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that nanotube FETs are so good that they even rival SETs as the best electrometers available today. In addition, high operation speed has been achieved in nanotube FETs.
SUPERCONDUCTING SWNT TRANSISTOR
R. Danneau, P. Hakonen, E. Kauppinen, P. Queipo, T. Tsuneta, and F. Wu
We have continued our investigations on proximity induced superconductivity in carbon nanotubes. We studied a SWNT sample contacted by superconducting Ti/Al leads, 300 nm apart from each other. Initially, the sample showed Fabry-Perot pattern with some asymmetry in the contact transparency (Fig. 2 a). The supercurrent, reaching a maximum of 4.5 nA, was found to vary periodically with the gate voltage. The results could be fitted using Breit-Wigner resonance model for the normal state conductance and phase diffusion model for describing the voltage biased IV-curve in the superconducting state.
We were able to reach the Kondo regime (i.e. intermediate contact resistance) with the very same sample by cycling the cryostat several times form 290 K to 4.2 K. In the Kondo regime, both the critical current and the zero-bias conductance were smaller than in the Fabry-Perot regime; a comparison of the critical currents over typical conductance resonances is displayed in Fig. 3 a. We also investigated the dependence of the critical current on the Kondo temperature TK. As seen in Fig. 3 b, there are two distinct branches of IC(TK). Theoretical explanation of these effects is lacking at present.
The multiple Andreev peaks are quite smeared in our SWNT Josephson junctions, though the value of the transmission coefficient at the contact interfaces should be favorable for their appearance. Therefore, we have studied excess current Iex, which is a quantity integrating over all orders of multiple Andreev reflections. We find a quite linear dependence between Iex and IC, as predicted by the Josephson weak link theory.
Fig. 2. Differential conductance versus bias voltage and gate voltage in (a) Fabry-Perot regime, and (b) Kondo regime. These have been obtained on the same sample in separate cooldowns.
Annual Report 2007