New insight into the physics of iron pnictides from optical and penetration depth data

TL;DR

This paper combines theoretical and experimental optical data to analyze the electronic properties of iron pnictides, revealing strong many-body effects, significant polarizability, and implications for superconductivity mechanisms.

Contribution

It provides new estimates of plasma frequencies, polarizability, and electron-boson coupling constants in iron pnictides, highlighting many-body effects beyond standard DFT calculations.

Findings

Large renormalization of plasma frequencies indicates many-body effects.

Estimated arsenic polarizability reduces Coulomb repulsion U_d to 2 eV or below.

Weak to intermediate coupling regime found in LaFeAsO_0.9F_0.1, with potential for strong coupling in multiband scenarios.

Abstract

We report theoretical values for the unscreened plasma frequencies Omega_p of several Fe pnictides obtained from DFT based calculations within the LDA and compare them with experimental plasma frequencies obtained from reflectivity data. The sizable renormalization observed for all considered compounds points to the presence of many-body effects beyond the LDA. From the large empirical background dielectric constant of about 12-15, we estimate a large arsenic polarizability of about 9.5 +- 1.2 Angstroem^3 where the details depend on the polarizabilities of the remaining ions taken from the literature. This large polarizability can significantly reduce the value of the Coulomb repulsion U_d about 4 eV on iron known from iron oxides to a level of 2 eV or below. In general, this result points to rather strong polaronic effects as suggested by G.A. Sawatzky et al., in Refs. arXiv:0808.1390 and arXiv:0811.0214 (Berciu et al.). Possible consequences for the conditions of a formation of bipolarons are discussed, too. From the extrapolated muon spin rotation penetration depth data at T= 0 and the experimental Omega_p we estimate the total coupling constant lambda_tot for the el-boson interaction within the Eliashberg-theory adopting a single band approximation. For LaFeAsO_0.9F_0.1 a weak to intermediately strong coupling regime and a quasi-clean limit behaviour are found. For a pronounced multiband case we obtain a constraint for various intraband coupling constants which in principle allows for a sizable strong coupling in bands with either slow electrons or holes.