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P. Hakonen, J. Hassel, L. Korhonen, A. Puska, J. Sarkar, and H. Seppä
The first fully superconducting version of a Bloch oscillating transistor was measured in autumn 2007. The new design has a SIS junction on the base instead of a NIS junction. The resistance of the chromium thin film resistor in the collector was 40 kΩ, and the tunneling resistances of the junctions in the emitter and in the base were about 10 kΩ. The emitter consisted of two Josephson junctions in a SQUID-like geometry in order to tune the ratio EJ/EC.
Despite the low, not yet optimized chromium resistance, the device gave promising results when compared with the original devices. The collector-emitter IV curves showed hysteresis that was suppressed when EJ decreased. P(E) peak consisted of two peaks instead of one as in the previous measurements of the former design. To be exact, the peak became doubled when EJ increased to be of the same order as the EJ of the base junction. However, the operation region, meaning the region where the gain is highest, is reached with EJs of the same order. The highest observed current gain was about 30, when the base current was 200 pA. In noise measurements, 1/f noise seemed to increase when approaching the value of EJ at which the IV curves become hysteretic. At the operation point, the input referred current noise level was on the order of 2 fA/√Hz.
Fig. 1. Measured collector-emitter IV curves with different ratios of EJ/EC. The base current is 200 pA and EJ/EC is of the order of 1, being highest in the blue curve. Hysteretic behaviour disappears when EJ decreases.
QUANTUM TRANSPORT IN CARBON NANOTUBES AND GRAPHENE
During the year 2007, we have successfully expanded our carbon-based quantum electron transport to samples of graphene. In our measurements, we have concentrated on studies of shot noise and superconductivity, both in CNTs and graphene. The current noise measurements together with conductance can be interpreted in terms of “transmission channel fingerprints” that have been a primary analysis method in the investigations of atomic point contacts. Moreover, ballistic Josephson junction theories can be then employed to test the observed supercurrents against the transmission channel eigenvalues. We find a rather good agreement between experiment and theory in our shot noise studies, both in single walled carbon nanotubes (SWNTs) and graphene. In semiconducting samples, our results indicate
Annual Report 2007