Raman Scattering in Iron-Based Superconductors

Abstract

Iron-based superconducting layered compounds have the second highest transition temperature after cuprate superconductors.Their discovery is a milestone in the history of high-temperature superconductivity and will have profound implications for high-temperature superconducting mechanism as well as industrial applications. Raman scattering has been extensively applied to correlated electron systems including the new superconductors due to its unique ability to probe multiple primary excitations and their coupling. In this review, we will give a brief summary of the existing Raman experiments in the iron-based materials and their implication for pairing mechanism in particular. And we will also address some open issues from the experiments.Keywords: Raman scattering; iron-based superconductor; electron-phonon coupling; electronic Raman scattering; two-magnon process. BackgroundRaman scattering, also known as inelastic light scattering, was discovered in 1928 by C. V. Raman. It can be used to detect many kinds of excitations in solids, such as phonons, 1 magnons, 2 excitons 3 etc.With the invention of laser and the continuous improvement of monochromator and new-generation detectors, Raman scattering has become a conventional and fundamental technique in many fields.Superconductivity is one of the most important frontiers in condensed matter physics and much Raman scattering work has been done in the field so far. Basically, Raman scattering can probe phonons and related electron-phonon coupling. The information on superconducting gap size can be drawn from cooper-pair-breaking peak in electronic Raman scattering spectra. Symmetry analysis is a unique advantage of Raman scattering. In a non-isotropic superconductor, the position, shape and low-energy behavior of pair-breaking peak will substantially change in different symmetry channels, which gives a unique way to explore pairing symmetry. Raman scattering has also been employed to detect two-magnon procedure in spin-ordered systems, from which we can obtain accurate exchange energies and learn the evolution of spin fluctuation. Actually, Raman scattering plays a crucial role in the study of cuprate superconductors.Since its discovery in 2008, iron-based superconductor has attracted a lot of interests due to its second highest transition temperature after cuprate superconductor. So far, all known iron-based superconductors can be divided into six categories in structure: 1, "1111", represented by F-doped LaFeAsO, is the first iron-based superconductor. The highest Tc reaches up to 56 K by rare-earth substitution. 4 2, "122", represented by Co or K-doped BaFe 2 As 2 , is the most studied system due to available high-quality crystals and tunable doping levels. 5 3, "111" refers to LiFeAs with the maximum Tc of 18 K. 6 4, "11" is Fe(Se,Te), the prototype of iron-based superconductor without poisonous element As. 7 5, "21311" is isostructural to 122 system with a divalent group instead of Ba. 8 6, A 0.8 Fe 1.6 Fe 2 (A=K, Tl, Cs…), which has ordered Fe-vacancies was d...

Coupling of the AsA1gphonon to magnetism in iron pnictides

“…1,3,8,42 No phonon peak is observed because of symmetry in the B 1g polarization geometry in the paramagnetic state whereas a phonon anomaly clearly emerges in the magnetic phase. 8,42,43,65 In BaFe 2 As 2 the B 1g instensity is larger than the one corresponding to the A 1g polarization symmetry. 8,42 No peaks are observed in the B 2g symmetry in either state.…”

“…Note that we address the onset of the magnetic state as a function of the interaction U at zero temperature, while in an experiment the varying parameter is the temperature and the interaction U remains constant. Experimentally, the A 1g As phonon appears in the A 1g polarization symmetry in the paramagnetic state as a small or nonidentifiable peak, 1,8,42,43 depending on the compound. This peak is strongly enhanced in 122 compounds when decreasing the temperature through the magnetostructural transition.…”

“…36 Raman response represents a powerful tool for investigating the properties of lattice dynamics. 1,3,5,8,[37][38][39][40][41][42][43] A significant narrowing of the A 1g As phonon linewidth at the onset of magnetism has been reported, whereas both softening and hardening of the phonon frequency with decreasing temperature have been observed. 1,39,41 Crucial information is also encoded in the intensity of the phonon resonances.…”

Theory of two-magnon Raman scattering in alkaline iron selenide superconductors

“…Based on the local moment Heisenberg picture for the spin-cluster configuration, two-magnon energies were estimated to be [42] It can be seen from column I-III and row VII-X in Table 1 that the theoretical Raman shifts are commonly 200 cm À1 larger than the experimental results. Besides, there is an obvious difference of more than 100 cm À1 in the calculated peak positions between A g and B g channels using the parameters from column III.…”

Electron—phonon coupling in cuprate and iron-based superconductors revealed by Raman scattering

“…And a review on Raman scattering in Fe superconductors has been made in Ref. 33. In this paper, we summarize the existing experimental and theoretical results in both cuprate and iron-based superconductors from the point of view of Raman scattering, and discuss the connection between EPC and HTSC paring mechanism.…”