\documentstyle [12pt,psfig]{article} \addtolength{\evensidemargin}{-6ex} \addtolength{\oddsidemargin}{-6ex} \addtolength{\textwidth}{+14ex} \addtolength{\textheight}{+29ex} \setlength{\parindent}{+5ex} \setlength{\topmargin}{-5em} \def\baselinestretch{1.0} \begin{document} \title{Possible 400 K superconductivity of carbon nanotubes} \author{C.Y. Yau\\ \\ Department of Physics \\ University of Cincinnati \\ Cincinnati, Ohio 45221\\[1em]} \date{March 6, 2002 } \maketitle \begin{abstract} Room temperature superconductivity is a goal of scientists and engineers dealing with superconductors. As room temperature superconductivity is very important to electronic, levitation and national defense technology, achieving room temperature superconductivity can cause a big change to the world. Recently Zhao \textit{et al} [G.M. Zhao, Y.S. Wang,arXiv:cond-mat/0111268 v2 19 Nov 2001] has reported results of measurements of superconductivity of carbon nanotubes and yield a $T_{c}$ higher than room temperature. In this paper, I will review their results and the results published by other groups on same research field. \\[1em] \end{abstract} \pagebreak \def\baselinestretch{1.1} Superconductivity phenomenon accounts for disappearance of resistance and total expulsion of magnetic (so-called Meissner effect) at transition temperature $T_{c}$. Type I and type II superconductors behave differently under magnetic field. For type I, or low $T_{c}$ superconductor, the magnetic field is completely expelled from the inside when the temperature is higher than $T_{c}$. However for type II superconductors, or high T$_{c}$ superconductors, they will have magnetic fields trapped inside and will not expel them immediately when temperature is higher than $T_{c}$. This is due to the flux pinning at the grain boundaries of high $T_{c}$ superconductors.Flux pinning makes the flow of supercurrent unimpeded and let the supercurrent flows for a temperature \section{What is carbon nanotube} Carbon nanotube (CNT) was first discovered by Sumio Iijima [1] in 1991. It is a tolled-up graphite sheets with caps on both ends. The tube is composed of hexagons of carbons on the tube with pentagons and heptagons (which are called ``defects'') on both ends. There are 3 types of structures of carbon nanotubes, viz. armchair, zigzag and chiral tubes. They have different electronics structures. Armchair tubes have zero bandgap and hence a metal, zigzag has a bandgap of 1.1 eV like silicon as a semiconductor, and chiral tubes have the bandgap between those two former tubes. Then the electronic properties of carbon nanotubes depend on the molecular structure of carbon nanotubes. The nanotubes can have several shells. And the structure is generally distinguished to be single-walled nanotube (SWNT) and multi-walled nanotubes (MWNT). SWNT is usually a defectless version of carbon nanotubes. In MWNT there are many cross-links or bonds between the shells, and the structure in MWNT is not always perfect. Hence a MWNT is usually called a defective nanotube. \section{Proximity effect induced superconductivity of CNT} Proximity induced superconducting behaviors of Single molecule SWNT and ropes of SWNTs are first reported by Kasumov [2] and the $T-{c}$ of CNT was recorded as < 1K for ropes (as shown in Fig. 2) and $\sim$ 0.4 K for single molecule of SWNT (as shown in Fig. 3). Proximity effect induced superconductivity makes use of S/N/S junction with the sample in the middle N layer, as shown in Fig 1. \begin{figure}[htb] \centerline{\psfig{figure=fig1.ps,height=1.5in}} \caption{\em{Proximity induced superconductivity measurement of ropes.}} \end{figure} \begin{figure}[htb] \centerline{\psfig{figure=fig2.ps,height=1.5in}} \caption{\em{R-T curve of proximity induced superconductivity ropes}} \end{figure} \begin{figure}[htb] \centerline{\psfig{figure=fig3.ps,height=1.5in}} \caption{\em{Arrangement of proximity induced superconductivity measurement of single SWNT.