d-wave pairing in superconducting organics

We now have significance experimental evidence which should end any debate about whether the superconductivity in organic charge transfer salts is unconventional.

There is a nice preprint
In-plane superfluid density and microwave conductivity of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br: evidence for d-wave pairing
S. Milbradt, A. A. Bardin, C. J. S. Truncik, W. A. Huttema, P. L. Burn, S.-C. Lo, B. J. Powell, D. M. Broun

They provide definitive measurements of the temperature dependence of the superfluid density and of the quasi-particle scattering rate. What is distinctly new and exciting about these very careful and precise measurements is the absolute determination of these quantities.

The results also highlight some of the similarities of the organics with the cuprates: a d-wave superconductor "derived" from an antiferromagnetic Mott insulator and that quasi-particle scattering is largely due to electron-electron interactions. Indeed, the key temperature dependences shown below are essentially the same as what is seen in the cuprates.

There are three clear experimental signatures of d-wave pairing:

• the linear temperature dependence of the superfluid density (see figure above)
• the T^3 dependence of the scattering rate in the superconducting state
• the absence of a BCS "coherence" peak in the conductivity near Tc

I particularly like the figure below. It shows how the quasi-particle scattering rate drops dramatically in the superconducting state because the electronic degrees of freedom  responsible for the scattering are being "gapped out" by the superconductivity.