Insight into the physics of Fe-pnictides from optical and T= 0 penetration depth data

Abstract

We report theoretical values for the unscreened plasma frequencies Ω p of several Fe pnictides obtained from density functional theory (DFT) based calculations within the local density approximation (LDA) and compare them with experimental plasma frequencies obtained from reflectivity measurements on both polycrystalline samples and single crystals. The sizable renormalization observed for all considered compounds points to the presence of significant many-body effects beyond the LDA. From the large values of the empirical background dielectric constant ε ∞ ≈ 12-15 derived from reflectivity data, we estimate a large arsenic polarizabilityα As ≈ 9.5 ± 1.2Å 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 ∼4 eV on iron known from iron oxides to a level of 2 eV or below. In general, independently on such details, this result points to rather strong polaronic effects as suggested by G.A. Sawatzky et al., in references arXiv:0808.1390 [11] and arXiv:0811.0214 [12]. Possible consequences for the conditions of a formation of bipolarons are discussed, too. From the extrapolated µSR (muon spin rotation) penetration depth data at very low-temperature and the experimental value of the unscreened plasma frequency we estimate the total coupling constant λ tot for the electron-boson interaction within the framework of the Eliashberg-theory adopting an effective single band approximation. For LaFeAsO 0.9 F 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.

Theoretical investigation of optical conductivity in Ba(Fe1−xCox)Sanna1, Bernardini2, Profeta3 et al. 2011Phys. Rev. B192260View full textAdd to dashboardCite

“…The antiferromagnetic phases have the most isotropic ω p,z /ω p,x ratio, while the NM structure is the most anisotropic. Our non-magnetic plasma frequencies are in agreement with previous calculations [34][35][36] . Magnetic plasma frequencies in the Str.…”

“…However a strong dependence of the plasma frequency on temperature is not expected: in case of a similar system, BaNi 2 As 2 , the plasma frequency drops by only 5% of its value between 300 K and 150 K (above the structural/magnetic phase transition) 39 . For the non-magnetic phase, a disagreement between LDA calculated plasma frequencies and experimental estimation from optical conductivity has been already reported, and been interpreted as an indication of correlation effects 34,35,40,41 . However, assuming that the main reason for the discrepancy is correlation, our results indicate that renormalization effects are much stronger in the normal state than in the magnetic one.…”

Spin-density-wave-induced anomalies in the optical conductivity ofAFe2As2, (Charnukha1, Pröpper2, Larkin3 et al. 2013Phys. Rev. B2717531View full textAdd to dashboardCite

“…6 and Table II). This trend is also weakly present in first-principles calculations of the plasma frequency 64 but is significantly enhanced by renormalization in the real materials.…”

Electronic correlations in the iron pnictides

“…Despite the high conductivity, the reduction of the kinetic energy is significant. Therefore, the ground state wave-function of LaFePO is not the same as calculated by band theory, and dynamical correlation effects omitted by band theory have to be taken into account for 19,20,21). The error bars (not shown) for other materials are estimated to be at least the width of the symbols and at most ± 0.05.…”