  Non-stoichiometry effect on the extreme magnetoresistance is systematically
investigated for the Weyl semimetal WTe2. Magnetoresistance and Hall
resistivity are measured for the as-grown samples with a slight difference in
Te vacancies and the annealed samples with increased Te vacancies. The fittings
to a two-carrier model show that the magnetoresistance is strongly dependent on
the residual resistivity ratio (i.e., the degree of non-stoichiometry), which
is eventually understood in terms of electron doping which not only breaks the
balance between electron-type and hole-type carrier densities but also reduces
the average carrier mobility. Thus, compensation effect and ultrahigh mobility
are probably the main driving force of the extreme magnetoresistance in WTe2.
