%&LaTeX
\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{textcomp}

\begin{document}

\begin{thebibliography}{1}

\bibitem{Helm_etal1999} {\textquotedblleft}\textit{Continuum Wannier-Stark ladders strongly interacting with Zener resonances in semiconductor superlattices}{\textquotedblright}. Helm M, Hilber W, Strasser G, de Meester R, Peeters FM, Wacker A, Physical review letters \textbf{82}, 3120 (1999).   http://doi.org/10.1103/PhysRevLett.82.3120

\end{thebibliography}

\end{document}

