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

Journal name:
Nature Physics
Year published:
Published online

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.

At a glance


  1. Spin current generation and detection in BST/Cu/Py trilayer structure.
    Figure 1: Spin current generation and detection in BST/Cu/Py trilayer structure.

    a, ST-FMR measurement circuit and device design employing BST/Cu/Py heterostructures. White arrows on the surfaces of the BST layer show the polarization direction of spin accumulation. The static magnetic field (Hext) is tilted by θ = 45°. b, A typical ST-FMR spectrum measured for a BST (x = 0.7)/Cu/Py trilayer film at 10K. Red plot shows the experimental spectrum, which can be divided into symmetric (VSym, green line) and anti-symmetric (VAnti, blue line) parts. VSym and VAnti correspond to spin-current-induced FMR and Oersted-field-induced FMR, respectively.

  2. Transport properties of BST films and detected VSym as a function of Sb composition.
    Figure 2: Transport properties of BST films and detected VSym as a function of Sb composition.

    a, Hall coefficient RH for BST films measured at 10K. b, Mobility μ and carrier density n2D, p2D as a function of Sb composition at 10K. c, Symmetric voltage of ST-FMR as a function of Sb composition. The input rf frequency and power are 7GHz and 10dBm, respectively. The error bars are the standard deviation from five samples with different dimensions. d, Schematic of the energy dispersion in BST. e,f, (Top) Spin accumulation due to a Fermi circle shift at the surface state of n-type (e) and p-type (f) BST. Solid and dashed circles are the Fermi circles with Ex and without Ex, respectively. (Bottom) Difference in the Fermi distribution (ff0) on applying an electric field.

  3. Dependence of the charge-to-spin current conversion efficiency of BST on the Sb composition.
    Figure 3: Dependence of the charge-to-spin current conversion efficiency of BST on the Sb composition.

    a, Interface C–S conversion efficiency qICS as a function of Sb composition. Inset shows the band structure and Fermi level position for each Sb composition. Bulk insulating BST with 0.50 ≤ x ≤ 0.90 should have only surface transport. Bi2Te3 (x = 0) and Bi2Sb3 (x = 1) have both bulk and surface conduction paths. Error bars represent the standard deviation over five samples with different dimensions. b, Spin current conductivity as a function of Sb composition.


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Author information


  1. RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan

    • K. Kondou,
    • Y. Fukuma,
    • J. Matsuno,
    • K. S. Takahashi,
    • M. Kawasaki,
    • Y. Tokura &
    • Y. Otani
  2. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan

    • R. Yoshimi,
    • M. Kawasaki &
    • Y. Tokura
  3. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

    • A. Tsukazaki
  4. Frontier Research Academy for Young Researchers, Kyushu Institute of Technology, Iizuka 820-8502, Japan

    • Y. Fukuma
  5. Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan

    • Y. Otani


Y.O. and Y.T. conceived the project. K.K. made the devices and performed the spin torque ferromagnetic resonance measurements. R.Y. grew the topological insulator thin films and performed Hall measurements. K.K. analysed the data and wrote the manuscript with contributions from all authors. A.T., Y.F., K.S.T., J.M., M.K., Y.T. and Y.O. jointly discussed the results.

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