| The focus of this thesis is the study of superconducting resistive transition in 4-A carbon nanotube arrays embedded in the aligned, linear pores of AFI zeolite crystals. While superconductivity was discovered in this material system one decade ago, resistive transition was not observed at that time owing to inadequate sample quality. The present study remedies this situation with samples fabricated from a new approach. Two types of superconducting resistive behaviors were observed in different selective samples. The first is the quasi one dimensional (1D) fluctuation superconductivity, which exhibits a smooth resistance drop with decreasing temperature, initiating at 15 K. At low temperatures, the differential resistance also shows a smooth increase with increasing bias current. Both are not affected by a magnetic field up to 11 Tesla. Such manifestations are consistent with the 1D superconducting characteristics as qualitatively interpreted by the mechanism of thermally activated phase slips, within the framework of the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory. A competing mechanism with the 1D superconductivity, the Peierls distortion, is shown to manifest itself as a differential resistance peak (as a function of the driving current), appearing above a certain temperature. Theoretical calculations suggest the quasi-1D superconducting behavior to arise from thin arrays of (5,0) 4-A carbon nanotubes, each comprising a central member surrounded by at least six others, with a wall-to-wall separation of ∼1 nm as dictated by the molecular structure of the AFI zeolite crystal. These thin (5,0) nanotube arrays plausibly act as the critical electrical links bridging the measuring electrodes.;The second type of observed superconducting behavior is the one dimensional (1D) to three dimensional (3D) superconducting crossover transition. In four samples, the transition was observed to initiate at 15 K with a slow resistance decrease switching to a sharp, order of magnitude drop at ∼7.5 K. The latter exhibits anisotropic magnetic field dependence, and the differential resistance versus bias current show a rich variety of details that are consistent with the establishment of coherence, in stages, as the temperature is lower below 15 K. In particular, the resistance drop that starts at 7.5 K exhibits attributes that are consistent with the manifestations of a Berezinskii-Kosterlitz-Thouless transition that establishes quasi-long-range order in the plane transverse to the c-axis of the aligned nanotubes, thereby mediating a 1D to 3D crossover. The electrical transport data and their interpretation are supported by the magnetic and specific heat results obtained by the group of Prof. Rolf Lortz. |
