Extended Drude model and role of interband transitions in the midinfrared spectra of pnictides

Abstract

We analyze the outcomes of an extended-Drude-model approach to the optical spectra of pnictides, where the multiband nature of the electronic excitations requires a careful analysis of the role of interband processes in the optical conductivity. Through a direct comparison between model calculations of the intraband optical spectra and experimental data, we show that interband transitions, whose relevance is shown by first-principle calculations, give a non negligible contribution already in the infrared region. This leads to a substantial failure of the extended-Drude-model analysis on the measured optical data without subtraction of interband contributions.Optical studies are a useful experimental probe to analyze the interactions at play in different classes of correlated materials, 1 for which it would be desirable to devise a common theoretical scheme. A typical example is the so-called extended-Drude-model (EDM) analysis, where the experimental optical conductivity is analyzed in terms of a Drude-like model with a frequencydependent inverse lifetime and effective mass. 1This approach is well justified for a single-band system interacting with a continuum of bosonic excitations (such for instance phonons in a metal) where it can provide useful information on the relevant collective modes interacting with the electronic particle-hole excitations. 2The EDM has been widely used also for strongly correlated materials, such as the cuprate superconductors; here, intraband excitations are well separated from the optical interband transitions, which pose a natural cut-off (typically of order of 4000 cm −1 ) to the applicability of the EDM itself in these compounds. 2,3After the discovery of superconductivity in iron-based superconductors an intense experimental and theoretical research has been devoted to the EDM analysis in these materials as well. 4-8A general outcome of this analysis is a relatively large frequency-dependent in-plane inverse lifetime τ −1 (ω) ∼ ω,that, within the EDM, is interpreted in terms of a strong-coupling regime (λ ≃ 3 − 4) for the relevant bosonic excitations, 5,6 located around 20-60 meV. These are usually identified with the spin fluctuations between the hole and electron pockets of the Fermi surface, that from the very beginning have been suggested as the most promising candidates for the superconducting pairing. 9There are however two main open issues concerning these results. On one hand, a strong coupling to these low-energy bosonic modes would imply also an effective mass at low energy much larger than that measured by other probes, , even though some authors suggested that they are present at lower energies. 13-15Low-energy interband transitions may invalidate the conclusions of the EDM analysis, introducing spurious effects.In this paper, we quantify the relative role of interband and intraband transitions in the EDM analysis of the in-plane optical conductivity in pnictides, focusing on LaFePO where no magnetic transition occurs and where the coupling to spin fluctuations with...

Two-band conductivity of a FeSe$_{0.5}$Te$_{0.5}$ film by reflectance measurements in the terahertz and infrared range

“…45 Te 0.55 single crystal in a wide frequency range (2 meV-3.5 eV) and the transmission experiments in the sub-terahertz region (5-35 cm −1 ) reported by Pimenov et al [14] on a FeSe 0.5 Te 0.5 film, were well accounted for by using a one-band model conductivity, at least well above T c . On the contrary, in both papers a two-band, two-gap model was adopted in the superconducting state.…”

“…The contribution σ 1L of the lorentzian terms which were included in the fitting procedure shows an intense mid-infrared component (MIR in Fig.3b) at about 2500 cm −1 , which may be interpreted in terms of intersite electronic transitions between Fe atoms [39]. The component located in the THz region at about 75 cm −1 (THz in Fig.3b) is analoguos to the one observed in Co-doped BaFe 2 As 2 systems [36, 44,12]), which was interpreted as an intrinsic excitation [45] due to the onset of low-energy interband transitions.…”

“…As to the results obtained on different samples, Homes et al [33] find that well below T c the optical conductivity of a FeSe 0. 45 Te 0.55 single crystal is well modeled in the framework of a two-band, two-gap model with ∆ 1 = 20 cm −1 and ∆ 2 = 41 cm −1 . This result suggests either an s ± or a nodeless extended s-wave gap functions.…”

Magnetic interactions in iron superconductors: A review

“…In particular the suppression of the Drude peak spectral weight in optical conductivity experiments and that of the kinetic energy are not equal, even in a Fermi liquid description [119]. Note that the identification of the Drude peak spectral weight is also affected by interband transitions [119,191].…”

Fast recovery of the stripe magnetic order by Mn/Fe substitution in F-doped LaFeAsO superconductors

“…In the cuprates and in the electron-doped iron-based superconductors (IBS) the strength of the electronic correlations can be tuned either by charge doping or by applying an external or a chemical pressure. [10][11][12][13][14] In particular, upon increasing the charge doping, the strength of the electronic correlations tend to decrease [15][16][17][18][19][20][21][22] and a metallic Fermi liquid (FL) ground state is usually restored [23][24][25][26][27]. However significant electronic correlations may still be present even close to the charge doping levels yielding the maximum superconducting transition temperature T c and a convenient method to test their magnitude is to perturb the system with impurities.…”