Dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors*

The origin of superconductivity in Fe-based superconductors remains unclear. Although spin fluctuation plays an important role in Fe-based superconductors, other factors, such as orbital fluctuation, are also in play [1,2]. The intercalated iron selenide (Li,Fe)OHFeSe (FeSe11111) is a single-phase bulk superconductor, which can reach a high T_c of 42K [3,4]. Intriguingly, it shows remarkably similar electronic behaviors as those in monolayer FeSe with the T_c up to 65K [5], providing a bulk counterpart to explore the origin of high T_c in iron selenides. Here we performed an extensive study on FeSe11111 by using resonant inelastic x-ray scattering (RIXS) at Fe L₃-edge [6]. We have observed four Raman-like features at ∼ 0.1 eV, ∼ 0.3 eV, ∼ 0.7 eV, ∼ 2.5 eV, which are dispersionless versus momentum transfer. Moreover, these excitations show different temperature behaviors. Using atomic multiplet calculations we identify the excitations at ∼ 0.3 and 0.7 eV as local e_g - e_g and e_g - t_2g orbital excitations involving spin degrees of freedom. Our results uncover the excitations in FeSe11111 and provide an important insight to understand the superconductivity in iron selenides.

[1] Dai, P.  Rev. Mod. Phys. 87, 855-896 (2015).

[2] Kontani, H. & Onari, S. Phys. Rev. Lett.104,157001  (2010).

[3] Dong, X. et al. Phys. Rev. B 92, 064515 (2015).

[4] Lu,  X.  F.et al. Nature Ma-terials14, 325–329 (2015)

[5] Zhao, L. et al. Nature Communications 7, 10608 (2016).

[6] Xiao, Q. et al. arXiv:2110.05361 (2021).