Surface electronic structure and isotropic superconducting gap in (Li0.8Fe0.2)OHFeSe

Heavily electron-doped iron selenide superconductors, such as AxFe2-ySe2 (A = K, Rb, Cs, Tl/K) and single-layer FeSe on oxides (SrTiO3, BaTiO3), are currently the research focus in the field of iron-based superconductors. The absence of hole Fermi surfaces, together with the nodeless superconducting gap in these materials, pose great challenges on various pairing theories. Recently, a new intercalated FeSe-derived superconductor with Tc higher than 40 K, (Li0.8Fe0.2)OHFeSe, has been synthesized. The crystal structure consists of alternating stacking layers of FeSe and (Li0.8Fe0.2)OH [Fig. 1(a)], without antiferromagnetic phase or Fe-vacancy order in the FeSe layers. 

Using angle-resolved photoemission spectroscopy (ARPES), we found that (Li
0.8Fe0.2)OH layers dope electrons into FeSe layers. The electronic structure of surface FeSe layers in (Li0.8Fe0.2)OHFeSe resembles that of RbxFe2-ySe2 except that it only contains half of the carriers due to the polar surface, suggesting similar quasiparticle dynamics between bulk (Li0.8Fe0.2)OHFeSe and RbxFe2-ySe2 [Fig. 1(b-d)]. Superconducting gap (~10 meV) is clearly observed below Tc, with an isotropic distribution around the electron Fermi surface [Fig. 1(e, f)]. Compared with AxFe2-ySe2, the higher Tc in (Li0.8Fe0.2)OHFeSe could be due to the superior quality of FeSe layer in this material. The (Li0.8Fe0.2)OH layer may also play an important role for elevating Tc.

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Reference
X. H. Niu, et al., Surface electronic structure and isotropic superconducting gap in (Li0.8Fe0.2)OHFeSe, Physical Review B 92, 060504(R) (2015). editor’s choice)
URL: http://link.aps.org/doi/10.1103/PhysRevB.92.060504