  Traditional solid-state physics has long correlated the optical properties of
materials with their electronic structures. However, recent discoveries of
intrinsic gapped metals have challenged this classical view. Gapped metals
possess electronic properties distinct from both metals and insulators, with a
large concentration of free carriers without any intentional doping and an
internal band gap. This unique electronic structure makes gapped metals
potentially superior to materials designed by intentional doping of the wide
band gap insulators. Despite their promising applications, such as transparent
conductors, designing gapped metals for specific purposes remains challenging
due to the lack of understanding of the correlation between their electronic
band structures and optical properties. This study focuses on representative
examples of gapped metals and demonstrates the cases of (i) gapped metals
(e.g., CaN2) with strong intraband absorption in the visible range, (ii) gapped
metals (e.g., SrNbO3) with strong interband absorption in the visible range,
(iii) gapped metals (e.g., Sr5Nb5O17) that are potential transparent
conductors. We explore the complexity of identifying potential gapped metals
for transparent conductors and propose inverse materials design principles for
discovering new-generation transparent conductors.
