Research ArticleAPPLIED PHYSICS

Tailoring emergent spin phenomena in Dirac material heterostructures

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Science Advances  21 Sep 2018:
Vol. 4, no. 9, eaat9349
DOI: 10.1126/sciadv.aat9349
  • Fig. 1 Gr-TI heterostructure.

    (A) Schematic representation of a device consisting of a Gr-TI heterostructure channel and ferromagnetic (FM) tunnel contacts for spin injection and detection in a nonlocal transport geometry. The insets show the band structures of Gr and TI, as well as the splitting in Gr bands expected in a heterostructure region. (B) SEM micrograph of the fabricated device showing the Gr-TI heterostructure channel with FM tunnel contacts of TiO2 (1 nm)/Co (60 nm) on Gr. Scale bar, 2 μm.

  • Fig. 2 Electrical and spin transport in Gr-TI heterostructures.

    (A) Schematic of four-terminal local geometry used for measurement of electrical resistance and magnetotransport in TI materials. (B) Temperature dependence of the channel resistance for BS and BSTS. (C) Schematic of measurement configuration of vertical Gr-TI heterostructure channels. (D) Temperature dependence of two-terminal resistivity in a Gr-BSTS heterostructure and a Gr channel. (E) Schematic of nonlocal measurement configuration used for spin-valve and Hanle experiments. (F and G) Spin-valve and Hanle measurements in a Gr-BS heterostructure. Hanle data are fitted using Eq. 1 to extract spin precession parameters. (H and I) Spin-valve and Hanle measurements in a Gr-BSTS heterostructure.

  • Fig. 3 Gate dependence of spin-valve signals in Gr-TI heterostructures.

    (A) Schematic of the Gr-TI heterostructure with an applied gate voltage (Vg) over the SiO2 layer, using n++ Si as a gate electrode. This mainly affects the Fermi level position in Gr, with smaller variation in the TIs due to their higher doping and screening of gate voltage by Gr. (B) Spin-valve signals ΔRNL in Gr-BS heterostructure measured at different gate voltages Vg. (C) Spin-valve signals in Gr-BSTS heterostructures measured at different gate voltages. (D and E) Gate dependence of spin-valve signal amplitude ΔRNL in Gr-BS and Gr-BSTS heterostructures, respectively.

  • Fig. 4 Gate dependence of the Hanle signal in Gr-TI heterostructures.

    (A) Hanle spin signal ΔRNL, measured in a Gr-BS device at different back gate voltages (Vg). (B to D) Gate voltage dependence of the channel sheet resistivity ρ, Hanle signal amplitude ΔRNL, and spin lifetime τs measured in the Gr-BS device. Inset shows the Gr-BS band structures at two representative gate voltages. (E) Nonlocal Hanle spin precession measurements for a Gr-BSTS device at different Vg. (F to H) Gate voltage dependence of ρ, ΔRNL, and τs for the Gr-BSTS device. Inset shows the Gr-BSTS band structures at two representative gate voltages.

  • Fig. 5 Calculation of SOC strength and spin relaxation in Gr-TI heterostructures.

    (A) Calculated band structure of the Gr-BS hybrid structure, showing the BS surface state in proximity to Gr in blue, the opposite BS surface state in green, and Gr states in red. Inset: Zoom-in of the Gr bands around the Gr Dirac point, showing a bandgap and spin splitting on the order of millielectron volts. Open circles are ab initio results, while solid lines are the fit to a tight-binding model. (B) Calculated spin texture induced in Gr in proximity with BS. In the inset, the inner (outer) circle corresponds to an energy contour 14 (155) meV away from the Dirac point. The color scale indicates out-of-plane spin polarization, showing primarily out-of-plane (in-plane) spin polarization near (far from) the Gr Dirac point. The main panel shows the average in-plane spin polarization in Gr as a function of energy (gate voltage). Away from the Gr Dirac point, the spin polarization remains primarily in-plane. (C) Estimated spin lifetime in Gr arising from the ab initio band structure and spin texture, assuming spin relaxation dominated by the EY and DP mechanisms (solid line). The dashed line shows the predicted spin lifetime when including absorption of spins into the BS layer. (D) Estimation of SOC strength in Gr-SiO2 (green), Gr-BS (blue), and Gr-BSTS (orange) devices extracted from experimental data in Fig. 4, using Eq. 2. The black dashed line shows the expected scaling based on the ab initio simulations.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/9/eaat9349/DC1

    Section S1. Gate voltage dependence of TI resistance

    Section S2. Doping of Gr by TIs

    Section S3. Effect of spin absorption by TIs

    Fig. S1. Gate dependence of TI resistance.

    Fig. S2. Magnetotransport measurements in BS and BSTS.

    Fig. S3. Electrical transport in Gr-TI heterostructures.

    Fig. S4. Atomic force microscopy and Raman spectroscopy measurements of BS, BSTS, Gr, and Gr-TI heterostructures.

    Fig. S5. Gate dependence of spin signals in another Gr-BS heterostructure device at room temperature.

    Fig. S6. Gate dependence of spin signals in another Gr-BSTS heterostructure device at room temperature.

    Fig. S7. Gr doping.

    Fig. S8. Calculated spin lifetime due to spin absorption as a function of the Gr square resistance.

    References (4952)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Gate voltage dependence of TI resistance
    • Section S2. Doping of Gr by TIs
    • Section S3. Effect of spin absorption by TIs
    • Fig. S1. Gate dependence of TI resistance.
    • Fig. S2. Magnetotransport measurements in BS and BSTS.
    • Fig. S3. Electrical transport in Gr-TI heterostructures.
    • Fig. S4. Atomic force microscopy and Raman spectroscopy measurements of BS, BSTS, Gr, and Gr-TI heterostructures.
    • Fig. S5. Gate dependence of spin signals in another Gr-BS heterostructure device at room temperature.
    • Fig. S6. Gate dependence of spin signals in another Gr-BSTS heterostructure device at room temperature.
    • Fig. S7. Gr doping.
    • Fig. S8. Calculated spin lifetime due to spin absorption as a function of the Gr square resistance.
    • References (4952)

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