Tombros Nikolaos, Jozsa Csaba, Popinciuc Mihaita, Jonkman Harry T, van Wees Bart J
Physics of Nanodevices, Zernike Institute for Advanced Materials, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
Nature. 2007 Aug 2;448(7153):571-4. doi: 10.1038/nature06037. Epub 2007 Jul 15.
Electronic transport in single or a few layers of graphene is the subject of intense interest at present. 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, supercurrent transport has also been observed. Graphene might also be a promising material for spintronics and related applications, such as the realization of spin qubits, owing to the low intrinsic spin orbit interaction, as well as the low hyperfine interaction of the electron spins with the carbon nuclei. Here we report the observation of spin transport, as well as Larmor spin precession, over micrometre-scale distances in single graphene layers. The 'non-local' spin valve geometry was used in these experiments, employing four-terminal contact geometries with ferromagnetic cobalt electrodes making contact with the graphene sheet through a thin oxide layer. We observe clear bipolar (changing from positive to negative sign) spin signals that reflect the magnetization direction of all four electrodes, indicating that spin coherence extends underneath all of the contacts. No significant changes in the spin signals occur between 4.2 K, 77 K and room temperature. We extract a spin relaxation length between 1.5 and 2 mum at room temperature, only weakly dependent on charge density. The spin polarization of the ferromagnetic contacts is calculated from the measurements to be around ten per cent.
目前，单层或几层石墨烯中的电子输运是一个备受关注的课题。石墨烯独特的能带结构，具有独特的能谷结构以及将空穴态与电子态分开的狄拉克中性点，导致了新的电子输运现象的观测，如反常量子化霍尔效应、弱局域化的缺失以及最小电导率的存在。除了耗散输运外，还观测到了超流输运。由于低本征自旋轨道相互作用以及电子自旋与碳核的低超精细相互作用，石墨烯也可能是自旋电子学及相关应用（如实现自旋量子比特）的一种有前途的材料。在此，我们报告在单层石墨烯中微米尺度距离上自旋输运以及拉莫尔自旋进动的观测结果。在这些实验中使用了“非局域”自旋阀几何结构，采用四端接触几何结构，铁磁钴电极通过薄氧化层与石墨烯片接触。我们观测到清晰的双极（从正号变为负号）自旋信号，该信号反映了所有四个电极的磁化方向，表明自旋相干在所有接触下方延伸。在4.2 K、77 K和室温之间，自旋信号没有显著变化。我们在室温下提取出1.5至2微米之间的自旋弛豫长度，仅微弱地依赖于电荷密度。根据测量结果计算出铁磁接触的自旋极化率约为10%。
