Strong Correlations, Strong Coupling and s-wave Superconductivity in Hole-doped BaFe2As2 Single Crystals

F. Hardy, A. E. Böhmer, L. de' Medici, M. Capone, G. Giovannetti, R. Eder, L. Wang, M. He, T. Wolf, P. Schweiss, R. Heid, A. Herbig, P. Adelmann, R. A. Fisher, C. Meingast

The figures below summarise some of the key physics. The top is the phase diagram.

The bottom shows the specific heat coefficient gamma as a function of alkali metal content (Cs to Rb to K, and then fractional K content (doping x).

Note that

a. The black curve shows values calculated from density functional theory (DFT) based calculations. The blue points are experimental data, which are as much as an order of magnitude larger, reflecting

**strong correlations.**

b. As one goes K to Rb to Cs the correlations are enhanced, somehow reflecting the "negative pressure" associated with the increasing ion size.

c. The experimental trend is captured nicely by calculations using slave spins (SS) to treat the relevant multi-band Hubbard model with Hund's rule coupling and band structure from DFT.

The thermal expansion alpha is particularly interesting because it is dominated by electronic effects (unlike in most metals) and shows a coherent-incoherent crossover from a Fermi liquid (where alpha/T is constant) to a bad metal at a temperature T*.

As one goes K to Rb to Cs alpha/T is enhanced reflecting the increased correlations.

One reason I am particularly interested in the manifestation of strong correlations in the thermal expansion because this also occurs in organic charge transfer salts, as discussed at length in a recent paper I published with Jure Kokalj. But, we did struggle to obtain a detailed quantitative description of the experiments, partly because of the crystallographic complexity.

It would be nice to see if a DMFT + LDA treatment of the relevant model for these iron compounds could describe the data above.

I thank Christoph Meingast for bringing this work to my attention and helpful discussions about it.

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