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Register a new userPseudospin-driven spin relaxation mechanism in graphene
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The possibility of transporting spin information over long distances in graphene, owing to its small intrinsic spin-orbit coupling (SOC) and the absence of hyperfine interaction, has led to intense research into spintronic applications. However, measured spin relaxation times are orders of magnitude smaller than initially predicted, while the main physical process for spin dephasing and its charge-density and disorder dependences remain unconvincingly described by conventional mechanisms. Here, we unravel a spin relaxation mechanism for nonmagnetic samples that follows from an entanglement between spin and pseudospin driven by random SOC, which makes it unique to graphene. The mixing between spin and pseudospin-related Berrys phases results in fast spin dephasing even when approaching the ballistic limit, with increasing relaxation times away from the Dirac point, as observed experimentally. The SOC can be caused by adatoms, ripples or even the substrate, suggesting novel spin manipulation strategies based on the pseudospin degree of freedom.
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A principal motivation to develop graphene for future devices has been its promise for quantum spintronics. Hyperfine and spin-orbit interactions are expected to be negligible in single-layer graphene. Spin transport experiments, on the other hand, show that graphenes spin relaxation is orders of magnitude faster than predicted. We present a quantum interference measurement that disentangles sources of magnetic and non-magnetic decoherence in graphene. Magnetic defects are shown to be the primary cause of spin relaxation, while spin-orbit interaction is undetectably small.
Electronic states in 2D materials can exhibit pseudospin degrees of freedom, which allow for unique carrier-field interaction scenarios. Here, we investigate ultrafast sublattice pseudospin relaxation in graphene by means of polarization-resolved photoluminescence spectroscopy. Comparison with microscopic Boltzmann simulations allows to determine a lifetime of the optically aligned pseudospin distribution of $12pm 2,text{fs}$. This experimental approach extends the toolbox of graphene pseudospintronics, providing novel means to investigate pseudospin dynamics in active devices or under external fields.
By analytically constructing the matrix elements of an electron-phonon interaction for the $D$ band in the Raman spectra of armchair graphene nanoribbons, we show that pseudospin and momentum conservation result in (i) a $D$ band consisting of two components, (ii) a $D$ band Raman intensity that is enhanced only when the polarizations of the incident and scattered light are parallel to the armchair edge, and (iii) the $D$ band softening/hardening behavior caused by the Kohn anomaly effect is correlated with that of the $G$ band. Several experiments are mentioned that are relevant to these results. It is also suggested that pseudospin is independent of the boundary condition for the phonon mode, while momentum conservation depends on it.
Spin relaxation in graphene is investigated in electrical graphene spin valve devices in the non-local geometry. Ferromagnetic electrodes with in-plane magnetizations inject spins parallel to the graphene layer. They are subject to Hanle spin precession under a magnetic field $B$ applied perpendicular to the graphene layer. Fields above 1.5 T force the magnetization direction of the ferromagnetic contacts to align to the field, allowing injection of spins perpendicular to the graphene plane. A comparison of the spin signals at B = 0 and B = 2 T shows a 20 % decrease in spin relaxation time for spins perpendicular to the graphene layer compared to spins parallel to the layer. We analyze the results in terms of the different strengths of the spin orbit effective fields in the in-plane and out-of-plane directions.
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.
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