Record-sensitivity SET made using a semiballistic MWCNT http://boojum.hut.fi/nano/Nanotube.html
We have demonstrated that it is possible to construct ultra-low-noise single-electron transistors (SETs) using free-standing multiwalled carbon nanotubes. The 1/f-noise of our devices, 6 . 10-6 e/Sqrt{Hz} at 45 Hz, outperforms the best metallic SETs of today.
Our record device is based on a design in which we avoid contact of the central island with any dielectric material. For our device, this is achieved by using atomic force microscope to move a multiwalled carbon nanotube (MWNT) on top of two adjacent gold electrodes (See Fig. http://boojum.hut.fi/nano/tube_elmtr.jpg ). Vacuum brazing at 700 C for 30 sec was employed to embed the tube 6 nm into the gold. In the final structure, the MWNT (d =14 nm) has a 275 nm long free-standing section hanging at a distance of 17 nm above the substrate.
http://boojum.hut.fi/nano/joose.html
A Bloch oscillating transistor (BOT) is a new type of a mesoscopic transistor (three terminal device, see Fig. http://boojum.hut.fi/nano/bot2_www.jpg ) that combines single particle tunneling and Cooper pair tunneling. When a BOT resides on an upper band (superconducting junction is in a finite-voltage zero-current state), just single tunneling event (either clocked or spontaneous) in the normal-state junction triggers the device momentarily into Bloch-oscillating state (until Zener tunneling returns it to the upper band) so that a finite current pulse is obtained. According to the semiclassical simulations, a BOT provides high current gain (~ 10), large input impedance (Zin ~ 500 k), and a band width of 100 MHz. On the basis of thermal voltage noise of the base tunnel junction and the shot noise of the bias current, one can estimate <100 mK for the noise temperature of a BOT.
We have succeeded in making the first working BOTs. In our experimental realization of the BOT, the base electrode is connected via an SIN junction, the collector has a Cr-resistance of 50 k, and on the emitter there is a Josephson junction with EJ/EC ~ 1. In our experiments we find a significantly asymmetric IV-curve, the analysis of which indicates that the principle works. We obtain current gains of ~ 35 under the best biasing conditions. Our BOT-samples illustrate the state of the art manufacturing that we are able to make (see Fig. http://boojum.hut.fi/nano/bot1_www.jpg ).
In Josephson junctions, so called secondary (macroscopic) quantum effects are due to the operator nature of the phase and charge Q, which satisfy a canonical commutation relation. Hence, these two variables are related by the Heisenberg uncertainty relation Q ~ e. In other words, Coulomb repulsion results in the delocalization of the phase variable and, thereby, leads to the formation of energy bands as first shown by Averin, Likharev and Zorin in 1985.
We have investigated the energy bands of a Josephson junction using incoherent Cooper-pair tunneling as a probe. According to the environmental fluctuation theory, non-coherent Cooper pair tunneling is allowed only if energy is exchanged with the surroundings. As this is based on general principles, inelastic Cooper pair tunneling provides a good tool to do energy level spectroscopy also in a non-linear system made out of a tunable Josephson junction (SQUID loop) In our experiments, we find a sequence of resonance peaks in the subgap-conductance (See Fig. http://boojum.hut.fi/nano/spectrum.jpg ). These peaks are identified as coming from transitions between the excited states of a Josephson junction.
In Addition, there are planned activities on:
Electron-electron and electron-phonon interactions and energy relaxation in diffuse metallic and heavily doped silicon micro- and nanostructures at very low temperatures.
Coherent Cooper pair manipulations in networks of small Josephson junctions.
Solid state microcooling.
Under the direction of Prof. Jukka Pekola.
