Observation of Open-Orbit Fermi Surface Topology in Extremely Large Magnetoresistance Semimetal MoAs$_2$

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