Fermi-level-dependent charge-to-spin current conversion by Dirac surface states of topological insulators

Abstract

The spin–momentum locking of Dirac surface states offers intriguing possibilities for converting between charge and spin currents. Experiments show that fine tuning of the Fermi level is critical for maximizing the efficiency of such conversions. Spin–momentum locking in the Dirac surface state of a topological insulator (TI)1,2,3,4,5,6 offers a distinct possibility for highly efficient charge-to-spin current (C–S) conversion compared with spin Hall effects in conventional paramagnetic metals7,8,9,10,11,12,13. For the development of TI-based spin current devices, it is essential to evaluate this conversion efficiency quantitatively as a function of the Fermi level position EF. Here we introduce a coefficient qICS to characterize the interface C–S conversion effect by means of the spin torque ferromagnetic resonance (ST-FMR) for (Bi1−xSbx)2Te3 thin films as EF is tuned across the bandgap. In bulk insulating conditions, the interface C–S conversion effect via the Dirac surface state is evaluated as having large, nearly constant values of qICS, reflecting that qICS is inversely proportional to the Fermi velocity vF, which is almost constant. However, when EF traverses through the Dirac point, the qICS is remarkably reduced, possibly due to inhomogeneity of kF and/or instability of the helical spin structure. These results demonstrate that fine tuning of EF in TI-based heterostructures is critical in maximizing the efficiency using the spin–momentum locking mechanism.