  Three-dimensional (3D) Dirac semimetals are new quantum materials and can be
viewed as 3D analogues of graphene. Many fascinating electronic properties have
been proposed and realized in 3D Dirac semimetals, which demonstrates their
potential applications in next generation quantum devices. Bismuth-antimony
Bi1-xSbx can be tuned from a topological insulator to a band insulator through
a quantum critical point at x ~ 4%, where 3D Dirac fermions appear. Here, we
report on a magnetotransport study of Bi1-xSbx at such a quantum critical
point. An unusual magnetic-field induced semimetal-semiconductor phase
transition was observed in the Bi0.96Sb0.04 single crystals. In a magnetic
field of 8 T, Bi0.96Sb0.04 single crystals show giant magnetoresistances of up
to 6000% at low-temperature, 5 K, and 300% at room-temperature, 300 K. The
observed magnetoresistances keep linear down to approximate zero-field when the
temperature is below 200 K. Our experimental results are not only interesting
for the fundamental physics of 3D Dirac semimetals, but also for potential
applications of 3D Dirac semimetals in magnetoelectronic devices.
