Graphene's extremely small intrinsic spin-orbit (SO) interaction 1 makes the realization of many interesting phenomena such as topological/quantum spin Hall states 2,3 and the spin Hall effect 4 (SHE) practically impossible. Recently, it was predicted 1,5-7 that the introduction of adatoms in graphene would enhance the SO interaction by the conversion of sp 2 to sp 3 bonds. However, introducing adatoms and yet keeping graphene metallic, that is, without creating electronic (Anderson) localization 8 , is experimentally challenging. Here, we show that the controlled addition of small amounts of covalently bonded hydrogen atoms is sufficient to induce a colossal enhancement of the SO interaction by three orders of magnitude. This results in a SHE at zero external magnetic fields at room temperature, with non-local spin signals up to 100 ; orders of magnitude larger than in metals 9 . The non-local SHE is, further, directly confirmed by Larmor spin-precession measurements. From this and the length dependence of the non-local signal we extract a spin relaxation length of ∼1 µm, a spin relaxation time of ∼90 ps and a SO strength of 2.5 meV.Graphene 10 is an ideal two-dimensional (2D) system with large Young's modulus 11 and low bending rigidity 12 . Its extraordinary in-plane mechanical strength allows for large out-of-plane deformations, even at the atomic scale. This enables a broad class of chemical reactions/functionalizations, that are not practical with other 2D materials [13][14][15] . The out-of-plane distortion of the planar carbon bonds is unique to graphene and may allow for a strong enhancement in its otherwise weak intrinsic SO coupling strength 1 . This enhancement is unlike the SO enhancement in metals 16 and semiconductors 17 , and is even distinct from the curvatureinduced SO coupling in carbon nanotubes 18,19 . As the sp 3 -bond angle depends strongly on the graphene-substrate interaction, the hydrogenation of graphene allows for a controllable SO strength ranging from a few tens of microelectronvolts up to 7 meV (ref. 1). This allows the manipulation of electron/hole spins in graphene through SHE (refs 17,[20][21][22][23][24], thus eliminating the need for any magnetic elements or externally applied (local) magnetic fields in the device architecture.We introduce small amounts of covalently bonded hydrogen atoms to the graphene lattice by the dissociation of a hydrogen silsesquioxane (HSQ) resist 25 . The extent of hydrogenation for our samples is determined by Raman spectroscopy measurements 26,27 (see Supplementary Information) and gives ∼0.01-0.05% hydrogenation for a HSQ dose in the range 0.4-5 mC cm −2 LETTERS architecture. Last but not least, the demonstration of the non-local SHE due to impurity adatoms in graphene is a major step in the realization of a robust 2D topological states 6 and a SHE-based spin transistor at room temperature.
We report on the first systematic study of spin transport in bilayer graphene (BLG) as a function of mobility, minimum conductivity, charge density and temperature. The spin relaxation time τ s scales inversely with the mobility µ of BLG samples both at room temperature (RT) and at low temperature (LT). This indicates the importance of D'yakonov -Perel' spin scattering in BLG. Spin relaxation times of up to 2 ns at RT are observed in samples with the lowest mobility. These times are an order of magnitude longer than any values previously reported for single layer graphene (SLG). We discuss the role of intrinsic and extrinsic factors that could lead to the dominance of D'yakonov-Perel' spin scattering in BLG. In comparison to SLG, significant changes in the carrier density dependence of τ s are observed as a function of temperature.
Advances in large-area graphene synthesis via chemical vapour deposition on metals like copper were instrumental in the demonstration of graphene-based novel, wafer-scale electronic circuits and proof-of-concept applications such as flexible touch panels. Here, we show that graphene grown by chemical vapour deposition on copper is equally promising for spintronics applications. In contrast to natural graphene, our experiments demonstrate that chemically synthesized graphene has a strong spin-orbit coupling as high as 20 meV giving rise to a giant spin Hall effect. The exceptionally large spin Hall angle B0.2 provides an important step towards graphene-based spintronics devices within existing complementary metal-oxide-semiconductor technology. Our microscopic model shows that unavoidable residual copper adatom clusters act as local spin-orbit scatterers and, in the resonant scattering limit, induce transverse spin currents with enhanced skew-scattering contribution. Our findings are confirmed independently by introducing metallic adatoms-copper, silver and gold on exfoliated graphene samples.
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