Atomically thin MoS2 crystals have been recognized as a quasi-2D semiconductor with remarkable physical properties. This letter reports our Raman scattering measurements on multilayer and monolayer MoS2, especially in the low-frequency range (<50 cm −1 ). We find two low-frequency Raman modes with contrasting thickness dependence. With increasing the number of MoS2 layers, one shows a significant increase in frequency while the other decreases following a 1/N (N denotes the number of unit layers) trend. With the aid of first-principles calculations we assign the former as the shear mode E 2 2g . The latter is distinguished as the compression vibrational mode, similar to the surface vibration of other epitaxial thin films. The opposite evolution of the two modes with thickness demonstrates novel vibrational modes in atomically thin crystal as well as a new and more precise way to characterize thickness of atomically thin MoS2 films. In addition, we observe a broad feature around 38 cm −1 ( 5 meV) which is visible only under near-resonance excitation and pinned at the fixed energy independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in conduction band at K points in their Brillouin zone.
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...
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