  The plateau-to-plateau transition in quantum Hall effect under high magnetic
fields is a celebrated quantum phase transition between two topological states
through either sweeping the magnetic field or tuning the carrier density. The
recent realization of the quantum anomalous Hall (QAH) insulators with tunable
Chern numbers introduces the channel degree of freedom to the dissipation-free
chiral edge transport and makes the study of the quantum phase transition
between two topological states under zero magnetic field possible. Here, we
synthesized the magnetic topological insulator (TI)/TI penta-layer
heterostructures with different Cr doping concentrations in the middle magnetic
TI layers using molecular beam epitaxy (MBE). By performing transport
measurements, we found a zero magnetic field quantum phase transition between
the C = 1 and C = 2 QAH states. In tuning the transition, the Hall resistance
monotonically decreases from h/e2 to h/2e2, concurrently, the longitudinal
resistance exhibits a maximum at the critical point. Our results show that the
ratio between the Hall resistance and the longitudinal resistance is greater
than 1 at the critical point, which indicates that the original chiral edge
channel from the C = 1 QAH state coexists with the dissipative bulk conduction
channels. Subsequently, these bulk conduction channels appear to self-organize
and form the second chiral edge channel in completing the plateau phase
transition. Our study will motivate further investigations of this novel Chern
number change-induced quantum phase transition and advance the development of
the QAH chiral edge current-based electronic and spintronic devices.
