Optical and Raman spectroscopy studies on Fe-based superconductors

Abstract

A brief review of optical and Raman studies on the Fe-based superconductors is given, with special emphasis on the competing phenomenon in this system. Optical investigations on ReFeAsO (Re=rare-earth element) and AFe 2 As 2 (A=alkaline-earth metal) families provide clear evidence for the gap formation in the broken symmetry states, including the partial gaps in the spin-density wave states of parent compounds, and the pairing gaps in the superconducting states for doped compounds. Especially, the superconducting gap has an s-wave pairing lineshape in hole-doped BaFe 2 As 2 . Optical phonons at zone center detected by Raman and infrared techniques are classified for several Fe-based compounds. Related issues, such as the electron-phonon coupling and the effect of spin-density wave and superconducting transitions on phonons, are also discussed. Meanwhile, open questions including the T-dependent mid-infrared peak at 0.6-0.7 eV, electronic correlation, and the similarities/differences between high-T c cuprates and Fe-based superconductors are also briefly discussed. Important results from other experimental probes are compared with optical data to better understand the spin-density wave properties, the superconductivity, and the multi-band character in Fe-based compounds.

Effective medium approximation and the complex optical properties of the inhomogeneous superconductorK0.8Fe2−ySe2Homes1, Xu2, Wen3 et al. 2012Phys. Rev. B172160View full textAdd to dashboardCite

“…In this work we apply the Bruggeman effective-medium approximation dielectric function 32 to the normal-state optical properties of K 0.8 Fe 2−y Se 2 . The metallic inclusions appear to comprise about 10% of the total sample volume, resulting in a Drude plasma frequency that is significantly higher than the volume-averaged value 28 but is still much smaller that the values observed in other (homogeneous) ironbased superconductors, [33][34][35][36][37][38][39][40] unless volume fractions of less than 1% are considered. Interestingly, and in agreement with another recent study of the optical properties of this material, 25 the EMA can not be applied to the data successfully without assuming that the inclusions are extremely distorted, suggesting the formation of filamentary conducting networks.…”

“…Given that the insulating matrix accounts for about 90% of the sample volume, it is not surprising that the vibrational parameters should remain essentially unchanged. Previous Drude-Lorentz fits to the volume-averaged optical conductivity at 35 observed in these materials. [25][26][27] If we attribute this average plasma frequency to the fraction f of the sample that is metallic, then 33-39 Indeed, for ω p,D to rival these values would require a volume fraction of less than 1%.…”

Optical properties of the iron arsenic superconductorBaFe1.85Co0.15As2

“…18 The emergence of superconductivity and the suppression of the magnetic and structural transitions may be achieved through the application of pressure 19 or through chemical substitution; 20-23 K doping results in a hole-doped material with a maximum T c Ӎ 38 K while Ni and Co doping result in electron-doped materials with a somewhat lower maximum T c Ӎ 29 K. The electron-and hole-doped materials have been the subject of numerous investigations, including thermodynamic and transport studies, [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] angle-resolved photoemission [36][37][38][39][40][41][42][43][44][45] ͑ARPES͒, and Raman [46][47][48][49] and optical studies. [50][51][52][53][54][55][56][57][58][59][60][61][62][63][64]…”

Optical properties of FeAs-based parent compound: A comparative study for polycrystalline EuFe2As2 and LaFeAsO

“…density wave state) [30,35], i.e., the SDW gap is determined by the peak position in σ 1 (ω). When the SDW gap is anisotropic, or there are multi gaps on different FS sheets, the sharp asymmetry peak in σ 1 (ω) might be broadened into a rounded peak without any clear edge-like feature.…”