Electrical Control of Valley-Zeeman Spin-Orbit-Coupling–Induced Spin Precession at Room Temperature

The ultimate goal of spintronics is achieving electrically controlled coherent manipulation of the electron spin at room temperature to enable devices such as spin field-effect transistors. With conventional materials, coherent spin precession has been observed in the ballistic regime and at low temperatures only. However, the strong spin anisotropy and the valley character of the electronic states in 2D materials provide unique control knobs to manipulate spin precession. Here, by manipulating the anisotropic spin-orbit coupling in bilayer graphene by the proximity effect to ${\mathrm{WSe}}_2$, we achieve coherent spin precession in the absence of an external magnetic field, even in the diffusive regime. Remarkably, the sign of the precessing spin polarization can be tuned by a back gate voltage and by a drift current. Our realization of a spin field-effect transistor at room temperature is a cornerstone for the implementation of energy efficient spin-based logic.

Figure 1

Device working principle and BLG-${\mathrm{WSe}}_2$ spin transistor operation. (a) Sketch of a BLG-${\mathrm{WSe}}_2$ heterostructure. Out-of-plane valley-Zeeman SOF (black arrows) with opposite sign at the $K$ and $K^{\prime }$ valleys induce in-plane spin precession with a period ${\tau }_{\mathrm{VZ}}$. Spins can scatter between the valleys via intervalley scattering (${\tau }_{\mathrm{iv}}$). (b) Time dependence of the spin accumulation ${\mu }_{sy}$ for different ${\tau }_{\mathrm{iv}}$ values (see Ref. [39] for details). ${\mu }_{sy}$ undergoes net precession for ${\tau }_{\mathrm{iv}}\ge 0.5$${\tau }_{\mathrm{VZ}}$. (c) and (d) Sketch of a spin field-effect transistor operating at the strong SOC regime where the valley-Zeeman induced spin precession is tuned by $V_{\mathrm{bg}}$ to control the sign reversal.

Figure 2

Diffusive spin transport at 50 K. (a) Optical image of the measured device. The BLG flake is the dark horizontal stripe and ${\mathrm{WSe}}_2$ is the bright flake in the middle. The scale bar is $2\mu \mathrm{m}$. The bottom panel shows a sketch of the device with the ${\mathrm{WSe}}_2$-covered BLG region shown in green. The circuit corresponds to the standard nonlocal spin diffusion measurement configuration. Contacts 1 and 7 are not magnetic (Ti/Au) and 2 to 6 are spin-polarized ${\mathrm{TiO}}_x$/Co contacts. (b) Nonlocal spin valve measurement across the ${\mathrm{WSe}}_2$-covered BLG region as a function of the magnetic field applied along $y$ ($B_y$) for $V_{\mathrm{bg}}=50\mathrm{V}$. The horizontal arrows represent the $B_y$-sweep direction and the vertical ones the magnetization of contacts 3 and 4. (c) Nonlocal spin precession measurements with the magnetic field applied along $x$ ($B_x$) in the parallel (P) and antiparallel (AP) configurations for $V_{\mathrm{bg}}=50\mathrm{V}$. Inset: low field detail. (d) Spin signal ($\mathrm{\Delta }{R}_{\mathrm{nl}}$) and charge diffusivity ($D_c$) as a function of $V_{\mathrm{bg}}$. (e) Spin-polarized band structure of BLG-${\mathrm{WSe}}_2$ at zero electric field. The red lines represent spin-up (along $+z$) and the blue ones spin-down (along $-z$). (f) Spin splitting ($2{\lambda }_{\mathrm{VZ}}$) of the valence and conduction bands obtained from the band structure in panel e.

Figure 3

Controlling spin transport with drift at 50 K. (a) Sketch of the device with the spin drift measurement configuration. (b) Nonlocal spin valve measurements for $V_{\mathrm{bg}}=-50\mathrm{V}$ at $I_{\mathrm{DC}}=-40$, 0, and $+40\mu \mathrm{A}$. The curves have been shifted for clarity. (c) $I_{\mathrm{DC}}$ dependence of $\mathrm{\Delta }{R}_{\mathrm{nl}}$ at $V_{\mathrm{bg}}=-50$ and $-30\mathrm{V}$. (d) $\mathrm{\Delta }{R}_{\mathrm{nl}}$ vs $V_{\mathrm{bg}}$ at $I_{\mathrm{DC}}=-40\mu \mathrm{A}$. The vertical light blue line is the charge neutrality point of the ${\mathrm{WSe}}_2$-covered BLG region. (e),(f) Nonlocal spin precession measurements with $B_x$ in the P and AP configurations for $I_{\mathrm{DC}}=-40\mu \mathrm{A}$ and $V_{\mathrm{bg}}=-30\mathrm{V}$ and $-50\mathrm{V}$, respectively. The lines are obtained by averaging over a window of eleven points.

Figure 4

Room temperature electrical control of spin transport. $V_{\mathrm{bg}}$ dependence of $\mathrm{\Delta }{R}_{\mathrm{nl}}$ for $I_{\mathrm{DC}}=\pm 40\mu \mathrm{A}$. The blue area represents the charge neutrality point of the ${\mathrm{WSe}}_2$-covered BLG region. $\mathrm{\Delta }{R}_{\mathrm{nl}}$ reverses sign upon changing the sign of $I_{\mathrm{DC}}$.