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Fig. 3. a) Correlation between critical current IC and normal state conductance GN. Black and red dots represent Fabry-Perot regime and Kondo regime, respectively. Lines are theoretical fits with 2-channel Breit-Wigner model, with I0 = 4.5nA for F-P and I0 = 3nA for Kondo. b) Critical current IC versus Kondo temperature TK. The Kondo temperature is taken from the width of normal state zero-bias conductance.

SEMICONDUCTING NANOTUBE FET

ENS-group, P. Hakonen, and L. Lechner

In collaboration with ENS (Paris), we have worked on microwave operation of top-gated single carbon nanotube transistors. From transmission measurements in the 0.1-1.6 GHz range, we obtained a large and frequency-independent transconductance gm ~ 20 S on short devices, which meets the best dc results. The capacitance per unit gate length of 60 aF/m is typical of top gates on a conventional oxide with ~ 10. This value is a factor of 3-5 below the nanotube quantum capacitance which, according to recent simulations, favors high transit frequencies fT = gm/2Cg. For the smallest devices, we find a large fT ~ 50 GHz with no evidence of saturation in length dependence.

FABRY-PEROT CARBON NANOTUBE RF-SET

S. Andresen, R. Danneau, , P. Hakonen L. Lechner, and F. Wu

In addition to regular rf-SET operation of SWNT quantum dots, we have investigated similar rf operation of nanotubes in the Fabry-Perot regime. We have reached an excellent charge sensitivity of 2.0.10-6 e/Hz1/2 with a carrier frequency of 715 MHz over a bandwidth of 80 MHz at 4.2 K. Unlike previously demonstrated SETs in the Coulomb blockade regime, our device can, however, work as an electron interferometer up to temperatures of 20 K above which the dephasing length becomes gradually too short.

Our device consists of a CVD-grown single wall carbon nanotube contacted by superconducting electrodes. To prevent parasitic capacitances that would limit the rf-operation, the SWNT is grown from patterned catalyst islands on an insulating (sapphire) substrate using an Al2O3-insulated gate electrode deposited on top of the tube. We obtain a charge sensitivity of δq = δVg/(2*SNR*BW)1/2 = 2.0.10-6 e/Hz1/

Annual Report 2007

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