ABSTRACT
Here, Bardeen-Cooper-Schrieffer (BCS)-type SC is a pheno-menon arising from attractive Coulomb interaction via phonon, which is in contradiction to the Coulomb repulsion leading to the TLL state. This leads naturally to the question of how the SC can develop in the CNTs, with the consequent transition from the TLL [7-11, 14]. The interplay between the TLL state and intrinsic SC has been only reported in our MWNTs with a significant number N of shells participating in the conduction [2]. The results were qualitatively consistent with low-energy theories of CNTs [8], which had already predicted the breakdown of the TLL at low temperatures in nanotube bundles with a strong intertube electrostatic coupling between the different 1D conduction channels.In the MWNTs, however, the interplay of the SC with the TLL and the value of Tc as high as 12 K still had to be quantitatively understood, as the samples were not intentionally doped and the number of 1D conduction channels was unknown. On the other hand, the high Tcvalue was in approximate agreement with Tc values of CaC6 and boron
(B)-doped diamond, which is approximately equal to 10 K. Indeed, low-energy theories of MWNTs have suggested the importance of having a large number of 1D conduction channels for the development of an SC instability [10]. Therefore, it is crucial to clarify the correlation between the appearance of the SC phase and carrier doping in MWNTs. B-doping has been actually successfully achieved following known methods [15,16].In this section, I describe about SC in array of the MWNTs and explain its correlation with TLL state [2]. I find an abrupt resistance drop due to SC at a Tc as high as 12 K in the MWNTs entirely end-bonded by metal electrodes. In contrast, the partially end-bonded MWNTs show very small resistance drop at low temperatures. It reveals that a gradual transition from TLL behavior to a SC regime can start only from low eV/kT values at low temperatures, but the development of the SC enhances the regime to high eV/kT region. Phase diagram obtained from low-energy theory reveals that such a transition is actually possible. Nuclear magnetic resonance (NMR) measurements ascertain the presence of substitutional boron doping for SC in the MWNTs. 3.2.2 Superconductivity in Entirely End-Bonded MWNTs
At first, I show the possibility of unintentional B-doping in our MWNTs, which were synthesized by chemical vapor deposition using Fe/Co catalyst and methanol gas in the nanopores of the alumina template [2,19]. Boron has been intentionally used only for the enhancement of the chemical reaction of the Fe/Co catalyst [2,19]. Figure 3.1 shows results of NMR measurements of our MWNTs with three different boron concentrations (NB) in the catalyst. Evident peaks can be observed at 0-5 ppm in individual samples. The peak position implies the presence of chemical bonds of B-C and, hence, substitutional B doping in the carbon network of the MWNTs. Moreover, at least five-times higher intensity of NMR signal peak for the B-C was detected in the B-MWNTs with NB = ∼2 atomic % (at.%), compared with other B-MWNTs with different NB values. Very importantly, we find that only the B-MWNTs with NB = ∼2 at.% showed SC with high Tc [Fig. 3.3(a)] and also present signature of SC [Fig. 3.3(b)] with the highest reproducibility. This
result implies that the most suitable condition for substitutional B doping into the MWNTs, which leads to appearance of the SC, is ∼2 at.%. This is consistent with our recent discovery [20] and a theory [21]. The small B concentration allows the CNTs remaining a 1D ballistic charge transport regime.
