* Title: Carbon Nanotube Superconductivity: An Update
* Speaker: Prof. Ping Sheng
* Affiliation: Hong Kong University of Science and Technology
* Date: July 6(Mon) PM 3:00
* Place: Hogil Kim Memorial Bldg. 512, POSTECH
* Hosted by APCTP, POSTECH Center for Theoretical Physics
Superconductivity in carbon nanotubes is a topic of intriguing interest. While the small-diameter nanotubes are predicted to have enhanced electron-phonon coupling--a key element responsible for nanotube superconductivity--the associated increase in fluctuation effects is unfavorable to the manifestation of a superconducting transition. The possibility of a Peierls transition in thin nanotubes is a further deterrent to superconductivity. It follows that the existence of coupling between the nanotubes is important to the realization of its superconducting behavior, since the transverse coherence can suppress fluctuations and lower the Peierls transition temperature, thereby making the appearance of a superconducting transition possible. Owing to its ordered and closely-spaced pore structure (with a 13.6 Å center-to-center separation between the 0.7 nm diameter pores), the AFI zeolite (composition: Al12P12O48) with embedded 4-Å carbon nanotubes constitutes an ideal material for the observation of nanotube superconductivity.
In this talk I will describe the eight-year effort that resulted in the two new developments in 2007 (on improved sample fabrication) and 2008 (on making electrical contacts), leading to reasonably easy observation of nanotube superconductivity with high reliability and repeatability. In particular, we have measured the specific heat signal for the superconducting transition as well as the resistive superconducting transition. The coupled carbon nanotube arrays can yield both nearly 1D superconducting behavior as well as the Josephson array behavior, with their attendant magnetic characteristics. The overall physical picture that emerges is that of a coupled Josephson array consisting of aligned nanotubes crossing over from an individually fluctuating 1D system to a coherent 3D superconductor, mediated by a Kosterlitz-Thouless (KT) transition which establishes quasi long range order in the lateral plane perpendicular to the c-axis of the nanotubes. The attainment of global coherence is seen at 5K and below, accompanied by the appearance of a well-defined supercurrent gap at 2K. While the existence of the superconducting transition in nanotube arrays is now beyond reasonable doubt, there are still many aspects of the data which have yet to be theoretically understood. I will present some important lessons learned from the eight-year effort and what they tell us about the mesoscopic electronic states in most carbon nanotubes. In particular, these lessons may shed light on why nanotube superconductivity was not observed and/or confirmed earlier.
 Superconducting Characteristics of 4 Ǻ Carbon Nanotube--Zeolite Composite, Lortz, R., Zhang, Q. C., Shi, W., Ye, J. T., Qiu, C. Y., Wang, Z., He, H. T., Sheng, P., Qian, T. Z., Tang, Z. K., Wang, N., Zhang, X. X., Wang, J. N. & Chan, C. T., PNAS 106, 7299-7303 (2009).
 Meissner Effect in a System of Coupled One-Dimensional Superconducting Wires Monte Carlo Simulations, C. Qiu, T, Qian and Ping Sheng, Phys. Rev. B75, 024504 (2007).
 Superconductivity in 4 Angstrom Single-Walled Carbon Nanotubes, Z. K. Tang, L. Zhang, N. Wang, X. X. Zhang, G. H. Wen, G. D. Li, J. N. Wang, C. T. Chan and Ping Sheng, Science 292, 2462-2465 (2001).