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

Abstract

We report on a comparative optical investigation on EuFe2As2 and LaFeAsO polycrystalline samples. The spin-density wave (SDW) partial gap is observed for both compounds. The gap-like absorption peaks in the real part of conductivity for EuFe2As2 and LaFeAsO scale with their respective TSDW. Most remarkably, the study reveals a substantial difference in optical reflectance spectra for the two prototype compounds of 122 and 1111 systems: EuFe2As2 shows a much higher reflectivity with fewer phonon modes than that of LaFeAsO. This yields optical evidence for much stronger anisotropy between ab-plane and c-axis in 1111-type compounds.Keywords FeAs-based superconductor, optical properties, spin-density wave PACS numbers 75.30.Fv Understanding the competing orders in a superconducting system provides essential information in revealing the microscopic mechanism for superconductivity. Since the discovery of superconductivity up to 55 K in electron-doped 1111-type ReFeAsO (Re=rare-earth element) [1−4], considerable efforts have been devoted to exploring the nature of magnetism in the FeAs-based parent compound [5−10]. Neutron diffraction measurements on LaFeAsO directly observe a lattice distortion below 155 K, and the establishment of a long-range SDW-type antiferromagnetic (AFM) order for the Fe layer below 137 K [11]. Further experimental investigations for AFe 2 As 2 (A=alkaline-earth metal) compound [12−14] found that the structural and magnetic transition happen simultaneously in this 122 system. Although no clear signature of the structural/magnetic transition can be found for LiFeAs in the early stages [15−17], a recent study indeed finds evidence for separated structural and magnetic transitions in nearly stoichiometric Na 1−δ FeAs single crystal [18], suggesting the magnetic instability as a common feature for the FeAs-based system.The driving mechanism for the magnetic instability in the FeAs-based parent compound is interpreted from either an itinerant or a strong coupling approach. The SDW in LaFeAsO was first suggested to arise from a Fermi surface (FS) nesting between the hole and electron pockets [19]. Then it was alternatively proposed that the superexchange interaction of Fe ions mediated through the off-plane As atom plays a key role [20−25], and a stripe-type AFM order will arise when the superexchange interaction between next-nearest-neighbor Fe sites is larger than half of the nearest neighbor exchange interaction. The itinerant picture is supported by the optical measurements which find evidence for the partial gap [19,26] and the reduction of the carrier scattering rate in the SDW state [26]. However, the welldefined linear T -dependent normal state in-plane resistivity/susceptbility in BaFe 2−x Co x As 2 [27] also suggests the importance of local antiferromagnetic correlation in the FeAs-based system [28]. Recently, an inelastic neutron scattering study on CaFe 2 As 2 points out that the local moment picture is only limited to small wave vectors, while the high energy magnetic excitations have...

Interpretation of optical conductivity in normal state of Iron-Based Superconductors CeOFeAs

“…However, the low-frequency reflectance still increases fast towards unity at zero frequency, indicating a metallic behavior. Further more, the spin-density wave (SDW) partial gap is observed for EuFeAs and LaOFeAs compounds [5]. The gap-like absorption peaks in the real part of conductivity for both the compounds scale with their respective T SDW .…”