  Topological Dirac and Weyl semimetals, which support low-energy
quasiparticles in condensed matter physics, are currently attracting intense
interest due to exotic physical properties such as large magnetoresistance and
high carrier mobilities. Transition metal diarsenides such as MoAs2 and WAs2
have been reported to harbor very high magnetoresistance suggesting the
possible existence of a topological quantum state, although this conclusion
remains dubious. Here, based on systematic angle-resolved photoemission
spectroscopy (ARPES) measurements and parallel first-principles calculations,
we investigate the electronic properties of TAs2 (T = Mo, W). Importantly,
clear evidence for switching the single-Dirac cone surface state in MoAs2 with
the cleaving plane is observed, whereas a Dirac state is not observed in WAs2
despite its high magnetoresistance. Our study thus reveals the key role of the
terminated plane in a low-symmetry system, and provides a new perspective on
how termination can drive dramatic changes in electronic structures.
