Quantized transport in topological insulator n-p-n junctions

The half-integer quantum Hall effect is one of the most iconic features of Dirac fermions. Yet, band-structure degeneracy in graphene (spin and valley) and topological insulators(top-bottom surface symmetry) prevents a direct observation of this effect. To this end, we fabricate dual gated devices of the bulk insulating topological insulator BiSbTe_1.25Se_1.75 where gate electrodes are configured to produce n-p-n junctions across the sample surface and also break the top-bottom surface inversion symmetry. In the quantum Hall regime, these junctions reveal plateaus in longitudinal resistance at 3/2 and 4/3 times the resistance quantum, h/e². We model the electrostatics of the system using four distinct filling factors which give rise to six quantum Hall edge modes. Mode mixing between these channels in the unipolar and bipolar regimes gives rise to the observed resistance fractionalization. Specifically, the fractions observed in experiment arise as a direct consequence of the 1/2 integer quantum Hall effect of the Dirac fermions, thereby providing a method to access zero-LL physics in Dirac systems.