Firstly, spin pumping is used to generate a spin current in CoFeB and to detect SCC by inverse Edelstein effect. The maximum SCC efficiency (SCE) is measured as large as 0.035 nm in a 6 nm thick BSG sample, which shows a strong decay when tBSG increases due to the increase of BSG surface roughness. The second method is the THz time-domain spectroscopy, which reveals a small tBSG dependence of SCE, validating the occurrence of a pure interface state related SCC. Furthermore, our angle-resolved photoemission spectroscopy data show dispersive two-dimensional surface states that cross the bulk gap until to the Fermi level, strengthening the possibility of SCC due to the amorphous TI states. Our studies provide a new experimental direction towards the search for topological systems in the amorphous solids.