(By: LION/EA)
Nature published an article on a paradoxical discovery in superconductivity. Leiden physicist Jan Zaanen wrote a News & Views article about this article in the same issue of August 19.

Superconductivity is a bizarre but useful physical phenomenon. By cooling a material down to below a critical temperature, its electrical resistance suddenly disappears completely. That way, you can easily send electricity through a wire without any loss of energy. This comes in very handy for example in windmills or MRI scanners.

News & Views
The cooling however poses a problem, which finally does cost energy. That is why physicists are on the hunt for a material with superconductive behavior at not-too-low temperatures. In a News & Views article in Nature, Leiden physicist Jan Zaanen describes how a new discovery leads to an interesting paradox.

Pairs
The discovery, made by Ivan Božović from Yale University, has to do with copper oxides. In principle these don’t conduct any current. Their electrons have too strong of an interaction and retain each other, like cars in a traffic jam. But taking away some electrons gives the rest some room to manoeuver. They do this in pairs. These electron pairs can actually move so well that superconductivity occurs, even though the temperature is relatively high.

Paradox
By removing too many electrons, a surplus of empty spots arises. Electrons now have difficulty finding each other to form pairs, and the value for the critical temperature drops. Everything seems to indicate that the famous Bardeen-Cooper-Schrieffer (BCS) theory from 1957 is applicable in this case. This theory describes quantum physics of conventional superconductors very precisely. BCS counter-intuitively predicts that all electrons take part in superconductivity, even if the critical temperature value is extremely low. However, Božović sees the number of participating electrons diminish proportionally to the critical temperature value. Like Zaanen describes, this as an apparent paradox which cannot be explained with our current understanding of quantum physics.