Quantum phase transition between the quantum anomalous Hall liquid and insulator states

The quantum anomalous Hall (QAH) effect has been discovered in magnetically doped topological insulator (TI) thin films in zero magnetic field. A fundamental question concerning the QAH effect is whether it is merely a zero-magnetic-field quantum Hall (QH) effect, or if it can host unique quantum phases and phase transitions that are unavailable elsewhere. Here we perform transport studies on more than 80 QAH samples with different level of disorders. We discover two novel quantum phases, namely the QAH liquid and anomalous Hall (AH) insulator, and the quantum phase transition between them driven by magnetic field. Surprisingly, a universal quantum resistance h/e2 is observed at the coercive field of QAH liquid samples in the low disorder limit. We propose that the transmission between chiral edge states, tunable by disorder and magnetic field, is the key for unraveling the peculiar quantum transport phenomena in magnetic TIs.