Charging Effects in Niobium Nanostructures
Three types of metallic nanostructures comprising niobium were investigated experimentally; in all three types, electric transport at very low temperatures was governed by Coulomb blockade effects.
1. Thin film strips of niobium could be tuned into resistor strips by an electrochemical anodisation process, using microfabricated masks and in situ resistance monitoring. These resistors showed a transition from superconducting to insulating behaviour with increasing sheet resistance, occurring at a value approximately equal to the quantum resistance for Cooper pairs, h/(4e2).
2. Combining the anodisation technique with lateral size minimisation by shadow evaporation, devices in a single electron transistor-like configuration with two weak links and a small island between these were made. Direct evidence for the Coulomb blockade in the anodisation thinned niobium films was found when the transport characteristics could be modulated periodically by sweeping the voltage applied to a gate electrode placed on top of the structure.
3. Conventional single electron transistors with Al base electrodes, AlOx barriers formed in situ by oxidation, and Nb top electrodes were made by angular evaporation. The output current noise of such a transistor was measured as a function of bias voltage, gate voltage, and temperature. The low frequency noise was found to be dominated by charge input noise. The dependence of the noise on the bias voltage is consistent with self-heating of the transistor activating the noise sources.