Graphene spintronics: puzzling controversies and challenges for spin manipulation

摘要

This article presents the current puzzling controversy between theory and experimental results concerning the mechanisms leading to spin relaxation in graphene-based materials. On the experimental side, it is surprising that regardless of the quality of the graphene monolayer, which is characterized by the carrier mobility, the typical Hanle precession measurements yield spin diffusion times (τs) in the order of τs ～ 0.1-1 ns (at low temperatures), which is several orders of magnitude below the theoretical estimates based on the expected low intrinsic spin-orbit coupling in graphene. The results are weakly dependent on whether graphene is deposited onto SiO_2 or boron-nitride substrates or is suspended, with the mobility spanning 3 orders of magnitude. On the other hand, extraction form two-terminal magnetoresistance measurements, accounting for contact effects results in τs ～ 0.1μs, and corresponding diffusion lengths of about 100 μm up to room temperature. Such discrepancy jeopardizes further progress towards spin manipulation on a lateral graphene two-dimensional platform. After a presentation of basic concepts, we here discuss state-of-the-art literature and the limits of all known approaches to describe spin transport in massless-Dirac fermions, in which the effects of strong local spin-orbit coupling ceases to be accessible with perturbative approaches. We focus on the limits of conventional views of spin transport in graphene and offer novel perspectives for further progress.