}} \end{figure} As the inter-tube Josephson tunneling (or coupling) effect can cause the 1D superconductor becomes 3D superconductor and increase the $T_{c}$, The $T_{c}$ of the rope may not be accurate and could be higher than the actually value of a single tube. Also the inset in Fig. 3 shows the hallmark of type I superconductor and this confirms why the $T_{c}$ of the samples are so low. \begin{figure}[htb] \centerline{\psfig{figure=fig4.ps,height=2in,}} \caption{\em{R-T curve of single molecule of SWNT. The inset shows the H-T curve indicates a type I superconductor behavior of single molecule of SWNT.}} \end{figure} Kasumov's results were further confirmed by Morpurgo's group [3]. Kasumov [4] also confirmed his result was not due to the superconducting contact pads by introducing defects in nanotubes by electron irradiation during $R-T$ measurement and caused an increase of 41 ohm in resistance after irradiation. Tang's group [5] reported a $T_{c}< 20$ K of individual single molecule of SWNT by Meissner effect and showed that SWNT could be a type I superconductor. Kociak \textit{et al} [6] reported a $T_{c}$ of 0.2 which matches the result of 0.4 K of Kasumov's group. \section{400 K or more superconductivity of carbon nanotubes} In November 2001, Zhao et al [7] reported a larger than 400 $T_{c}$ of multi-walled carbon nanotubes. The arrangement of the R-T measurement is shown in Fig 5. \begin{figure}[htb] \centerline{\psfig{figure=fig5.ps,height=1.5in}} \caption{\em{Suspended arrangement for R-T measurement of rope of MWNTs with silver contact.}} \end{figure} The reported R-T results as shown in Fig. 6 fits an insulating behavior of semiconductors. However, after eliminating the signals of insulators, the sample shows a resistance drop like an inhomogeneous superconductor, as shown in Fig. 7. \begin{figure}[htb] \centerline{\psfig{figure=fig6.ps,height=2in}} \caption{\em{R-T curve of sample showing the insulating behavior.}} \end{figure} \begin{figure}[htb] \centerline{\psfig{figure=fig7.ps,height=2in}} \caption{\em{After eliminating the signals of insulators, the curve shows a resistance drop similar to inhomogeneous superconductors with $T_{c,mid}$ $\sim$ 650 K}} \end{figure} Zhao also measured the remnant magnetic field of the sample with increasing temperatures. And they found the $T_{c}$ $\sim$ 400 K as shown in Fig. 8, consistent with the data shown in the $R-T$ curve. If the sample is ferromagnetic, the curve will drop very quickly instead of slowly. Then this eliminates the possibility of the ferromagnetic properties of samples. The remnant magnetization would probably totally disappeared at $\sim$ 800K. The $T_{c}$ at 400 K is supposed by the group to be the accepted result for the sample. However, the $T_{c}$ is too high for many scientists to accept. A question normally arises: will there be any unknown effect causing this big change? \begin{figure}[htb] \centerline{\psfig{figure=fig8.ps,height=2in}} \caption{\em{Measurement of remnant magnetization with increasing temperature. If the sample is ferromagnetic, the trend of the curve will go like the bold solid line that the M$_{r}$ will drop quickly.}} \end{figure} \section{Conclusion} The data of proximity-induced superconductivity cannot prove the nanotubes are superconductors. Josephson tunneling effect will eliminate the 1D nature of nanotubes and convert it into 3D nature and this can bring forth a much higher $T_{c}$. Kausmov's data are undeniable, but we will question: will there be any unknown effects causing this big change? Further experiments will be highly encouraged to confirm the results. \begin{thebibliography}{99} \bibitem{refA} S.\ Ijima, Nature\ {\bf 354}, 56(1991) \bibitem{refB} A.\ Yu. Kasumov et al, Science\ {\bf 284}, 1508(1999) \bibitem{refC} A.F.\ Morpurgo ,et al,Science\ {\bf 286}, 263(1999) \bibitem{refD} A.\ Yu. Kasumov, TNT 2001, September 3-7, Segovia-Spain \bibitem{refE} Z.K.\ Tang, et al. Science {\bf 292}, 2462(2001) \bibitem{refF} M.\ Kociac, et al, Phys. Rev. Lett. \ {\bf 86}, 2416(2001) \bibitem{refG} G.M.\ Zhao, Y.S.\ Wang, arXiv:cond-mat/0111268 v2 19 Nov 2001 \end{document}