  EuCd2As2 was theoretically predicted to be a minimal model of Weyl semimetals
with a single pair of Weyl points in the ferromagnet state. However, the
heavily p-doped EuCd2As2 crystals in previous experiments prevent direct
identification of the semimetal hypothesis. Here we present a comprehensive
magneto-transport study of high-quality EuCd2As2 crystals with ultralow bulk
carrier density (10^13 cm-3). In contrast to the general expectation of a Weyl
semimetal phase, EuCd2As2 shows insulating behavior in both antiferromagnetic
and ferromagnetic states as well as surface-dominated conduction from band
bending. Moreover, the application of a dc bias current can dramatically
modulate the resistance by over one order of magnitude, and induce a periodic
resistance oscillation due to the geometric resonance. Such nonlinear transport
results from the highly nonequilibrium state induced by electrical field near
the band edge. Our results suggest an insulating phase in EuCd2As2 and put a
strong constraint on the underlying mechanism of anomalous transport properties
in this system.
