Chirality reversal quantum phase transition in flat-band topological insulators.

Quantum anomalous Hall effect generates dissipationless chiral conductive edge states in materials with large spin-orbit coupling and strong, intrinsic, or proximity magnetisation. The topological indexes of the energy bands are robust to smooth variations in the relevant parameters. Topological quantum phase transitions between states with different Chern numbers require the closing of the bulk bandgap:|C|=1→C=1/2corresponds to the transition from a topological insulator to a gapless state ink=0- quantum anomalous semimetal. Within the Bernevig-Hughes-Zhang (BHZ) model of 2D topological quantum well, this study identifies another type of topological phase transition induced by a magnetic field. The transitionC=±1→C=∓1occurs when the monotonic Zeeman field reaches the threshold value and thus triggers the reversal of edge modes chirality. The calculated threshold depends on the width of the conduction and valence bands and is more experimentally achievable the flatter the bands. The effect of the topological phase transition|ΔC|=2can be observed experimentally as a jump in magnetoresistance.