While recent advances in band theory and sample growth have expanded theseries of extremely large magnetoresistance (XMR) semimetals in transitionmetal dipnictides $TmPn_2$ ($Tm$ = Ta, Nb; $Pn$ = P, As, Sb), the experimentalstudy on their electronic structure and the origin of XMR is still absent.Here, using angle-resolved photoemission spectroscopy combined withfirst-principles calculations and magnetotransport measurements, we performed acomprehensive investigation on MoAs$_2$, which is isostructural to the $TmPn_2$family and also exhibits quadratic XMR. We resolve a clear band structure wellagreeing with the predictions. Intriguingly, the unambiguously observed Fermisurfaces (FSs) are dominated by an open-orbit topology extending along both the[100] and [001] directions in the three-dimensional Brillouin zone. We furtherreveal the trivial topological nature of MoAs$_2$ by bulk parity analysis.Based on these results, we examine the proposed XMR mechanisms in othersemimetals, and conclusively ascribe the origin of quadratic XMR in MoAs$_2$ tothe carriers motion on the FSs with dominant open-orbit topology, innovating inthe understanding of quadratic XMR in semimetals.
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