\begin{tabular}{ | m{5em} | m{1cm}| m{1cm} | }
but if I use revtex4 it doesn't work anymore. Any ideas ? I really need to keep revtex4. Thanks !Here a dummy example :
\documentclass[aps,amsmath,showkeys,superscriptaddress]{revtex4}
%\documentclass{report}
\usepackage{booktabs}
\usepackage{array}
\begin{document}
\begin{tabular}{ | m{5em} | m{1cm}| m{1cm} | }
\hline
cell1 dummy text dummy text dummy text& cell2 & cell3 \\
\hline
cell1 dummy text dummy text dummy text & cell5 & cell6 \\
\hline
cell7 & cell8 & cell9 \\
\hline
\end{tabular}
\end{document}