  Fundamental insight into the nature of the quantum phase transition from a
superconductor to an insulator in two dimensions, or from one plateau to the
next or to an insulator in quantum Hall effect, has been revealed through the
study of its scaling behavior. Here, we report on the experimental observation
of a quantum phase transition from a quantum-anomalous-Hall (QAH) insulator to
an Anderson insulator in a magnetic topological insulator by tuning the
chemical potential. Our experiment demonstrates the existence of scaling
behavior from which we extract the critical exponent for this quantum phase
transition. We expect that our work will motivate much further investigation of
many properties of quantum phase transition in this new context.
