Universal scaling relations for exotic superconductors

There is an interesting paper
Do organic and other exotic superconductors fail universal scaling relations?
S. V. Dordevic,  D. N. Basov, and C. C. Homes

It has been found previously that a wide range of superconductors obey certain scaling relations involving their superfluid density. These are a generalisation of a scaling between the superfluid density and transition temperature Tc that Uemura originally found for underdoped cuprates. They have been particularly promoted by these authors. But, in a 2005 PRL Frances Pratt and Steve Blundell argued that molecular superconductors did not obey them. In 2004 Ben Powell and I pointed out that a number organic transfer salts with relatively low Tc had much lower superfluid densities than the Uemura relation.

In this paper the authors stress how tricky it is to measure the superfluid density and the corresponding conductivity at the transition temperature. It is important to measure these quantities on the same sample with the same technique. [Their preferred technique is the microwave surface impedance]. They show that when this is done for a range of organics that the data then do lie on the universal curve shown below.

A few comments.

1. It remains to be shown whether the low-Tc organics of particular concern to Ben and I lie on the curve.
2. It is a worry to me that the elemental superconductors also lie on the universal curve. This suggests a non-exotic explanation. The authors flag this issue too. 
3. The issues here highlight the pro's and con's of listening to experimentalists, as I discussed recently.

I thank Ben Powell for bringing the paper and a helpful discussion about it.