Large oscillating non-local voltage in multi-terminal single wall carbon nanotube devices

Spin field-effect transistor has been recently realized in single wall carbon nanotube (SWCNT) devices contacted with NiPd alloy [1]. In order to separate charge related effects from that of pure spin transport we measure a non-local voltage in SWCNTs by using a four-terminal structure. The four contacts divide the tube into three quantum dots (QD) which we control by the back-gate voltage V$_{g}$. We inject the current through the first QD by using excitation voltage of 200 $\mu $V and measure the non-local signal V$_{nl}$ across the third QD. We measure large \textit{oscillating} non-local voltage as a function of V$_{g}$ with amplitude of V$_{nl}\sim $ 2$\mu $V [2]. While the classical resistor model can account for the negative sign of the non-local voltage its large amplitude needs deeper understanding. We discuss the origin of this large non-local signal and its effect on the non-local spin transport measurements in this type of devices. [1] S. Sahoo, et al. Nature Phys. \textbf{1}, 99 (2005). [2] G. Gunnarsson et al., arXiv:0710.0365v1.