Coherent Control of Quantum Phase in Single- and Multi-Wall Carbon Nanotubes Combined System

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The low temperature magnetoresistance (MR) associated with the nano-scale contact to a single-walled carbon nano-tube (SWNT), has been studied for multi-walled carbon nano-tube (MWNT) FET devices fabricated by SPM-manipulation. In a case of SWNT, the rounding configuration of the surface spin is expected to produce a nano-scale effect for the transport as well as a Rashba type potential. Such an effect should influence the weak localization MR behavior related to quantum coherence of the electron waves in the carbon nano-tube (CNT). In previous years, we have reported AAS type oscillations in the MR at a similar nano-scale contact formed by the crossing of two MWNTs. However, in this year, anti-localization type oscillations have been observed in a hybrid contact between a SWNT and a MWNT in spite of absence of spin-orbit interaction for carbon. In our project, two kinds of the hybrid devices have been prepared for our MR measurements. These are a short bar or a single ring of a SWNT bundle that lies beneath the current path of a MWNT. Low temperature MR measurements have been performed using He-3 cryostat. With change of the source-drain bias, a negative and/or positive MR peak can be observed in the same hybrid CNT device. In the case where there is no underlying SWNT, two or three kinds of AB oscillations have been observed in the low temperature MR of one MWNT wire. The coherence of the electron spreads over the whole region of the MWNT between the voltage lead contacts. Therefore, the results show that underlying SWNT is strongly affected to the quantum coherent property of MWNT. Such interference behaviors in the both CNTs should be utilized as a powerful tool on the analyzing of nano-scale weak localization in a small spin concentration system of the SWNT.