Spectroscopic and thermodynamic properties in a four-band model for pnictides

TL;DR

This paper uses a four-band Eliashberg model based on ARPES data to explain various spectroscopic and thermodynamic properties of optimally-doped Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$, bridging weak and intermediate coupling regimes.

Contribution

It demonstrates that a four-band Eliashberg model accurately describes the hierarchy of gaps and low-energy signatures in pnictides, clarifying the role of coupling strength.

Findings

The model reproduces the measured gap hierarchy.

It explains band dispersion kinks and effective mass enhancements.

Thermodynamic behavior aligns with weak-coupling BCS despite intermediate coupling.

Abstract

In this paper we provide a comprehesive analysis of different properties of pnictides both in the normal and superconducting state, with a particular focus on the optimally-doped Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ system. We show that, by using the band dispersions experimentally measured by ARPES, a four-band Eliashberg model in the intermediate-coupling regime can account for both the measured hierarchy of the gaps and for several spectroscopic and thermodynamic signatures of low-energy renormalization. These include the kinks in the band dispersion and the effective masses determined via specific-heat and superfluid-density measurements. We also show that, although an intermediate-coupling Eliashberg approach is needed to account for the magnitude of the gaps, the temperature behavior of the thermodynamic quantities does not show in this regime a significant deviation with respect to weak-coupling BCS calculations. This can explain the apparent success of two-band BCS fits of experimental data reported often in the literature.