– 35 –
group of N.O. Birge from the Michigan State University (MSU). With the RHUL group, the attention was both in the thermoelectric response of superconducting proximity systems and in the nonequilibrium properties of multi-terminal normal-superconducting systems. Driving the system out of equilibrium with an external control current, the Josephson junctions realized by pairs of normal-superconductor contacts may turn into the pi-state where the supercurrent flows opposite to the gradient of the order parameter phase. In multi-terminal setups, it is possible to create many such pi-states, and depending on the range of control currents, a different combination of “0” and pi-states is possible (see Fig. 1 below). To our great regret, our fruitful collaboration with the RHUL group was terminated by the completely unexpected death of Dr Igor Sosnin in August 2007. We are all grateful to him for the many years of collaboration.
Our project with MSU continued with an article on nonequilibrium supercurrent and especially the effect of the supercurrent on the nonequilibrium state of the system. In this work, we also studied in detail the effect of a magnetic field on the SNS supercurrent in a previously unexplored regime.
We have generalized our earlier work on supercurrent transistors to the case of an all-superconducting SIS’IS system. Here S and S’ mark two different superconductors and I is an insulating layer between them. One interesting aspect of this work, not yet found experimentally, is the nonequilibrium-induced hysteretic superconductor-normal metal transition.
When discussing nonequilibrium or thermal effects in nanostructures, a very relevant topic is the characteristics of energy relaxation. At low temperatures the electrons and phonons are effectively decoupled and additional relaxation mechanisms become important. It was found that especially relevant is the coupling between the electrons and the surrounding electromagnetic modes. We showed how the heat current between the two systems can be calculated directly from the microscopic Hamiltonian and showed how this relaxation mechanism works in a nonequilibrium system.
Very recently, we have suggested a novel type of a radiation detector based on the strong temperature dependence of the supercurrent in superconductor-normal metal-superconductor (SNS) systems. This “Proximity Josephson Sensor” seems to outperform the widely used superconducting transition edge sensors in some device aspects. This work was done together with the VTT Quantronics group, and groups from SNS-Pisa and University of Naples.
CURRENT FLUCTUATIONS AND THEIR MEASUREMENT
Tero Heikkilä, Teemu Ojanen, and Pauli Virtanen
Collaborators: the NANO and PICO groups, and M. P. Stenberg (Laboratory of Physics, TKK)
Our research on the nature of current fluctuations in mesoscopic systems concentrated during 2007 on the study of two effects: the effect of supercurrent on the nonequilibrium shot noise in multi-terminal normal-superconducting systems and to the study of the contact effects on the noise in carbon nanotubes, the latter in collaboration with the NANO group. In the work on carbon nanotubes, by taking into account the contact effects on the noise we were able to show that the intrinsic Fano factor of the disordered multi-wall nanotubes is very small, in contrast to previous
Annual Report 2007