Fig. 1

Energy band dispersion of (a) Pb\(_2\)As\(_2\)O\(_7\), (b) Pb\(_2\)Sb\(_2\)O\(_7\) and (c) Pb\(_2\)Sn\(_2\)O\(_7\) from first-principles calculations. The purple crosses are the energy bands obtained by the first-principles calculations, and the green solid line are the bands constructed from the Wannier functions. The orange hatched area contains PB1, and the blue hatched area contains PB2. In the panel (c), a quadratic band touching occurs at \(E_\textrm{F}\), shown by a red circle. \(E_\textrm{F}\) is set to 0 eV.