Raman scattering in FeSe-based superconductors

Abstract

Raman scattering is one of the fundamental techniques in modern condensed matter physics. Its technical uniqueness allows to probe a variety of excitations in condensed matter and their couplings. FeSe-based superconductors have abundant structural phases like bulk, monolayer, Fe-deficient phases, and intercalated phases. Meanwhile, they exhibit rich electronic and magnetic phase diagrams and easily tunable superconducting transition temperatures ranged from a few Kelvins up to more than 40 K. This leads to the formation of an ideal research platform for superconductivity and has attracted a great deal of interest in the field. In this paper, we made a brief introduction to the structural phases, magnetism and electronic structures of FeSe-based superconductors. And we focused on the recent progress on Raman scattering studies of FeSe-based superconductors. For phononic Raman scattering, we introduced the Raman scattering measurement of the phonons in the Fe(Se,Te) system firstly. Next, in order to further explore the influence of doping on T c, we discussed the doping/substitution of A-sites and Fe-sites of A x Fe2− y Se2. Finally, we studied the correlation between superconductivity and various structures. We then discussed two-magnon and spin-phonon coupling in the materials. We discussed the relationship between magnetic order and superconductivity, and the effect of magnetic order on phonons. For electronic Raman response, we presented the discussions and analysis on electronic nematic phases. We introduced the observation of nematic phases by different measurement methods. And we discussed the origin of nematic phases: spin fluctuations or/and orbital (charge) fluctuations, different measurement methods have different conclusions. Finally, we introduced the studies in monolayer FeSe and some Raman scattering results there. We introduced the discovery of T c above 77 K in monolayer FeSe/SrTiO3. And we discussed many novel physical properties such as the opening of energy gap in 1UC FeSe/NbSTO and the charge transfer and ferroelectric phase transition between 1-UC thin film and NbSTO substrate.