\documentclass{article}
\usepackage{nicematrix}
\newcolumntype{C}{c}
\begin{document}
\begin{equation*}
  A^{0}=
  \begin{pNiceArray}{C|CCC}[margin]
    -m^{00}& 0 & 0 & 0\\
    \hline
    0 & \Block{3-3}{-m^{ab}}& &\\
    0 & & & \\
    0 & & & \\
  \end{pNiceArray}
  \qquad
  A^{k}=
  \begin{pNiceArray}{C|CCC}[margin]
    -2m^{0k}& m^{1k} & m^{2k} & m^{3k}\\
    \hline
    m^{1k}& \Block{3-3}{0_{3\times3 }}& &\\
    m^{2k}& & & \\
    m^{3k}& & &
  \end{pNiceArray}
\end{equation*}
\end{document}
