$_{b}^{a}$

documentclass{article}
usepackage{stackengine}
stackMath
begin{document}
% defstacktype{S}% DEFAULT TYPE, constant gap between stacked elements
[
x stackon{g}{b} y stackon{b}{g} zquad
x stackunder{g}{b} y stackunder{b}{g} zquad
x stackanchor{g}{b} y stackanchor{b}{g} z
]
defstacktype{L}% constant baselineskip between stacked elements
[
x stackon{g}{b} y stackon{b}{g} zquad
x stackunder{g}{b} y stackunder{b}{g} zquad
x stackanchor{g}{b} y stackanchor{b}{g} z]
end{document}

Put $a$ over $b$: $a atop b$