Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. However, its correlation with carrier doping has not been reported. We report on the Meissner effect found in thin films consisting of assembled boron (B)-doped single-walled CNTs (B-SWNTs). We find that only B-SWNT films consisting of low boron concentration leads to evident Meissner effect with Tc=12 K and also that a highly homogeneous ensemble of the B-SWNTs is crucial. The first-principles electronic-structure study of the B-SWNTs strongly supports these results.

We report Meissner effect for type-II superconductors with a maximum T c of 19 K, which is the highest value among those in new-carbon related superconductors, found in the honeycomb arrays of multi-walled CNTs (MWNTs). Drastic reduction of ferromagnetic catalyst and efficient growth of MWNTs by deoxidization of catalyst make the finding possible. The weak magnetic anisotropy, superconductive coherence length (∼ 7 nm), and disappearance of the Meissner effect after dissolving array structure indicate that the graphite structure of an MWNT and those intertube coupling in the honeycomb array are dominant factors for the mechanism.New carbon-based superconductors, such as calcium -intercalated graphite with a transition temperature (T c ) of 11.5 K (1,2) and highly boron-doped diamond with T c = 4 K (3), have been recently found and attracted considerable attention, because a small mass of carbon may lead to high-T c superconductivity. Superconductivity in carbon nanotubes (CNTs), which are typical nano-carbon materials, has also attracted increasing attention (4-7). Three groups have experimentally reported superconductivity in different kinds of CNTs as follows; 1. with a T c as low as 0.4 K for resistance drops (T cR ) in ropes of single-walled CNTs (SWNTs) (4)(6), 2. with a maximum T cR as high as 12 K for an abrupt resistance drop in arrays of our multi-walled CNTs (MWNTs) entirely end-bonded by gold electrode (7), and 3. with a T c of 15 K for magnetization drops (T cH ) in thin SWNTs with diameters as small as 0.4 nm (5). However, no groups could report the observation of both the Meissner effect and the resistance drop down to 0 Ω in their respective systems (21), although these are indispensable factor to identify the occurrence of superconductivity in any materials. Moreover, it is extremely interesting to reveal how shielding current for Meissner effect or vortex occur and behave in one-dimensional space of CNTs. One of main reasons for difficulty in observation of Meissner effect in CNTs is that most high-quality CNTs have been synthesized using ferromagnetic catalysts (e.g., Fe, Co, Ni) in previous studies. Such catalysts remain in the CNTs even after synthesis in some cases and destroy superconductivity. Hence, reduction of amount of the catalysts remaining after the synthesis of CNTs is crucial for observation of superconductivity.In the present study, we have synthesized honeycomb arrays of high-quality MWNTs in nanopores of alumina template (Al 2 O 3 ) by chemical Figure 1: (a) Schematic cross-sectional view of the sample, which consist of aluminum substrate, nano-porous alumina membrane, MWNTs in the nano-pores, and Fe/Co catalyst remaining in some parts. (b) -(d) : SEM top view images of nano-porous alumina templates with array of MWNTs, which were synthesized using the smallest volume of Fe/Co catalyst (b) without deoxidization, (c) with deoxidization at 550 o C for 1 hour and (d) for 4 hours right before CVD process.

We show formation of paperlike thin films ͑Buckypaper͒ consisting of pseudo-two-dimensional network of individual boron-doped single-walled carbon nanotubes ͑B-SWNTs͒, by sufficiently dissolving as-grown ropes of SWNTs and densely assembling them on silicon substrate. We find superconducting transition at temperature ͑T c ͒ of 8 K under absent pressure and that it can be induced as large as 2.4 times up to 19 K by applying a small pressure of 20 MPa. A significant frequency increase in the radial breathing phonon mode ͑RBM͒ is also found with applying pressure in Raman spectrum measurements. Moreover, B-SWNTs with a diameter of 0.6 nm are confirmed in the Buckypaper. These imply the strong correlation of the pressure-induced T c with electron-phonon ͑RBM͒ coupling of 0.6-nm-diameter B-SWNTs.

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We show creation of paperlike thin film ͑Buckypaper͒ consisting of pseudo-two-dimensional network of boron-doped single-walled carbon nanotubes ͑B-SWNTs͒ within weakly intertube van der Waals coupling ͑IVDWC͒ state. It was formed by sufficiently dissolving as-grown ropes of B-SWNTs and densely assembling them on silicon substrate. We find that superconducting transition temperature ͑T c ͒ of 8 K under absent pressure can be induced up to 19 K by applying a small pressure to the film and that a frequency in the radial breathing phonon drastically increases with applying pressure. Discussion about IVDWC and distribution of B-SWNTs diameter imply the strong correlation.