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We theoretically reveal the possibility of a specular Andreev reflection in a thin film topological insulator normal-superconductor (N/S) junction. We demonstrate the determining effects of the gate-induced potential difference of the up and down surfaces of the thin film ($U$), and the coupling parameter of the two surfaces ($\omega$) on the probability of the electron-hole conversion in a specular Andreev reflection. The probability of the specular Andreev reflection increases with $U$ and the electron-hole conversion with unit efficiency happens in a wide experimental range of the potential difference $U$. This perfect Andreev reflection is found to show strong robustness with respect to increasing the coupling parameter $\omega$ for normally incident electrons to the N/S interface with the excitation energy $\varepsilon=\Delta_S$ ($\Delta_S$ is the superconducting gap), while it decreases with $\omega$ for $\varepsilon<\Delta_S$. We further demonstrate that this perfect specular Andreev reflection can be occurred for all angles of incidence to the proposed N/S interface with $\varepsilon=\Delta_S$. Our results reveal the potential of the proposed topological insulator thin film-based N/S structure for the realization of the intra-band specular Andreev reflection.
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