Goldilocks on superconductivity

Manifesto for a higher Tc by Dimitri Basov and Andrey Chubukov is an interesting Perspective in Nature Physics. It contains a nice comparison of the new "iron pnictide" superconductors with the cuprates.

They identify three key questions:

(1) Do all high-Tc materials superconduct for the same reason? 

(2) Are the rather anomalous normal-state properties of exotic superconductors a necessary prerequisite for high-Tc superconductivity? 

(3) Is there a generic route to increase Tc? 

They claim that the answer to (1) is yes, exchange of spin fluctuations associated with nesting of different parts of the Fermi surfaces. I am not sure if the majority of people would agree with them on this point.

A few things the Basov and Chubukov also highlight

• The reduction of the kinetic energy by strong correlations deduced from the optical conductivity is a convenient way to characterise the strength of interactions. They claim a 50-70% reduction is optimum for a high Tc. There is a balance ["just right" as found by Goldilocks!]. Stronger interactions increase the pairing interaction but also decrease the mobility of the Cooper pairs.
• The connection between the superfluid density and the loss of low energy spectral weight in the optical conductivity. [Strong dissipation reflected in the Homes scaling where the superfluid density is proportional to the product of Tc and the intralayer dc conductivity.]

In passing, I note that the reduction of the kinetic energy has been measured and discussed for a family of superconducting organic charge transfer salts in this PRL. [Although for idiosyncratic historical/experimental reasons it is plotted as the effective number of charge carriers].

I thank Ben Powell for bringing the paper to my attention.