In all finesse

The goal of the research, whose results have just been published (1), was therefore to determine precisely at what size the material loses its superconducting properties and where the transition lies between thermal fluctuations and quantum fluctuations. To achieve this – and this is a major first – Xavier Baumans and his colleagues from ULg and KUL first reduced the wire's fabrication limit to less than one nanometre! "It is important to realise that an atom is a tenth of a nanometre", the young researcher tells us excitedly. "A 10 nm wire is 100 atoms wide and a 1 nm wire is approximately 10 atoms wide!" How can this be achieved? Thanks to the phenomenon of electromigration, which is normally destructive but in this case, it worked to the advantage of the researchers.

This happens when a current is applied to conducting wires, in this case, an aluminium wire. The current causes the metallic ions to migrate inside the wire and the phenomenon is self-sustaining and runs away. The smaller the size of the wire, the more the current heats the wire because it is thinner and, therefore, the ions migrate more and the size reduces further, and so on. This phenomenon limits the lifetime of electronic circuits. "We used this to our advantage", Xavier Baumans explains. "We domesticated it by preventing runaway. A small current starts to make the ions migrate and reduces the size of the wire. If you're not careful, the wire can break in just a few seconds. By controlling it through a computer, we can regulate the current in the wire so that if we see indications of runaway, we can react in less than a thousandth of a second by reducing the value of the current in the wire; by doing this, we reduce the migration and the narrowing of the wire. Therefore, it is necessary to always maintain a sufficient current so that the ions migrate (otherwise, there is no reduction in the wire) but not too great so that the reaction doesn't run away (otherwise, the wire breaks)." This is one of the research's major contributions: the perfect mastery of the process to continuously reduce the size of a conducting wire.  

(1)Thermal and quantum depletion of superconductivity in narrow junctions created by controlled electromigration, Xavier D.A. Baumans et al. NATURE COMMUNICATIONS | 7:10560 | DOI: 10.1038/ncomms10560 |