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\begin{document}
\begin{equation*}
J =\frac{E_R}{2} \exp (- \frac{\pi^2}{4} \sqrt{ \frac{V_0}{E_R}})
  \left[\sqrt{\frac{V_0}{E_R}} + \left( \frac{V_0}{E_R}
  \right)^{3/2}\right] 
\end{equation*}
\end{document}