project title: NIS junctions for microrefrigeration and thermometry
project leader: Dr. Herve Courtois
access given (in days): 40
access used (in days): 41
local host: Prof. Jukka Pekola
home institution: CNRS, Grenoble
country of institution: France
starting date (yyyy-mm-dd): 2008-02-20
The aim is to investigate experimentally the following two topics during Courtois' stay in Helsinki: (1) The influence of Andreev processes on NIS microrefrigeration. Thereare theoretical predictions but the picture on the experimental side is not yet clear. (2) SNS thermometry down to very low temperatures. The aim is to be able tomeasure temperatures below 10 mK using a long SNS Josephson junction.
(1) The experiments on Andreev processes in NIS junctions were conducted during the Dr. Courtois' visit. In particular, a systematic high resolution set of measurements on leakage was made in NIS junctions which had different physical dimensions, but which were fabricated in a single batch. The results indicated that the ballistic Andreev result holds only in the smallest tunnel junctions that we can fabricate. (2) We have made a set of experiments where SNS Josephson junctions of different lengths have been measured. The goals have been: (i) To identify the reason of hysteresis in SNS junctions, which has been a puzzle for some time for many research groups. By inserting a SINIS thermometer into the normal part of the SNS Josephson junction, we managed to demonstrate that overheating is the source of hysteresis. A manuscript on this measurement is under final preparation. (ii) We want to make SNS a thermometer down to 10 mK electronic temperatures. This work is going on, but we have already seen that in a rather long (2 um) SNS junction, one can expect to have sufficient temperature sensitivity down to 10 mK and below. The advantage of this thermometer is that it is dissipationless up to the switching current of the junction. We are quite optimistic that temperatures down to 10 mK can be reached by SINIS refrigeration and that they can be measured by a SNS Josephson junction.