High energy pseudogap and its evolution with doping in Fe-based superconductors as revealed by optical spectroscopy

Abstract

We report optical spectroscopic measurements on electron- and hole-doped BaFe2As2. We show that the compounds in the normal state are not simple metals. The optical conductivity spectra contain, in addition to the free carrier response at low frequency, a temperature-dependent gap-like suppression at fairly high energy scale near 0.6 eV. This suppression evolves with the As–Fe–As bond angle induced by electron or hole doping. Furthermore, the feature becomes much weaker in the Fe-chalcogenide compounds. We elaborate that the feature is mainly caused by the strong Hund's rule coupling effect between the itinerant electrons and localized electron moment arising from the multiple Fe 3d orbitals. The coupling strength changes with the environment of the Fe atom. Our experiments demonstrate the coexistence of itinerant and localized electrons in iron-based compounds, which would then lead to a more comprehensive picture of the metallic magnetism in the materials.

Anomalous phonon behavior in superconducting CaKFe4As4 : An optical study

“…6, the lowtemperature spectral weight of these two materials has been normalized to the integrated spectral weight at room temperature. We observe that in both materials, at low temperatures, the spectral weight at low frequency ( 1 000 cm −1 ) is suppressed and gradually recovers at roughly 12 000 cm −1 ; such behavior is widely ascribed to the Hund's rule coupling effect [37], by which the itinerant carriers are localized and polarized by the local moment. Nevertheless, compared with CaFe 2 As 2 , the spectral weight transfer is weaker in CaKFe 4 As 4 , and in KFe 2 As 2 the spectral weight transfer tends to vanish [37].…”

“…We observe that in both materials, at low temperatures, the spectral weight at low frequency ( 1 000 cm −1 ) is suppressed and gradually recovers at roughly 12 000 cm −1 ; such behavior is widely ascribed to the Hund's rule coupling effect [37], by which the itinerant carriers are localized and polarized by the local moment. Nevertheless, compared with CaFe 2 As 2 , the spectral weight transfer is weaker in CaKFe 4 As 4 , and in KFe 2 As 2 the spectral weight transfer tends to vanish [37]. As Wang et al [37] pointed out, with increasing K doping, the pnitogen height relative to the Fe layer (h As ) increases.…”

“…Nevertheless, compared with CaFe 2 As 2 , the spectral weight transfer is weaker in CaKFe 4 As 4 , and in KFe 2 As 2 the spectral weight transfer tends to vanish [37]. As Wang et al [37] pointed out, with increasing K doping, the pnitogen height relative to the Fe layer (h As ) increases. As a result, the Hund's rule coupling is enhanced [38,39], so that most of the itinerant carriers are localized at high temperature and the spectral weight transfer could not be seen from room temperature down to 5 K. Since the pnictogen height of CaKFe 4 As 4 (1.41Å) is between that of CaFe 2 As 2 (1.37Å) and KFe 2 As 2 (1.42Å), we infer that the Hund's coupling effect in CaKFe 4 As 4 also lies between that of CaFe 2 As 2 and KFe 2 As 2 .…”

Interlayer Coherence and Superconducting Condensate in thec-Axis Response of Optimally DopedBa(Fe1−xCoxMoon1, Schafgans2, Tanatar3 et al. 2013Phys. Rev. Lett.40040View full textAdd to dashboardCite

“…It is worth pointing out that the evolution of σ 1ab (ω) with T is similar to that of σ 1c (ω) in OPD BaCo122 [15,17]. Whereas a Drude-like peak becomes enhanced with decreasing T, the total SW below 3000 cm -1 is suppressed and is shifted to the higher energies in both σ 1ab (ω) and σ 1c (ω) [17].…”

“…Most of the infrared studies of the Fe-SCs are focused on the ab-plane response [8][9][10][11][12][13][14][15][16][17] and only two works explored the c-axis response of the superconducting compounds, optimally doped (OPD) Ba 0.67 K 0.33 Fe 2 As 2 (Ref. [18]) and FeTe 0.55 Se 0.45 (Ref.…”