Figure 1

Laser ARPES data of heavily OD BaK ($x\sim$ 0.7, $T_c\sim$ 20 K): (a) FS map in the $k_x$-$k_y$ plane measured at $T\sim$ 7 K with circularly polarized light. The intensity was integrated within a window of $\pm$5 meV around $E_F$ and fourfold symmetrization was done. The dots indicate the $k_F$ points determined from MDCs fitting and the colored curves are guides to the eye. The cuts are labeled by white lines. (b)–(d) ARPES $E$-$k$ intensity maps showing the band dispersions of BaK ($x\sim$ 0.7) at $T\sim$ 7 K taken along cut 5 in panel (a) using different polarization directions of the incident laser ($c$, $v,$ and $h$). The corresponding MDC (white curves) are shown in top of panels (c) and (d) with their Lorentzian fitting (solid black curves). The white arrows represent the $k_F$ points determined from the MDC peak positions. (e)–(g) The symmetrized EDCs corresponding to several cuts taken on the FS contour [panel (a)]. (e), (f), and (g) correspond to the inner $\alpha$, middle $\beta$ and outer $\gamma$ bands respectively. The black solid lines represent the EDCs fitting to the BCS function (refer to text).Reuse & Permissions

Figure 2

Laser ARPES data of BaK with several dopings: $E$-$k$ plots taken at a cut along or close to the high-symmetry line in momentum space measured in the SC state at low temperature ($T\sim$ 7K) with circularly polarized light. The energy distribution curves (EDCs) corresponding to these $E$-$k$ plots where (a), (b), and (c) display the inner, middle, and outer band EDCs respectively and after symmetrization they are displayed in panels (d), (e), and (f). The black bars indicate the position of an additional peak observed at higher binding energies compared to the SC peak. The $x=0.4$ data were taken from Ref. 15.Reuse & Permissions

Figure 3

Doping dependence of the SC gap size (left $y$ axis) in BaK compound on the different holelike bands around the BZ center determined from the data displayed in Fig. 2 and the SC transition temperature $T_c$ (right $y$ axis). The $x=0.4$ data were taken from Ref. 15.Reuse & Permissions

Figure 4

FS maps of BaK with $x\sim$ 0.5 (a1) and 0.6 (a2) together with the corresponding magnified images around the $X$ point in panels (b1) and (b2). These maps were taken by He-discharge source with $h\nu$ $=$ 21.218 eV at $T\sim$ 7 K. The red solid lines enclose the first Brillouin zone. (c1), (c2) $E$-$k$ plots corresponding to cuts taken near the $X$ point and represented by white arrows [(c1) at $T\sim$ 35 K for $x\sim$ 0.5 and (c2) at $T\sim$ 7 K for $x\sim$ 0.6)]. Magnified $E$-$k$ plots and MDC second-derivative plots corresponding to the windows enclosed by black lines are respectively shown in panels (d1), (e1) for cut c1 ($x\sim$ 0.5) and in panels (d2), (e2) for cut c2 ($x\sim$ 0.6). The dots indicate the MDC peak positions determined from Lorentzian fitting and the black dotted lines are guides to the eye.Reuse & Permissions

Figure 5

Schematics of BaK FSs with the possible intra-orbital interaction channels 1 and 2 between the disconnected sheets through the wave vector $Q$ corresponding to BaK with $x\lesssim$ 0.5 (a) and $x\gtrsim$ 0.6 (b).Reuse & Permissions

Figure 6

Polarization-dependent laser ARPES data of BaK with $x\sim$ 0.6: The $E$-$k$ plots corresponding to cuts A and B taken at different polarization directions are displayed in the upper and lower panels respectively. The inner, middle, and outer bands are respectively labeled $\alpha$, $\beta ,$ and $\gamma$. The white curves represent the corresponding MDCs at $E_F$. The directions of cuts A and B (black solid lines), which also represent the analyzer slit direction are illustrated in the left-hand-side panels together with the FS contours and the polarization direction of the incident light.Reuse & Permissions

Figure 7

Symmetrized ARPES data corresponding to OD BaK taken below (upper panel) and above (lower panel) $T_c$.Reuse & Permissions

Figure 8

Three-dimensional Fermi surfaces of KFe${}_2$As${}_2$ obtained from DFT calculations browsing the contribution of $X^2-Y^2$ and $XZ$/$YZ$ orbitals to the Fermi surfaces at $X$ point. Panels (a) and (b) correspond to UD BaK obtained by shifting $E_F$ towards higher positions.Reuse & Permissions