Tuesday, October 16, 2012

Connecting the pseudogap to superconductivity in organics

For the cuprates an outstanding question concerns whether there is a connection between the pseudogap and superconducting states. The same question arises in organic charge transfer salts, although it has received little attention. For the organics evidence for the pseudogap is rather indirect, mostly coming from NMR, which shows a reduction in the Knight shift and the relaxation rate below about 50 K. There is no direct evidence of nodes in the pseudogap.

A recent PRB presents evidence from ultrasound measurements that there is an intimate connection between the two.
Symmetry-imposed signatures at the pseudogap crossover in κ-(BEDT-TTF)2X organic superconductors
by Mario Poirier, Maxime Dion, and David Fournier

The figure below shows the temperature dependence of different elastic moduli near the temperature at which the pseudogap opens.
A post from last year considered in detail an earlier paper by the same group [plus their theory colleague A.M. Tremblay] that considered the anisotropy in the acoustic anomalies at the superconducting transition.
Note that the anisotropies are the same for both transitions, suggesting that the coupling of the relevant order parameters to the different components of the elastic tensor is the same.

This earlier paper presented a theory for the elastic anomalies near the superconducting transition. This theory would mean that the pseudogap and the superconducting order parameter have the same symmetry.
My earlier post raised some concerns about the theory, including that it predicts a symmetry distinct from the B2g symmetry predicted by RVB and spin fluctuation calculations (and this recent experimental paper).
Hence, I think there is a need for a slightly different theory of the acoustic anomalies. Nevertheless, I don't think that will change the main conclusion of the current paper: that the pseudogap and the superconducting order parameter have the same symmetry.
Indeed, this was the last sentence of a 2005 PRL that Ben Powell and I wrote about an RVB theory of these materials.

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