Fig. 3: Fermi surface evolution with Se ratio x.

a Measured Fermi surfaces for different Se ratio x at 15 K except that for x = 0, which is measured at 80 K above its magnetic transition. b Simultaneously measured constant energy contours at −60 meV showing no change. c Corresponding band dispersions and momentum distribution curves (MDCs) along the Γ-X-Γ direction. The markers are peaks fitted from MDCs. The MDCs are for binding energies from 0 to 60 meV. d Integrated energy distribution curve (EDC) around X of FeTe_1−xSe_x with the integral range marked by the red dashed lines in (e). The energy positions where the integrated EDCs show a maximum curvature are marked by the red bars. The leading edge marked by the blue and red circles represents the energy position where the EDC intensity is half of where at the maximum curvature. e Band dispersion of FeTe_0.56Se_0.44 at 15 K along the Γ-X-Γ direction. f Leading edge determined in (d) versus Se content of FeTe_1−xSe_x. g Band dispersions (second derivative) measured with 22 eV photons at 10 K along the Γ-M direction around the Brillouin zone center for different x. Parabolic fittings of the d_yz and d_xy hole bands are overlaid. The red arrow in left-most panel of (g) points at the hybridization of d_yz and d_xy orbitals. h Extracted band enhancement factor m^*/m_DFT for d_yz and d_xy plotted as a function of x represented by circles. The shaded lines are guides to the eyes. Diamond markers show the inverse of the d_xy mass enhancement where the shaded pink line is a linear fit. The empty markers are reproduced from Liu et al.⁶¹.