Spin transport in high-mobility graphene on WS2 substrate with electric-field tunable proximity spin-orbit interaction

Graphene supported on a transition metal dichalcogenide substrate offers a novel platform to study the spin transport in graphene in the presence of a substrate-induced spin-orbit coupling while preserving its intrinsic charge transport properties. We report the first nonlocal spin transport measurements in graphene completely supported on a 3.5-nm-thick tungsten disulfide (WS2) substrate, and encapsulated from the top with an 8-nm-thick hexagonal boron nitride layer. For graphene, having mobility up to 16 000 cm(2) V-1 s(-1), we measure almost constant spin signals both in electron and hole-doped regimes, independent of the conducting state of the underlying WS2 substrate, which rules out the role of spin-absorption by WS2. The spin-relaxation time tau(s) for the electrons in graphene-on-WS2 is drastically reduced down to similar to 10 ps from tau(s) similar to 800 ps in graphene-on-SiO2 on the same chip. The strong suppression of tau(s) along with a detectable weak antilocalization signature in the quantum magnetoresistance measurements is a clear effect of the WS2-induced spin-orbit coupling (SOC) in graphene. Via the top-gate voltage application in the encapsulated region, we modulate the electric field by 1 V/nm, changing tau(s) almost by a factor of four, which suggests electric-field control of the in-plane Rashba SOC. Further, via the carrier-density dependence of tau(s), we also identify the fingerprints of the D'yakonov-Perel' type mechanism in the hole-doped regime at the graphene-WS2 interface.