Surface electronic structure and isotropic superconducting gap in $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$

Using angle-resolved photoemission spectroscopy (ARPES), we revealed the surface electronic structure and superconducting gap of $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$, an intercalated FeSe-derived superconductor without antiferromagnetic phase or Fe-vacancy order in the FeSe layers, and with a superconducting transition temperature $\left(T_c\right)\sim 40$ K. We found that $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}$ layers dope electrons into FeSe layers. The electronic structure of surface FeSe layers in $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$ resembles that of ${\mathrm{Rb}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ except that it only contains half of the carriers due to the polar surface, suggesting similar quasiparticle dynamics between bulk $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$ and ${\mathrm{Rb}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$. Superconducting gap is clearly observed below $T_c$, with an isotropic distribution around the electron Fermi surface. Compared with $A_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2(A=\text{K},\mathrm{Rb},\mathrm{Cs},\mathrm{Tl}/\text{K})$, the higher $T_c$ in $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$ might be attributed to higher homogeneity of FeSe layers or to some unknown roles played by the $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}$ layers.

Figure 1

(a) The crystal structure and (b) magnetic susceptibility of $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$. (c), (d) False color plots of the photoemission intensity maps at the Fermi energy ($E_F$) of $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$ and ${\mathrm{Rb}}_{0.76}{\mathrm{Fe}}_{1.87}{\mathrm{Se}}_2$, respectively. The intensity was integrated over an energy window of ($E_F-15$ meV, $E_F$ + 15 meV). The Fermi surface maps are fourfold symmetrized plotted over the projected two-dimensional Brillouin zone for the unit cell of two iron ions. The Fermi surfaces of the $\delta$ band are shown by the dashed circles. In this figure and hereafter, the data of $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$ and ${\mathrm{Rb}}_{0.76}{\mathrm{Fe}}_{1.87}{\mathrm{Se}}_2$ were taken with 21.2 eV photons from an in-house helium discharge lamp and 64 eV photons at Beamline I05 of Diamond Light Source, respectively. Taking the inner potential of 11 eV [9] to calculate the $k_z^{\prime }\mathrm{s}$ of ${\mathrm{Rb}}_{0.76}{\mathrm{Fe}}_{1.87}{\mathrm{Se}}_2$, we measured at the $\Gamma \text{−}M$ plane of ${\mathrm{Rb}}_{0.76}{\mathrm{Fe}}_{1.87}{\mathrm{Se}}_2$. The Fermi surface of $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$ shows negligible $k_z$ dependence in photon-energy-dependent ARPES study. The data were taken at $T<15$ K.

Figure 2

(a) Photoemission intensity $I(k$, $\omega )$ (upper panel) of $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$ and its second derivative $[{\partial }_{\omega }^{2}I(k$, $\omega$)] (lower panel) along the $\Gamma \text{−}M$ direction (cut #1) in the right inset. The data were measured at T = 14 K. (b) The EDCs for the photoemission intensity in panel (a). (c), (d) Same as in panels (a), (b), but the data were taken from ${\mathrm{Rb}}_{0.76}{\mathrm{Fe}}_{1.87}{\mathrm{Se}}_2$ at T = 6 K.

Figure 3

(a) The symmetrized (with respect to $E_F$) photoemission intensity of $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}\mathrm{Fe}\mathrm{Se}$ at T = 45, 40, 30, and 7 K along the cut #2 in the right inset of Fig. 2. At T = 40, 30, and 7 K, the spectral weight suppression at $k_F$ near $E_F$ indicates a gap opening. (b) Temperature dependence of the spectral weight integrated over the small momentum region around $k_F$. (c) The spectral weight in (b) is symmetried with respect to $E_F$ to show the variation of gap size. (d) Temperature dependence of the gap size extracted from (c).

Figure 4

(a) Photoemission intensity map of the $\delta$ band around the zone corner M. The data were integrated over an energy window of ($E_F-15$ meV, $E_F$ + 15 meV). (b) The symmetrized (with respect to $E_F$) EDCs taken at various Fermi crossings at the $\delta$ band, as indicated by the dots in (a). (c) Gap distribution of the $\delta$ band around M in polar coordinates, where the radius indicates the gap size, and the polar angle $\theta$ represents the angle on the $\delta$ band with respect to M, with $\theta =0$ being the $\Gamma$-M direction.