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Register a new userElectronic spin transport and spin precession in single graphene layers at room temperature
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The specific band structure of graphene, with its unique valley structure and Dirac neutrality point separating hole states from electron states has led to the observation of new electronic transport phenomena such as anomalously quantized Hall effects, absence of weak localization and the existence of a minimum conductivity. In addition to dissipative transport also supercurrent transport has already been observed. It has also been suggested that graphene might be a promising material for spintronics and related applications, such as the realization of spin qubits, due to the low intrinsic spin orbit interaction, as well as the low hyperfine interaction of the electron spins with the carbon nuclei. As a first step in the direction of graphene spintronics and spin qubits we report the observation of spin transport, as well as Larmor spin precession over micrometer long distances using single graphene layer based field effect transistors. The non-local spin valve geometry was used, employing four terminal contact geometries with ferromagnetic cobalt electrodes, which make contact to the graphene sheet through a thin oxide layer. We observe clear bipolar (changing from positive to negative sign) spin signals which reflect the magnetization direction of all 4 electrodes, indicating that spin coherence extends underneath all 4 contacts. No significant changes in the spin signals occur between 4.2K, 77K and room temperature. From Hanle type spin precession measurements we extract a spin relaxation length between 1.5 and 2 micron at room temperature, only weakly dependent on charge density, which is varied from n~0 at the Dirac neutrality point to n = 3.6 10^16/m^2. The spin polarization of the ferromagnetic contacts is calculated from the measurements to be around 10%.
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We demonstrate a large enhancement of the spin accumulation in monolayer graphene following electron-beam induced deposition of an amorphous carbon layer at the ferromagnet-graphene interface. The enhancement is 10^4-fold when graphene is deposited onto poly(methyl metacrylate) (PMMA) and exposed with sufficient electron-beam dose to cross-link the PMMA, and 10^3-fold when graphene is deposited directly onto SiO2 and exposed with identical dose. We attribute the difference to a more efficient carbon deposition in the former case due to an increase in the presence of compounds containing carbon, which are released by the PMMA. The amorphous carbon interface can sustain very large current densities without degrading, which leads to very large spin accumulations exceeding 500 microeVs 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 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.
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 $tau_s$ scales inversely with the mobility $mu$ of BLG samples both at room temperature and at low temperature. This indicates the importance of Dyakonov - Perel spin scattering in BLG. Spin relaxation times of up to 2 ns 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 Dyakonov-Perel spin scattering in BLG. In comparison to SLG, significant changes in the density dependence of $tau_s$ are observed as a function of temperature.
We find efficient spin transport in Si at room temperature in lateral spin valves (LSVs). When the crystal orientation of the spin-transport channel in LSVs is changed from $langle$110$rangle$, which is a conventional cleavage direction, to $langle$100$rangle$, the maximum magnitude of the spin signals is markedly enhanced. From the analyses based on the one-dimensional spin diffusion model, we can understand that the spin injection/detection efficiency in Si$langle$100$rangle$ LSVs is larger than that in Si$langle$110$rangle$ ones. We infer that, in Si-based LSVs, the spin detection efficiency of the pure spin current is related to the crystallographic orientation of the valley structures of the conduction band in Si.
Ballistic spin transport is studied through electronic tuners with double stubs attached to them. The spins precess due to the spin-orbit interaction. Injected polarized spins can exit the structure polarized in the opposite direction. A nearly square-wave spin transmission, with values 1 and 0, can be obtained using a periodic system of symmetric stubs and changing their length or width. The gaps in the transmission can be widened using asymmetric stubs. An additional modulation is obtained upon combining stub structures with different values of the spin-orbit strength.
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