Anomalous quantization trajectory and parity anomaly in Co cluster decorated BiSbTeSe2

Through the application of Co clusters, quantum Hall effect is modulated for the topological insulator BiSbTeSe2, allowing an optimized surface transport. We show the extraction of two sets of converging points in the conductivity tensor space, revealing that the top surface exhibits an anomalous quantization trajectory, while the bottom surface retains the 1/2 quantization. Co clusters are believed to induce a sizeable Zeeman gap through antiferromagnetic exchange coupling, which delays the Landau level hybridization on the top surface for a moderate magnetic field.