Earlier posts have considered some of the subtleties, particularly in strongly correlated electron systems.
To me a couple of recent experimental papers present beautiful data but are not cautious enough in their interpretation. They need to rule out alternative explanations [see below] before I will be convinced of the explanations that they propose.
Fermi-surface reconstruction by stripe order in cuprate superconductors
F. Laliberté, J. Chang, N. Doiron-Leyraud, E. Hassinger, R. Daou, M. Rondeau, B.J. Ramshaw, R. Liang, D.A. Bonn, W.N. Hardy, S. Pyon, T. Takayama, H. Takagi, I. Sheikin, L. Malone, C. Proust, K. Behnia, and Louis Taillefer
They observe a sign change in the thermopower and associate this with a Fermi surface reconstruction, stripe formation, and a quantum phase transition.
Spin fluctuation driven enhancement of thermopower near the quantum critical point in Ba(Fe(1-x)Co(x))2As2
S. Arsenijević, H. Hodovanets, R. Gaál, L. Forró, S. L. Bud'ko, P. C. Canfield
In both papers, the fact that S/T for a specific doping has a logarithmic temperature dependence over about a decade in temperature is equated with quantum criticality.
Why am I not convinced?
1. A recent preprint shows the cuprate data can be explained using the semi-classical Boltzmann equation and a Functional Renormalisation Group treatment of the Hubbard model (see Figure 10). The sign change is associated with a the van Hove singularity and a Lifshitz transition.
2. Dynamical mean-field theory (DMFT) shows how the thermopower can change sign as a function of temperature due to strong correlations and the associated low coherence temperatures. (See e.g. Figure 4 in this preprint which I discussed in an earlier post).
I suspect that a DMFT treatment of the relevant multi-band Hubbard model with Hund's rule coupling [a la Park, Haule, Kotliar] will be able to explain the thermopower data for the pnictides.