Figure 1

(a) The Fermi surface topology of K${}_x$Fe${}_{2-y}$Se${}_2$ superconductor in the three-dimensional (3D) Brillouin zone corresponding to two iron ions per unit cell. (b) Schematic diagram of the $d$-wave theoretical gap symmetry with the nodal lines along the $(0,0)-(\pm \pi ,\pm \pi )$ directions. Positive and negative phases of the superconducting order parameter are denoted by different colors. The Fermi surface is based on the data in Fig. 2b.Reuse & Permissions

Figure 2

Fermi surface of K${}_x$Fe${}_{2-y}$Se${}_2$ superconductor. (a) The photoemission intensity map for the Fermi surface at $k_z$ $=$ 0 taken with 21-eV photons at 35 K. The $k_x$ and $k_y$ are defined along Fe-Fe directions. (b) The photoemission intensity map for the Fermi surface at $k_z$ $=$ $\pi$ taken with 31-eV photons at 35 K. (c) The photoemission intensity map taken along $k_y$-$k_z$ cross section of the 3D Brillouin zone. (d) The momentum distribution curves (MDCs) at Fermi energy ($E_F$) taken at different photon energies. (e), (f) The polarization dependence of the photoemission intensity maps taken in the $Z-A$ plane at 35 K with the $p$ and $s$ polarization geometries, respectively. The arrows in panel (a) represent the nesting wave vectors $q_1=(\pi$, 0.52$\pi$), $q_2=(0.52\pi$, $\pi$), $q_3=(0.52\pi$, $-$$\pi$), and $q_4=(-\pi$, 0.52$\pi$) between the relatively straight sections of neighboring electron pockets. Data were taken at SSRL for panels (a)–(d) and UVSOR for panels (e) and (f).Reuse & Permissions

Figure 3

The superconducting gap at the $\kappa$ pocket of K${}_x$Fe${}_{2-y}$Se${}_2$. (a) Symmetrized photoemission intensities for three-momentum cuts across the $\kappa$ pocket near $Z$, as shown by the blue lines labeled 1–3 in the inset. (b) Temperature dependence of symmetrized energy distribution curves (EDCs) at the Fermi crossing marked by the red dot in panel (a). (c) Symmetrized EDCs at the Fermi crossings of the $\kappa$ band with momenta counterclockwise along the $\kappa$ pocket as shown by the labeled polar angles. (d) Gap distribution of the $\kappa$ pocket around $Z$ in polar coordinates, where the radius represents the gap size, and the polar angle $\theta$ represents the position on the $\kappa$ pocket with respect to the $Z$ point, $\theta =0$ being the $Z\text{--}A$ direction. The gap is estimated through an empirical fit as described in detail in Ref. 6, and the error bars come from the standard deviation of the fitting process. The data were taken at NSRRC with 31-eV photons at 13 K for panels (a), (c), and (d) and at SSRL with 31-eV photons for panel (b).Reuse & Permissions

Figure 4

Summary of the superconducting gap distribution at $\kappa$, $\delta$, and ${\delta }^{\prime }$ pockets in K${}_x$Fe${}_{2-y}$Se${}_2$.Reuse & Permissions