Electronic structure of the A15 superconductors <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi mathvariant="normal">Nb</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:mi mathvariant="normal">Sb</mml:mi> </mml:math> and <mml

Superconductors with A15 structure have been studied for decades, with recent research focusing on their potential topological band structures and spin Hall effects. However, direct electronic structure studies are still lacking. Here, we have investigated the electronic structure of ${\mathrm{Nb}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Sb}$ and ${\mathrm{V}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Si}$ using angle-resolved photoemission spectroscopy. The band structure reveals pronounced three-dimensional features, which are overall consistent with the results of our density-functional theory calculations. Notably, a band renormalization factor of 1.44 is required for ${\mathrm{V}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Si}$, whereas no such renormalization is needed for ${\mathrm{Nb}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Sb}$, indicating stronger correlation effect in ${\mathrm{V}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Si}$. Flat bands are present in both samples, located at \ensuremath{-}0.28 eV below the Fermi level in ${\mathrm{Nb}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Sb}$ and nearly touching Fermi level in ${\mathrm{V}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Si}$, thereby enhancing the density of states near Fermi level and thus responsible for the higher superconducting critical temperature of ${\mathrm{V}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Si}$. Both ${\mathrm{Nb}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Sb}$ and ${\mathrm{V}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Si}$ feature four- and eightfold degenerate fermions at the $R$ point. Moreover, ${\mathrm{Nb}}_{3}\phantom{\rule{0.16em}{0ex}}\mathrm{Sb}$ is also characterized by a topological invariant of ${Z}_{2}$ = 1, confirming it as a strong topological material, which might be responsible for its nonsaturating magnetoresistance. Our results give fundamental understanding of the electronic structure and band topology of the A15 superconductors, fill the long-standing gap of the experimental electronic structure study of these materials.