Control of spin motion in DOS-controlled single-wailed carbon nanotubes

本研究的目的是(1)稳定控制单壁碳纳米管(SWNTs)中的态密度，以控制注入载流子；(2)在DOS控制的单壁碳纳米管中注入极化自旋。为了达到这两个目的，我们制定了三个重要的里程碑，即(A)提高基于SWNT的场效应晶体管的载流子注入效率，(B)通过载流子掺杂在FET中将SWNTs作为沟道层进行能带工程，(C)从铁磁电极向此类SWNTs中自旋注入。关于里程碑(A)和(B)，我们已经成功地用四氰基对奎诺二甲烷(TCNQ)和聚乙二烯亚胺制备了高性能和极性可控的SWNT-FET。关于里程碑(C)，我们首先用单壁碳纳米管制作了单电子晶体管，估计注入载流子(电子和空穴)的散射长度为200-300 nm。接下来，为了实现自旋注入，我们将电极从非磁体替换为铁磁体。此外，我们还引入了一种非局域四端磁阻测量方法，以获得可靠的测量结果。结果表明，在3.8K下，我们观察到了4个电极的单壁碳纳米管自旋阀的电阻滞后现象，其自旋相干长度估计为90 nm。

The purpose of this study is (1) stable control of density of states (DOS) in single-walled carbon nanotubes (SWNTs) in order to control injected carriers, (2) injection of polarized spins into DOS-controlled SWNTs. In order to achieve these two purposes, we have set 3 important milestones, that is, (a) increase of carrier injection efficiency in SWNT-based field effect transistors, (b) band engineering of SWNTs as a channel layer in FETs by carrier doping (c) spin injection into such SWNTs from ferromagnetic electrodes. Concerning the milestone (a) and (b), we have succeeded in fabricating SWNT-FETs with high performance and also polarity controlled SWNT-FETs using Tetracyano-p-quinodimethane (TCNQ) and polyethireneimine. Concerning the milestone (c), first we fabricated single-electron transistors using SWNT and estimated scattering length of injected carriers (electron and hole) to be 200-300 nm. Next we replaced electrodes from non-magnet to ferromagnet in order to achieve spin injection. In addition, we newly introduced a non-local 4-terminal magnetoresistance measurement method for obtaining reliable results. As a result, we have observed hysteresis in resistance in SWNT spin valve with 4 electrodes at 3.8 K. The spin coherent length was estimated to be about 90 nm.