Injection of Polarized Spins and Anti-Localization Caused by Slight Doping of Heavy Impurities into One End of Carbon Na
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INJECTION OF POLARIZED SPINS AND ANTI-LOCALIZATION CAUSED BY SLIGHT DOPING OF HEAVY IMPURITIES INTO ONE END OF CARBON NANOTUBES Junji Haruyama, Izumi Takesue, and Tetsuro Hasegawa Aoyama Gakuin University, 6-16-1 Chitosedai, Setagaya, Tokyo 157-8572 JAPAN ABSTRACT Electrode atoms are slightly diffused, with only about 5% volume-ratio, into the top end of multi- walled carbon nanotubes (MWNTs), standing in nano-pores of porous Alumina membranes. Diffusion of light- mass materials (carbon and aluminum) leads to weak localization in the Altshuler-Aronov-Spivak (AAS) oscillations, which is qualitatively consistent with previous works on MWNTs. In contrast, we find that diffusion of heavy materials (gold and platinum) changes this weak localization into an anti- localization in the MWNT bulk. This effect is only observable when electrons are injected through the diffusion region, and undergo a π −phase shift in their electron waves, caused by polarized injection of spin- flipped electrons due to spin-orbit interaction in the diffusion-region of the MWNT bulk. INTRODUCTION Single-walled Carbon nanotubes are conducting molecular nanowires that exhibit a variety of mesoscopic phenomena. There is a renewed interest in these nanostructures because the ir characteristics are very sensitive to materials deposited onto the tube inner space[1]. In contrast, the physical properties of multi-walled Carbon nanotubes (MWNTs) have been interpreted only in terms of quantum phase interference effects of electron waves in the diffusive regime at the single- molecular level (e.g., weak localization (WL)[2]-[4], Altshuler-Aronov-Spivak (AAS) oscillations[5]-[9], universal conductance fluctuation[2], possible metal- insulator transition[10], and strong spin coherence[11]). The investigation of correlation of such interference phenomena with dopant materials in the tube inner space or the MWNT itself has so far received no attention. AAS oscillations and WL are the typical phase interference effects of electron waves observed in MWNTs. Constructive phase interferences in current paths encircling MWNTs with time-reversal symmetry leads to WL, when the sample size is smaller than both the phase coherence length and the localization length[12]-[14]. The presence of a resistance maximum and a negative magnetoresistance (MR) around zero- magnetic field reflect WL in the AAS oscillations[15]. Previous works on MWNTs reported only on this type of AAS oscillations. On the other hand, the presence of strong spin-orbit interaction (SOI) in thin metallic cylinders formed by heavy mass atoms leads to anti- localization (AL), reflected by a resistance minimum and a positive MR around zero-magnetic field, in the AAS oscillations[16]. This is because the SOI causes electron-spin flipping, thereby leading to change of the electron phase by π[17]-[19]. No group, however, has yet successfully observed this electron-spin flipping and AL in the AAS oscillations in MWNTs. In this report, we find that diffusion of heavy- mass atoms, deposited onto the top end of
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