A. Anthore, J. Kivioja, M. Meschke, T. Nieminen, A. Niskanen, J. Pekola, A. Savin, A. Timofeev
Visitors: F. Giazotto, F. Hekking
We investigate mesoscopic physics and its sensor applications. The main focus is on charge transport and thermal properties of both metallic and semiconducting nano- and microstructures. Particular research topics include electronic cooling, nonequilibrium in electronic nanostructures, (nano)thermometry, small superconducting (Josephson) junction devices, quantized and coherent charge pumping, and noise and full counting statistics of charge transport. Samples and devices are fabricated in the clean rooms of the Low Temperature Laboratory and of Micronova centre for micro- and nanotechnology, experiments at low temperatures (0.01 - 4 K) are performed both in Micronova building and in the Low Temperature Laboratory.
F. Hekking, J. Kivioja, M. Meschke, T. Nieminen, J. Pekola
Josephson junctions switch from a superconducting state into a dissipative normal state when approaching their critical current. We have investigated how the cross-over from thermally activated switching into macroscopic quantum tunnelling is modified in Josephson junctions and SQUIDs with low critical current. We have observed, for the first time, a new regime, which we call underdamped phase diffusion in switching dynamics of intermediate Josephson junctions. Also, we have observed how quantum levels in a metastable potential influence the cross-over from tunnelling behaviour into thermal activation. This work is done in collaboration with CNRS Grenoble.
A. Anthore, F. Giazotto, T. Heikkilä, F. Hekking, J. Pekola, M. Meschke, A. Savin
Based on our encouraging results in refrigerating electrons down to about 50 mK using standard tunnelling refrigerators (NIS refrigeration), we have now focused on finding ways to achieve even lower temperatures. This can be accomplished by employing superconductors with lower critical temperature than the commonly used aluminium, and by improving thermalization of the superconducting reservoirs at the secondary side of the refrigerator. Another focus in this project is to find ways to create far from equilibrium electron energy distributions to exploit them in refrigerator-controlled cold electron transistors. Thermometry at the low end of the temperatures achieved by the microrefrigerators is problematic, because no calibration can typically be safely extrapolated into this regime: Coulomb blockade and noise measurements are now prime candidates to provide a way to determine temperatures in this regime.
J. Kivioja, A. Niskanen, J. Pekola
Single-electron pumps are known to produce extremely accurate current sources but their yield in terms of maximum achievable current is very low, far too small to be applied in metrology. Josephson pumps can produce larger current, but up to now they have suffered from leakage current, which is a consequence of coherent tunnelling in superconductors. We have recently performed experiments on a new type of a superconducting charge pump, which combines the high speed (current) and low leakage by making benefit of combining flux and charge control in a mesoscopic superconducting circuit. An improved current pump can be created by replacing the tunable single Josephson junctions by Josephson junction arrays in the device. Experiments on these structures are to be run soon. This is a joint effort between LTL, VTT Information Technology and Mikes.
J. Kivioja, M. Meschke, T. Nieminen, J. Pekola, A. Savin, A. Timofeev
The hysteretic Josephson junctions described above have been employed in 2004 in two experiments as non-invasive ammeters. In one of them, shot noise of a mesoscopic conductor could be measured by detecting the resonant activation of a Josephson junction due to fluctuations of current at the plasma frequency of the Josephson junction. This will provide a way to perform spectroscopic noise measurements in the future. In another experiment, topologically protected tetrahedral Josephson junction qubits have been measured by escape techniques: this project has been recently started in collaboration with ISSP and Landau Institutes in Chernogolovka.
In a European FP6 project “RSFQubit” (2004-2006), Josephson junction control and readout circuits for very low temperature operation are being developed. Our group participates in this project with aim to down-scale the device parameters to reduce the heat dissipation to a level tolerable at temperatures around 50 mK.
Finally, based on our long-term experience in thermal properties of mesoscopic structures and thermometry, a metrological Coulomb blockade thermometer is being developed in collaboration with Mikes and TKK Microfabrication group.