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 Abstract: Weyl semimetals are extremely interesting. Although the first Weyl semimetal was recently discovered in TaAs, research progress is still significantly hindered due to the lack of robust and ideal materials candidates. In order to observe the many predicted exotic phenomena that arise from Weyl fermions, it is of critical importance to find robust and ideal Weyl semimetals, which have fewer Weyl nodes and more importantly whose Weyl nodes are well separated in momentum space and are located close to the chemical potential in energy. In this paper, we propose by far the most robust and ideal Weyl semimetal candidate in the inversion breaking, single crystalline compound tantalum sulfide Ta$_3$S$_2$ with new and novel properties beyond TaAs. We find that Ta$_3$S$_2$ has only 8 Weyl nodes, all of which have the same energy that is merely 10 meV below the chemical potential. Crucially, our results show that Ta$_3$S$_2$ has the largest $k$-space separation between Weyl nodes among known Weyl semimetal candidates, which is about twice larger than TaAs and twenty times larger than the predicted value in WTe$_2$. Moreover, we predict that increasing the lattice by $<4\%$ can annihilate all Weyl nodes, driving a novel topological metal-to-insulator transition from a Weyl semimetal state to a topological insulator state. We further discover that changing the lattice constant can move the Weyl nodes and the van Hove singularities with enhanced density of states to the chemical potential. Our prediction provides a critically needed robust candidate for this rapidly developing field. The well separated Weyl nodes, the topological metal-to-insulator transition and the remarkable tunabilities suggest Ta$_3$S$_2$'s potential as the ideal platform in future device-applications based on Weyl semimetals. Publication Date: 10-Jan-2017 Citation: Chang, Guoqing, Xu, Su-Yang, Sanchez, Daniel S, Huang, Shin-Ming, Lee, Chi-Cheng, Chang, Tay-Rong, Zheng, Hao, Bian, Guang, Belopolski, Ilya, Alidoust, Nasser, Jeng, Horng-Tay, Bansil, Arun, Lin, Hsin, Hasan, M Zahid. (A strongly robust Weyl fermion semimetal state in Ta3S2. Science Advances, 2 (6). e1600295 (1 - 8). DOI: doi:10.1126/sciadv.1600295 Pages: e1600295 (1 - 8) Type of Material: Journal Article Journal/Proceeding Title: Science Advances Version: Author's manuscript