Development of tunnel junction thermometers and coolers

We have investigated possibilities to use novel superconducting materials for the NIS junctions. This is maasilta5.jpginteresting, since by increasing or decreasing the energy gap from the standard material, Al, thermometers and coolers can be developed which work better at higher of lower temperatures. We have succeeded in developing Nb [Nevala2012, Julin2016], NbN [Chaudhuri2013i], TaN [Chaudhuri2014] and TiN [Torgovkin2015] based NIS junctions, with most of them having energy gaps of the order of 1 meV as compared to 0.2 meV fro Al. Especially interesting are the future possibilities to develop NIS microrefrigerators for higher operational temperature than the current 0.3 K. Recently, we have also developed efficient NIS coolers where the normal metal consists of Ti-Au bilayer [Räisänen2018].  

The more complex nitride superconducting films NbN, TaN and TiN are typically deposited using infrared avalanche.jpgpulsed laser deposition (PLD) [Chaudhuri2011, Chaudhuri2013ii], but atomic layer deposition (ALD) was also used for TiN  [Torgovkin2015]. High quality PLD NbN films and devices have also been provided for collaborators at Oslo University for studies of thermomagnetic avalanches that occur when external magnetic fields penetrate into superconducting films [Mikheenko2016,Mikheenko2015,Mikheenko2013].

We have also discovered that by annealing Al/AlOx/Al tunnel junctions, they become more stable [Koppinen2007] and their 1/f-noise is lowered [Julin2010]. This might help to lower the decoherence in superconducting quantum bits in the future, where these kind of junctions are heavily used. Recent results also demonstrated that NIS-junction 1/f noise can be lowered by annealing, if the junction is made from manganese-doped aluminum (AlMn) as the normal metal and Al as the superconductor [Julin2016].

In terms of theoretical studies of NIS junctions, we have clarified the effect of non-idealities on their performance [Chaudhuri2012], as well as the effect of single-electron tunneling (charging) on NIS thermometry [Koppinen2009iii].